Wire gauge selection

I'm working on building out my electrical system for my RV conversion (2020 Sprinter 170 4x4) and there are a few questions I could really use help with.

Long term I will be adding much more to the build (including a battery bank), but my plan for this phase of the build is:

Battery
- Factory Auxiliary battery (AGM 92Ah)

Loads
-Water pump 1 - Shurflo (10A breaker, 7.5A max current)
-Water pump 2 - Shurflo (10A breaker, 7.5A max current)
-Macerating toilet - Tecma Thetford Elegance 2g (40A breaker, unknown max current)
-Fan - Maxxair 7500K (10A breaker, 5A max current)
-Heater - Espar D2 (20A breaker, 14A max current)
-Pico battery monitor and lights (10A breaker, 5A current)

I'll be using a Blue Sea DC 8 position breaker panel, and I'll be running a wire from the battery under the passenger seat to the panel in the back of the van. The individual circuits will then run from that panel. Here are the questions I'm stuck on:

1) When calculating the size of the panel feed wire (from the battery to the panel), I understand that I include the length of both the positive and negative runs, but do I also need to account for the length of wire from the panel to the devices? It seems to me that if I just use the length of the two wires from the battery to the panel, with acceptable 3% voltage drop, and then do this same thing for the individual circuits, wont those percentages add up?

2) Does the calculation change If I were to connect the negative wire to a local ground instead of running it all the way back to the battery? are there losses in the chassis that would need to be accounted for, or would I just count the total (now shorter) wire length? Does this mean that the wire could be a smaller diameter if I find a closer ground?

3) Do I size the feed wire to take the total max current, or the sum of all the breakers?

Thanks for any help!
 

elemental

Wherever you go, there you are.
1) When calculating the size of the panel feed wire (from the battery to the panel), I understand that I include the length of both the positive and negative runs, but do I also need to account for the length of wire from the panel to the devices? It seems to me that if I just use the length of the two wires from the battery to the panel, with acceptable 3% voltage drop, and then do this same thing for the individual circuits, wont those percentages add up?
It is my understanding that if you engineer your wire size across one distance for a 3% voltage drop at a particular expected current, then a measurement of the voltage potential at that distance from the origin will be 3% less than at the origin. So... if the battery is at a 12.5 volt potential, then you would be at 12.125 volts at the end of that run, if you pull the maximum design current across that run. Then... if you add another run with a 3% drop, you would be at 11.76 volts at the end of the second run, or about 6% - IF - you are pulling maximum design current across each run. I'm no expert, but I don't see how it could be otherwise.

However... that amount of loss only occurs if you are pulling the maximum design current across each run, which is unlikely, so your actual voltage will not be impacted that much. Chances are your branch circuits won't be at any of their maximum design loads, and even if one or two of them are it is unlikely your feed circuit would be unless you have been really tight with your estimated current flows. What you choose for your design maximums will make a difference, especially the maximum expected current flow for the feed wire.

For question 3) [feed wire size] - size it for the expected maximum current flow with the desired voltage potential loss, and put a breaker at the head end (where it comes off the battery) that protects that wire. If you do a very realistic sum of branch circuit currents based on likely simultaneous operation, then you might want to design for only a 1% drop on your feed circuit or else risk under voltage conditions on the branch circuits when your maximum simultaneous operation conditions are reached (i.e., refrigerator cycles on while heater is starting up, a bunch of lights are on and someone runs the toilet).

The breakers for your branch circuits should be sized to protect those wires; the breaker for the feed is sized to protect that wire. If something were to go wrong and all of the branch circuits pulled their maximum current without exceeding it (and flipping the individual branch circuit breakers) you would want the feed wire breaker to flip (unless you have sized the feed wire to carry the current of all branch circuits simultaneously at full load).
 

autostaretx

Erratic Member
It is my understanding that if you engineer your wire size across one distance for a 3% voltage drop at a particular expected current, then a measurement of the voltage potential at that distance from the origin will be 3% less than at the origin. So... if the battery is at a 12.5 volt potential, then you would be at 12.125 volts at the end of that run, if you pull the maximum design current across that run. Then... if you add another run with a 3% drop, you would be at 11.76 volts at the end of the second run, or about 6% - IF - you are pulling maximum design current across each run. I'm no expert, but I don't see how it could be otherwise.

However... that amount of loss only occurs if you are pulling the maximum design current across each run, which is unlikely, so your actual voltage will not be impacted that much. Chances are your branch circuits won't be at any of their maximum design loads, and even if one or two of them are it is unlikely your feed circuit would be unless you have been really tight with your estimated current flows. What you choose for your design maximums will make a difference, especially the maximum expected current flow for the feed wire.

For question 3) [feed wire size] - size it for the expected maximum current flow with the desired voltage potential loss, and put a breaker at the head end (where it comes off the battery) that protects that wire. If you do a very realistic sum of branch circuit currents based on likely simultaneous operation, then you might want to design for only a 1% drop on your feed circuit or else risk under voltage conditions on the branch circuits when your maximum simultaneous operation conditions are reached (i.e., refrigerator cycles on while heater is starting up, a bunch of lights are on and someone runs the toilet).

The breakers for your branch circuits should be sized to protect those wires; the breaker for the feed is sized to protect that wire. If something were to go wrong and all of the branch circuits pulled their maximum current without exceeding it (and flipping the individual branch circuit breakers) you would want the feed wire breaker to flip (unless you have sized the feed wire to carry the current of all branch circuits simultaneously at full load).

Elemental,

Thank you for the detailed response, that's very useful. I was searching through the articles on Blue Sea's website last night, and found some useful marine standards. ABYC E-11 includes an "Electrical Equipment Load Requirement Worksheet" for calculating the total electrical load. It's on page 14 of this link.

ABYC E-11

It's basically the sum (A) of all the loads for circuits needed for continuous duty basis, and then sum (B) of all remaining loads. You then compare 10%*sum(B) to the largest load in the B column, and take whichever one is larger, and add it to sum (A). For my circuit, I found all of the highest continuous current specs I could find, and I assumed that one of the pumps was running continuously.


1603028183378.png

Sum (A) = 19.5A
10% Sum (B) = 2.5A
Greatest B load = 17A

Since 17A is greater than 2.5A, my final number is 19.5A + 17A = 36.5A

I then used the "DC Circuit Wizard" on Blue Sea's website to calculate the feeder wire size:

1603028887530.png
This gave me a 1 AWG wire. I used 2% voltage drop because I found a standard saying that the combined voltage drop of the feeder circuit and branch circuit should not be greater than 5%. I'll use this same 2% on the critical branch circuits, which in my case I guess is only the battery monitor and Espar D2, as everything else is pumps and fans.

Does this sound reasonable to you?
 
For up to a 20 foot run (source to toilet), the toilet appears to want 10 gauge wire (so that's 40 wire feet)
(per Fig D (wire size chart) in this document: https://www.thetford.com/wp-content/uploads/2018/03/98269D-IO-2G-RV.pdf?x98523 )

The 40 amps probably means that a stalled pump will draw 30 amps (industry practice is to provide 25% excess fuse capacity).

--dick
Dick,

I'm a little confused about the circuit requirements on the Tecma toilet. Page for of this link, below the "Wire Gauge Size Chart" there is a list of "Vehicle Electrical Requirements" and one of those is "Every toilet requires a 12-VDC/40-AMP dedicated circuit with 8-gauge wire and 40-AMP breaker of fuse between the main bus or battery and a terminal located near the toilet." Is this describing the branch circuit, or the feeder circuit?

Thetford Tecma

Also, in the bottom left of the chart, it states "Distance measured assumes power and ground wires." Does that mean that you don't include the length of wire that is a part of the toilet wiring harness?

Thanks!
Phil
 
I believe the Blue Sea Panels have their own max amp rating. Thinking it was 30 or 50 amps.
Lynn,

The 8 position Blue Sea DC panel appears to have a maximum current of 100A:

Blue Sea DC Panel - 8 Positions

Am I correct in thinking this means the maximum feeder current would be 100A, but I could still install breakers that add up to more than 100A? My load calculations for the feeder wire are 36.5A so I shouldn't be anywhere near 100A, and could install a breaker or fuse on the feeder circuit before the panel. My breakers on the panel currently add up to 100A. Long term I'll be adding 2 more 15A breakers for fridge and freezer (Isothem 195 - I promise I'm not copying everything you did) but not until I have a larger battery bank.

Thanks
Phil
 

gltrimble

Well-known member
The 8 position Blue Sea DC panel appears to have a maximum current of 100A:
My DC breakers add up to 110 amps with the Espar D2 being the only 20 amp breaker. All together my total DC “operating” amps is less than 30 amps. My 60 amp DC air compressor is wired and fused separately. I believe my feed to the DC panel is #6 wire for 50 amps. My AC panel feed is #10 for 30 amps which is the limit of the panel.

1D428561-7CFD-49DF-A14C-690C01A3E1A8.jpeg
 

borabora

Active member
The problem I see here is that you are designing this for both the current fairly wimpy AGM battery and the future bigger presumably LIFEPO4 battery. Your voltage drop calculations are based on distance/wire gauge/current only and apply to both batteries but the AGM battery starts at a lower working voltage and will sag significantly if you are drawing anything close to what you can draw. If you design for the AGM battery you really need to over-size your wires and even then some devices may not operate due to battery voltage sag. If you design for the future LIFEPO4 battery you can pretty much ignore voltage drop due to wires and design wire size simply for max current since your starting voltage is nearly 1V higher and sag is much smaller. Breakers should be sized for max wire current not the load.
 
The problem I see here is that you are designing this for both the current fairly wimpy AGM battery and the future bigger presumably LIFEPO4 battery. Your voltage drop calculations are based on distance/wire gauge/current only and apply to both batteries but the AGM battery starts at a lower working voltage and will sag significantly if you are drawing anything close to what you can draw. If you design for the AGM battery you really need to over-size your wires and even then some devices may not operate due to battery voltage sag. If you design for the future LIFEPO4 battery you can pretty much ignore voltage drop due to wires and design wire size simply for max current since your starting voltage is nearly 1V higher and sag is much smaller. Breakers should be sized for max wire current not the load.
Thanks for the input. I know it isn't ideal to be doing this in stages. We are currently in Montreal, and on a time frame for the build, planning to leave for California in a month or so. When I do install a LiFEPO4 battery bank, it will be located within 5 feet of the panel (near the rear wheel well), whereas the current aux AGM is under the passenger seat, and I'm connecting to the connector under the Driver's seat, so there is quite a lot of wire length to reach the panel. My plan when I switch to LIFEPO4 is to repurpose this AGM wire as a charging wire from the alternator feeding a B2B charger. I'm not sure if voltage drop matters for that application, but the current load would be similar, so I think it should work.

Our current use case with the AGM will be driving 6 hours per day, and running the heater and fan during the night. Hopefully the AGM will be up to the task. The other loads will be temporary, so hopefully we have the capacity.

Another issue I'm running into with sourcing parts here in Canada is that it is so expensive, so I'm waiting to make some of the larger purchase in the states (batteries, fridge, solar, chargers, inverter).

That's interesting about the higher voltage of the LiFEPO4 making the voltage drop less critical, that's new information for me, thanks!
 
My DC breakers add up to 110 amps with the Espar D2 being the only 20 amp breaker. All together my total DC “operating” amps is less than 30 amps. My 60 amp DC air compressor is wired and fused separately. I believe my feed to the DC panel is #6 wire for 50 amps. My AC panel feed is #10 for 30 amps which is the limit of the panel.

View attachment 157569

If you are feeding the panel with a 6 AWG wire for 50 Amps, it must be quite short. If I use the Blue Sea Circuit Wizard for 50A, 3% Voltage drop, anything above 18 ft loop (9 feet per wire) gives a 4 AWG output. I know you have your batteries mounted under the van, are they quite close to the panel?
 

gltrimble

Well-known member
If you are feeding the panel with a 6 AWG wire for 50 Amps, it must be quite short. If I use the Blue Sea Circuit Wizard for 50A, 3% Voltage drop, anything above 18 ft loop (9 feet per wire) gives a 4 AWG output. I know you have your batteries mounted under the van, are they quite close to the panel?
My 50 amp DC supply is 10’ or less in length. My typical DC panel load rarely exceeds 10 amps. And with lithium batteries low voltage is not an issue. Yes, my batteries are located beneath the floor forward of the rear wheels. I have 2/0 wire feeding my inverter which is behind my Transit seat. The wire from the inverter to the DC panel, located above the driver’s side window is 6 AWG.
 
It is my understanding that if you engineer your wire size across one distance for a 3% voltage drop at a particular expected current, then a measurement of the voltage potential at that distance from the origin will be 3% less than at the origin. So... if the battery is at a 12.5 volt potential, then you would be at 12.125 volts at the end of that run, if you pull the maximum design current across that run. Then... if you add another run with a 3% drop, you would be at 11.76 volts at the end of the second run, or about 6% - IF - you are pulling maximum design current across each run. I'm no expert, but I don't see how it could be otherwise.

However... that amount of loss only occurs if you are pulling the maximum design current across each run, which is unlikely, so your actual voltage will not be impacted that much. Chances are your branch circuits won't be at any of their maximum design loads, and even if one or two of them are it is unlikely your feed circuit would be unless you have been really tight with your estimated current flows. What you choose for your design maximums will make a difference, especially the maximum expected current flow for the feed wire.

For question 3) [feed wire size] - size it for the expected maximum current flow with the desired voltage potential loss, and put a breaker at the head end (where it comes off the battery) that protects that wire. If you do a very realistic sum of branch circuit currents based on likely simultaneous operation, then you might want to design for only a 1% drop on your feed circuit or else risk under voltage conditions on the branch circuits when your maximum simultaneous operation conditions are reached (i.e., refrigerator cycles on while heater is starting up, a bunch of lights are on and someone runs the toilet).

The breakers for your branch circuits should be sized to protect those wires; the breaker for the feed is sized to protect that wire. If something were to go wrong and all of the branch circuits pulled their maximum current without exceeding it (and flipping the individual branch circuit breakers) you would want the feed wire breaker to flip (unless you have sized the feed wire to carry the current of all branch circuits simultaneously at full load).

Elemental, one more question - When choosing a fuse / breaker to connect the feeder wire at the battery terminal, I hear over and over to select the fuse / breaker size to protect the cable. If the cable has a much higher current capacity than I will be drawing for the panel (because it's a long run - cable is sized to minimize voltage drop) does it make sense to put the maximum size allowable for the cable, or should I install something closer to the max current?

In my case:

1 AWG wire with derating factors - 171 A max. If my max current is 50A, would it be better to install a 100A or 150A fuse / breaker?
 

RVBarry

Active member
"Every toilet requires a 12-VDC/40-AMP dedicated circuit with 8-gauge wire and 40-AMP breaker of fuse between the main bus or battery and a terminal located near the toilet." Is this describing the branch circuit, or the feeder circuit?
Branch.
 

elemental

Wherever you go, there you are.
Thank you for the detailed response, that's very useful. I was searching through the articles on Blue Sea's website last night, and found some useful marine standards. ABYC E-11 includes an "Electrical Equipment Load Requirement Worksheet" for calculating the total electrical load. It's on page 14 of this link.

ABYC E-11

It's basically the sum (A) of all the loads for circuits needed for continuous duty basis, and then sum (B) of all remaining loads. You then compare 10%*sum(B) to the largest load in the B column, and take whichever one is larger, and add it to sum (A). For my circuit, I found all of the highest continuous current specs I could find, and I assumed that one of the pumps was running continuously.

[...]

Sum (A) = 19.5A
10% Sum (B) = 2.5A
Greatest B load = 17A

Since 17A is greater than 2.5A, my final number is 19.5A + 17A = 36.5A

[...]
This gave me a 1 AWG wire. I used 2% voltage drop because I found a standard saying that the combined voltage drop of the feeder circuit and branch circuit should not be greater than 5%. I'll use this same 2% on the critical branch circuits, which in my case I guess is only the battery monitor and Espar D2, as everything else is pumps and fans.

Does this sound reasonable to you?
I think ABYC material is good stuff. It looks like you didn't make a mistake following the ABYC worksheet. Your assumptions about which loads represent "continuous loads", and which current specs you used, are all of course your local knowledge about how you will use your devices. The voltage drop choices are where you want to truly understand consequences and not just follow standards. My perspective is that the main cost of using lower voltage drop percentages (i.e., 2% instead of 3% or 5%, for example) is that the wire sizes end up larger rather than smaller. As long as the $ cost difference is reasonable to you (more copper = more $$), and as long as you can fit the wires where they need to go, then you probably don't need to worry much. The value of using the lower voltage percentages is that your wiring will be more energy-efficient (less loss) and your load devices will operate with higher voltage levels.

You should consider (if you aren't already) how your wiring will work once you put in your own battery storage system to replace/supplement the factory auxiliary battery. What will be the role of the 1 AWG feeder wire from the front to the back in the new configuration, for example, and will it be sufficient for this role? Given that the 95ah aux battery is very small compared to your projected usage (i.e., if you did draw 36.5 amps pretty regularly, you would less than 90 minutes of power from the aux battery before it was at 50% state of charge) it is really just a temporary bandaid. If your long-term plan is to use a larger battery storage system located in the rear of the van near the electrical panel(s), then you might be planning to use the feeder line to supply power from the van electrical system to a DC-DC charger that was also located near the battery storage system. Will the 1AWG be sized appropriately for the amount of current the DC-DC charger will draw? If it is too small, installing the bigger feed wire now would make a lot of sense.

Elemental, one more question - When choosing a fuse / breaker to connect the feeder wire at the battery terminal, I hear over and over to select the fuse / breaker size to protect the cable. If the cable has a much higher current capacity than I will be drawing for the panel (because it's a long run - cable is sized to minimize voltage drop) does it make sense to put the maximum size allowable for the cable, or should I install something closer to the max current?

In my case:

1 AWG wire with derating factors - 171 A max. If my max current is 50A, would it be better to install a 100A or 150A fuse / breaker?
I believe that if the cable has a much higher current capacity then you require, it is reasonable to install a smaller breaker. Don't size it immediately above your expected max current, however (unless that was what the wire required). Having headroom is useful in case you end up drawing more current than you thought you might but at a level that is still safe. Of course, it would never be ok to use a bigger breaker than the wire's current capacity.

In the example you gave, either a 100 or 150 amp fuse/breaker would work, and there probably isn't much cost difference between the two, so "gentlemen's choice" in my opinion. If another forum user feels otherwise they will probably jump in :).
 

autostaretx

Erratic Member
Elemental, one more question - When choosing a fuse / breaker to connect the feeder wire at the battery terminal, I hear over and over to select the fuse / breaker size to protect the cable. If the cable has a much higher current capacity than I will be drawing for the panel (because it's a long run - cable is sized to minimize voltage drop) does it make sense to put the maximum size allowable for the cable, or should I install something closer to the max current?

In my case:

1 AWG wire with derating factors - 171 A max. If my max current is 50A, would it be better to install a 100A or 150A fuse / breaker?
(first, as RVBarry wrote, the 40 amp, 8 (or 10) gauge is on the *branch* circuit to the toilet. The installation manual assumes that you've got "adequate" feeder wiring to the breaker panel.)
The rated current capacity for single runs of10 gauge wire with TW insulation in free air is 40 amps.
For three wires in a cable (such as romex) it drops to 30 amps. (all 3 wires carrying current)
As a 2-wire rubber jacketed extension cord, it's 28 amps.

There are three basic things to consider on the choice of stuff.
(a) the cable thickness is chosen to minimize voltage drop under the maximum (expected) load.
(b) the feeder breaker at the battery should not allow current that would be above that cable's capacity
(c) BUT ... it doesn't need to be bigger than 25% over your maximum expected load.

Thus it may well be smaller than the cable could handle.

Given your 1 AWG example, if you're only ever expecting to draw 50 amps, a 70 or 75 amp breaker would be more than adequate, and would blow (interrupt) in the case of a fault sooner than a 100 or 150 amp breaker might do.

If you look at your house's breaker panel, you'll probably see that the total of the branch breakers exceeds the main breaker by quite a bit (my old house panel had a "sub main" layout ... with a 70 amp breaker feeding it. The total of the branch circuits on that 70 amp system was 120 amps per side. Perfectly legal and safe.)

IF, in the future (adding a Hot Tub?) you find that you're popping the 75 amp breaker under "normal" use, then simply swap it out for a bigger value that is still under the limit set by (b).

--dick
 
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I think ABYC material is good stuff. It looks like you didn't make a mistake following the ABYC worksheet. Your assumptions about which loads represent "continuous loads", and which current specs you used, are all of course your local knowledge about how you will use your devices. The voltage drop choices are where you want to truly understand consequences and not just follow standards. My perspective is that the main cost of using lower voltage drop percentages (i.e., 2% instead of 3% or 5%, for example) is that the wire sizes end up larger rather than smaller. As long as the $ cost difference is reasonable to you (more copper = more $$), and as long as you can fit the wires where they need to go, then you probably don't need to worry much. The value of using the lower voltage percentages is that your wiring will be more energy-efficient (less loss) and your load devices will operate with higher voltage levels.

You should consider (if you aren't already) how your wiring will work once you put in your own battery storage system to replace/supplement the factory auxiliary battery. What will be the role of the 1 AWG feeder wire from the front to the back in the new configuration, for example, and will it be sufficient for this role? Given that the 95ah aux battery is very small compared to your projected usage (i.e., if you did draw 36.5 amps pretty regularly, you would less than 90 minutes of power from the aux battery before it was at 50% state of charge) it is really just a temporary bandaid. If your long-term plan is to use a larger battery storage system located in the rear of the van near the electrical panel(s), then you might be planning to use the feeder line to supply power from the van electrical system to a DC-DC charger that was also located near the battery storage system. Will the 1AWG be sized appropriately for the amount of current the DC-DC charger will draw? If it is too small, installing the bigger feed wire now would make a lot of sense.


I believe that if the cable has a much higher current capacity then you require, it is reasonable to install a smaller breaker. Don't size it immediately above your expected max current, however (unless that was what the wire required). Having headroom is useful in case you end up drawing more current than you thought you might but at a level that is still safe. Of course, it would never be ok to use a bigger breaker than the wire's current capacity.

In the example you gave, either a 100 or 150 amp fuse/breaker would work, and there probably isn't much cost difference between the two, so "gentlemen's choice" in my opinion. If another forum user feels otherwise they will probably jump in :).
Elemental,

Thanks for taking the time to look through my design choices, I really appreciate it. The factory Aux AGM is indeed a temporary band aid. I'm hoping it will be enough for our trip from Montreal to San Diego. We will be driving every day, so the batteries will be recharged, and the only real continuous use will be the D2 and fan at night, as well as some lights, and the occasional water pump.

I would love to size the feeder line to be repurposed as DC-DC charger. I suppose the expected load will depend on the size of the battery bank and whether or not I install an additional alternator, or rely on the factory alternator. I'm intending on a 200Ah LiFEPO4 battery system, and will also be installing solar, so won't be relying only on the alternator to charge the batteries, so maybe I can get away with the factory alternator. I think the factory alternator can supply 40-50A (which would work with the 1 AWG cable), but I'll dig into it today and see what I can find. Is the voltage drop critical with a DC-DC charger, or is it just a question of current?

One more question about the breakers / fuse. If I installed a fuse at the terminal (lets say it's fairly close to the load capacity of the cable), would it then be reasonable to install a breaker closer to the panel (and rated lower than the fuse), so it's more accessible, and could be also used to cut the power to the panel? I know it's important that there is a fuse or breaker at the terminal so you don't risk a short in an unprotected section of wire, and it seems like once the fuse is at the terminal, it is no longer critical where I place an additional breaker. Thoughts?
 
(first, as RVBarry wrote, the 40 amp, 8 (or 10) gauge is on the *branch* circuit to the toilet. The installation manual assumes that you've got "adequate" feeder wiring to the breaker panel.)
The rated current capacity for single runs of10 gauge wire with TW insulation in free air is 40 amps.
For three wires in a cable (such as romex) it drops to 30 amps. (all 3 wires carrying current)
As a 2-wire rubber jacketed extension cord, it's 28 amps.

There are three basic things to consider on the choice of stuff.
(a) the cable thickness is chosen to minimize voltage drop under the maximum (expected) load.
(b) the feeder breaker at the battery should not allow current that would be above that cable's capacity
(c) BUT ... it doesn't need to be bigger than 25% over your maximum expected load.

Thus it may well be smaller than the cable could handle.

Given your 1 AWG example, if you're only ever expecting to draw 50 amps, a 70 or 75 amp breaker would be more than adequate, and would blow (interrupt) in the case of a fault sooner than a 100 or 150 amp breaker might do.

If you look at your house's breaker panel, you'll probably see that the total of the branch breakers exceeds the main breaker by quite a bit (my old house panel had a "sub main" layout ... with a 70 amp breaker feeding it. The total of the branch circuits on that 70 amp system was 120 amps per side. Perfectly legal and safe.)

IF, in the future (adding a Hot Tub?) you find that you're popping the 75 amp breaker under "normal" use, then simply swap it out for a bigger value that is still under the limit set by (b).

--dick

Dick,

The thing that is confusing to me about the Tecma Thetford electrical requirements is - the table says 30A breaker, and then below the table it says 40A breaker. This is why I was wondering if it was referring to the feeder circuit rather than the branch circuit. Do you understand this discrepancy?

Tecma Thetford

Thank you for the info on the bundled wires decreasing the max current. If I were to use 10 AWG duplex (positive and negative wire bundled together), does that mean it would have 28A capacity?

I did notice that the sum of the breakers on my house panel is more than the 200A rating, which makes sense, as it's protected by the main breaker, and continuous use doesn't require running all circuits at the same time at maximum current.
 

elemental

Wherever you go, there you are.
Thanks for taking the time to look through my design choices, I really appreciate it. The factory Aux AGM is indeed a temporary band aid. I'm hoping it will be enough for our trip from Montreal to San Diego. We will be driving every day, so the batteries will be recharged, and the only real continuous use will be the D2 and fan at night, as well as some lights, and the occasional water pump.
The circumstances you have described sound auspicious. The D2 heater uses 34 watts (34 watts/12 volts = 2.84 amps) on its highest setting, so running on its highest setting for 12 hours would suck 34 amp-hours out of your 92 amp-hour factory aux battery. If the factory aux battery is a not a "true" deep cycle AGM then this worst-case scenario won't be kind to it (but it will work). Depending on overnight temps and your van's insulation, you may not even come close to this level of discharge, of course.

I would love to size the feeder line to be repurposed as DC-DC charger. I suppose the expected load will depend on the size of the battery bank and whether or not I install an additional alternator, or rely on the factory alternator. I'm intending on a 200Ah LiFEPO4 battery system, and will also be installing solar, so won't be relying only on the alternator to charge the batteries, so maybe I can get away with the factory alternator. I think the factory alternator can supply 40-50A (which would work with the 1 AWG cable), but I'll dig into it today and see what I can find. Is the voltage drop critical with a DC-DC charger, or is it just a question of current?
If your battery storage is limited to 200Ah LiFEPO4, then the factory alternator is a viable power source (while driving) even if you don't have solar. You need a DC-DC charger to get the correct charging profile for this battery technology; the DC-DC charger will determine how many amps it pulls off the vehicle. According to the BEG documents I've seen, pulling 40 amps off the vehicle is ok. A DC-DC charger will use what it can get voltage-wise (within its specified source range), and increase/decrease the voltage of its source to get the voltage correct for the battery it is charging. This doesn't come for free, however - the output current is reduced/increased proportionally with the voltage increase/decrease. A bigger (up to "big enough") feed wire is better because it runs more efficiently (the wire heats up less from the internal resistance and you get more power at the far end). Your DC-DC charger should be located close to the battery(ies) it is charging so that the charging voltages are as correct as possible. I'm assuming that your rearward location of the electrical panels is meant to support an eventual rearward location of your add-on battery system, so the feed wire from the factory aux battery will connect to the DC-DC charger once it is installed.

The Blue Sea calculator for 40 amps and a 480 minute (4 hour) fixed load suggested AWG 0 for 2% loss, and AWG 2 for 3% loss when I ran it, so your AWG 1 feed wire would be (in my non-expert opinion) reasonably ok. If you want to up the DC-DC charger load to 50 amps (faster charging) then you might consider putting in an AWG 0 feed wire for better efficiency (AWG 1 works in this scenario for 3% loss, but trying to get to 2% loss changes the recommendation to AWG 2/0 [double-aught] - seems like this combination results in the calculation being very sensitive to the amount of loss you are targeting).

Solar provides an alternative power source that complements the use of the alternator (and will keep you charged up even when you aren't driving). I use only the vehicle as a power source right now, but am prepared to add solar over this winter so that I can do more boondock camping.

What you haven't mentioned is how you will monitor your battery state of charge and the current flow in/out. Not having this capability is sort of like driving your vehicle without a working fuel gauge. You can do it, but you'll rarely be sure how much juice you have left. Some DC-DC charger systems come with monitoring capabilities (may be optional), or you can add them separately. Typically you have a "shunt" that attaches to the battery; the shunt connects to a monitoring device that uses the shunt to measure current flow/direction.

One more question about the breakers / fuse. If I installed a fuse at the terminal (lets say it's fairly close to the load capacity of the cable), would it then be reasonable to install a breaker closer to the panel (and rated lower than the fuse), so it's more accessible, and could be also used to cut the power to the panel? I know it's important that there is a fuse or breaker at the terminal so you don't risk a short in an unprotected section of wire, and it seems like once the fuse is at the terminal, it is no longer critical where I place an additional breaker. Thoughts?
I have something like this configuration in my system. As far as I know, it is ok safety wise. I don't know how it impacts efficiency (depending on how the current interruption devices work, they may add some resistance to the circuit) but I don't think it is not overly much as long as you are using high-quality components.
 
The circumstances you have described sound auspicious. The D2 heater uses 34 watts (34 watts/12 volts = 2.84 amps) on its highest setting, so running on its highest setting for 12 hours would suck 34 amp-hours out of your 92 amp-hour factory aux battery. If the factory aux battery is a not a "true" deep cycle AGM then this worst-case scenario won't be kind to it (but it will work). Depending on overnight temps and your van's insulation, you may not even come close to this level of discharge, of course.


If your battery storage is limited to 200Ah LiFEPO4, then the factory alternator is a viable power source (while driving) even if you don't have solar. You need a DC-DC charger to get the correct charging profile for this battery technology; the DC-DC charger will determine how many amps it pulls off the vehicle. According to the BEG documents I've seen, pulling 40 amps off the vehicle is ok. A DC-DC charger will use what it can get voltage-wise (within its specified source range), and increase/decrease the voltage of its source to get the voltage correct for the battery it is charging. This doesn't come for free, however - the output current is reduced/increased proportionally with the voltage increase/decrease. A bigger (up to "big enough") feed wire is better because it runs more efficiently (the wire heats up less from the internal resistance and you get more power at the far end). Your DC-DC charger should be located close to the battery(ies) it is charging so that the charging voltages are as correct as possible. I'm assuming that your rearward location of the electrical panels is meant to support an eventual rearward location of your add-on battery system, so the feed wire from the factory aux battery will connect to the DC-DC charger once it is installed.

The Blue Sea calculator for 40 amps and a 480 minute (4 hour) fixed load suggested AWG 0 for 2% loss, and AWG 2 for 3% loss when I ran it, so your AWG 1 feed wire would be (in my non-expert opinion) reasonably ok. If you want to up the DC-DC charger load to 50 amps (faster charging) then you might consider putting in an AWG 0 feed wire for better efficiency (AWG 1 works in this scenario for 3% loss, but trying to get to 2% loss changes the recommendation to AWG 2/0 [double-aught] - seems like this combination results in the calculation being very sensitive to the amount of loss you are targeting).

Solar provides an alternative power source that complements the use of the alternator (and will keep you charged up even when you aren't driving). I use only the vehicle as a power source right now, but am prepared to add solar over this winter so that I can do more boondock camping.

What you haven't mentioned is how you will monitor your battery state of charge and the current flow in/out. Not having this capability is sort of like driving your vehicle without a working fuel gauge. You can do it, but you'll rarely be sure how much juice you have left. Some DC-DC charger systems come with monitoring capabilities (may be optional), or you can add them separately. Typically you have a "shunt" that attaches to the battery; the shunt connects to a monitoring device that uses the shunt to measure current flow/direction.


I have something like this configuration in my system. As far as I know, it is ok safety wise. I don't know how it impacts efficiency (depending on how the current interruption devices work, they may add some resistance to the circuit) but I don't think it is not overly much as long as you are using high-quality components.

Thanks again for all your input, very useful.

The van is quite well insulated, with Thinsulate on all walls, doors, and roof panels. I'll have window covers as well which should also help. I don't plan on spending a lot of time in really cold climates with this initial set up, just a night or two on the way over the mountains, so hopefully all goes well.

Out of interest, how big of a battery bank do you have, and how much current does your DC-DC charger pull?

You're correct, I am installing the panel near the rear wheel well, which is where I plan to locate my 200Ah LiFEPO4 batteries, and I'd locate the DC-DC charger close to the batteries. I also noticed when messing around with the Blue Sea Wizard that the recommended cable size seems to go way up as you approach 2% voltage drop. I think you're right that it would probably make sense to go with 0 AWG to keep things efficient, especially if I intend to go over 40A continuous charging load.

I need to do a final length check for the cables, my current estimate is a conservative guess after my initial measurement. I am thinking of running the return cable to the local ground on the chassis in front of the rear axle, as it would be shorter than running it all the way back to the seat base. How does this impact the calculations when a part of the return is through the chassis? Do I include the length that the current runs through the sheet metal as well, or only the length of the two cables?
 

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