Critique my wiring diagram (please): attempting to create common ground between starter and house systems

[benhasavan]

Hi all, I've adapted a wiring diagram (attached below) from the "far out ride" site to suit my 2005 Dodge T1n. This is the first electrical thing I've ever done, so total noob here. There's a few questions I have...

1. The Sterling 60A B2B charger instructions call for a common ground between the starter and house batteries. I ran a 2/0 negative wire 20 feet from the house negative buss, but the 2/0 lug is too bulky to connect directly to the negative post on my starter battery (unless some sort of adapter exists). Is it adequate to connect to the same ground point on the chassis that the starter connects to? Would that constitute a common ground?

2. I ran 2/0 based on the ABYC recommendation that "the grounding wire is able to carry the maximum current that the current-carrying conductor can supply." Thus I decided on 2/0 since the largest conductor elsewhere in my system are the 2/0 wires between the house battery, shunt, switch and buss bars. Is the 2/0 ground size adequate, given it's a long 20 ft return to the starter battery chassis connection? I.e. Should I be worried about voltage drop in regards to the ground connection? I'm not sure if I'm totally misunderstanding the ABYC guidelines.

3. Per the last question, I've seen people on the forum add a second ground between the negative buss and chassis closer to the house battery for "redundancy." Would that be a good idea in my case?

I thank anyone in advance for their help in saving me and my van from a catastrophic diy fail. Also I apologize if these questions are redundant. I've been lurking this forum for a couple years now and have always found an answer in someone else's thread, but I'm having a hard time finding a post with these exact questions.

Thanks,

Ben

Photo of where I'll be connecting the 2/0 negative from house buss (to what looks to me like the chassis ground from starter negative):

 

[Zundfolge]

First, there will be a few different approaches as this thread develops, but I'll just outline mine. Second, this is likely to get moved to the RV section...

1) It is good practice to run your b2b battery charing cable the full round trip (side note I just use chassis ground near the house batt. as the "common ground," with no issue for many years).

2/0 is BEEFY. Cables should be sized for the loads that they will be carrying. Let's just address your sterling 60A since that's what the question is about. This line of thinking will also apply to any other 12V element in your system.



Using the 3% voltage drop column (what I think should be used in these van systems, just treat everything as critical and you'll be in the green).
-You have a max current of 60A for you sterling.
-20' one way, that's 40' round trip.
-40'-60A collide at 1AWG on the table.
-So, (2) 1AWG cables from the sterling to the house batt (obvs. +'s going through the breakers).
-Speaking of, those breakers in the diagram are 100A. They should be 60A.

Since you are self-proclaiming as a noob, first I recommend reading a lot more through others' diagrams and questions here before you start buying materials and cutting wires. Second:

Breakers protect wires.
Wires are sized based on current.
Current is determined by the load - by the device asking for power.

So if you have a b2b charger asking for a max of 60A, you size your wire as above, 40' round trip gives you 1AWG. Then the breakers are sized to protect the wire, again, 60A. That way if a wire shorts and asks for way many more than 60A the breaker trips, cutting power.

You always want your fuses or breakers as close as possible to the SOURCE of power. If the wire shorts, you want there to be no possibility of the short happening between the power source (battery) and the breaker. If that short did happen, there would be an uninterrupted flow of power from the source to ground until something welds, melts, or catches fire.

Anywho, I can't imagine a necessity to have 2/0 connecting your neg. busbar to the starter battery? At 20' 2/0 is capable of carrying 200A. Do you imagine a situation where you will require 200A from your starter battery? (Spoiler alert, the answer is no.) I suppose you might want to run the inverter off of the house battery in a pinch? Even still, you're never going to pull 200A out of it, that's something like 2C? (2x the Capacity of the battery)

All of my 12V systems tied into my house electrical system have two wires, + and -. My neg. busbar ties to the van chassis as common ground, as I said earlier, with in my case 1' of 4AWG (capable of 100A).

There's much more to say, but give us as many details about your system as possible, the diagram's ok but kinda clogged. Then we can offer more help.

 

[avanti]

1) It is good practice to run your b2b battery charing cable the full round trip

This is incorrect. A proper chassis ground provides vastly more ampacity than any practical ground wire. The common advice to use separate ground wires is a "feel good" technique that is neither based in science nor consistent with professional best practice. For example, your starter motor (which is rather an extreme case) almost certainly uses a chassis return.

 

[Zundfolge]

This is incorrect. A proper chassis ground provides vastly more ampacity than any practical ground wire. The common advice to use separate ground wires is a "feel good" technique that is neither based in science nor consistent with professional best practice. For example, your starter motor (which is rather an extreme case) almost certainly uses a chassis return.

Well I based my personal build on common ground for b2b charging and as I outlined, have no problems. But I've seen it here so many times to run round trip that I assumed that it was best practice.

However, it's also true in my opinion that a properly sized wire is no better nor worse than using the chassis as common ground. I don't see a need for "vastly more ampacity" if the wires are properly sized. It's possible for 2 seemingly competing things to be true at the same time in my book, so I don't see the necessity for something that works equally as well to be considered "incorrect." (In fact, our world might benefit from us being able to hold more than one truth at once).

 

[avanti]

Well I based my personal build on common ground for b2b charging and as I outlined, have no problems. But I've seen it here so many times to run round trip that I assumed that it was best practice.

However, it's also true in my opinion that a properly sized wire is no better nor worse than using the chassis as common ground. I don't see a need for "vastly more ampacity" if the wires are properly sized. It's possible for 2 seemingly competing things to be true at the same time in my book, so I don't see the necessity for something that works equally as well to be considered "incorrect." (In fact, our world might benefit from us being able to hold more than one truth at once).

What I said was "incorrect" was the claim that a return wire was "good practice". It is not.
It is true that if the wire is big enough, the two approaches are electrically equivalent (or close enough that it doesn't matter). However, "best practice" evolves not simply as a result of functionality, but also considers financial cost, space, weight, understandability and other pragmatic factors. When these factors are taken into consideration, separate ground returns are simply not best practice. You would be hard-pressed to find a professionally-engineered product that is designed this way. If it were really a toss of the coin, this would not be true.

My "vastly more ampacity" comment was meant to establish that there was no electrical disadvantage to a chassis return (which many people do not seem to understand). We seem to agree on this point.

It is not my goal to attack you. But, as you say, this claim is repeated over and over. It is just wrong.

 

[benhasavan]

First, there will be a few different approaches as this thread develops, but I'll just outline mine. Second, this is likely to get moved to the RV section...

1) It is good practice to run your b2b battery charing cable the full round trip (side note I just use chassis ground near the house batt. as the "common ground," with no issue for many years).

2/0 is BEEFY. Cables should be sized for the loads that they will be carrying. Let's just address your sterling 60A since that's what the question is about. This line of thinking will also apply to any other 12V element in your system.

View attachment 178164

Using the 3% voltage drop column (what I think should be used in these van systems, just treat everything as critical and you'll be in the green).
-You have a max current of 60A for you sterling.
-20' one way, that's 40' round trip.
-40'-60A collide at 1AWG on the table.
-So, (2) 1AWG cables from the sterling to the house batt (obvs. +'s going through the breakers).
-Speaking of, those breakers in the diagram are 100A. They should be 60A.

Since you are self-proclaiming as a noob, first I recommend reading a lot more through others' diagrams and questions here before you start buying materials and cutting wires. Second:

Breakers protect wires.
Wires are sized based on current.
Current is determined by the load - by the device asking for power.

So if you have a b2b charger asking for a max of 60A, you size your wire as above, 40' round trip gives you 1AWG. Then the breakers are sized to protect the wire, again, 60A. That way if a wire shorts and asks for way many more than 60A the breaker trips, cutting power.

You always want your fuses or breakers as close as possible to the SOURCE of power. If the wire shorts, you want there to be no possibility of the short happening between the power source (battery) and the breaker. If that short did happen, there would be an uninterrupted flow of power from the source to ground until something welds, melts, or catches fire.

Anywho, I can't imagine a necessity to have 2/0 connecting your neg. busbar to the starter battery? At 20' 2/0 is capable of carrying 200A. Do you imagine a situation where you will require 200A from your starter battery? (Spoiler alert, the answer is no.) I suppose you might want to run the inverter off of the house battery in a pinch? Even still, you're never going to pull 200A out of it, that's something like 2C? (2x the Capacity of the battery)

All of my 12V systems tied into my house electrical system have two wires, + and -. My neg. busbar ties to the van chassis as common ground, as I said earlier, with in my case 1' of 4AWG (capable of 100A).

There's much more to say, but give us as many details about your system as possible, the diagram's ok but kinda clogged. Then we can offer more help.

Thank you Zundfolge!

I drew out another wiring diagram below and adjusted the sterling connections to 1 awg. Not sure how I arrived at 2 awg before but that was obviously a mistake. Hopefully this is more clear, I've included only what I have in the current system. So far the only loads on the house battery are a 10A roof vent and a 3A composting toilet exhaust, and the only input is the Sterling B2B. However, I do plan to add more later.

My logic on the breaker size for the B2B was that it should be larger than the device draw but less than the ampacity of the wire. Is this incorrect thinking? I've been using the Blue Sea Circuit Wizard and it shows 1 awg as having a 245A capacity, so would the breaker not just need to be lower than that? Or should it just match the current being drawn?

I indeed do not need 200A from the starter battery. I was under the vague impression that a bigger wire for the ground was safer, but it seems I have misunderstood the ABYC recs. How should I size my ground then? I've seen people match the wire size to the input on the sterling (1 awg in my case). But if I want to add solar, shore and potentially more loads at some point, would the ground need to match the total current of the whole system? The desire to beef things up later is why I used 2/0 wires to connect the house battery to the buss bars. What I'm asking is, is overkill on the ground wire size a bad thing or just unnecessary? Side note: I already bought and crimped the 2/0 wires.

I understand your point on studying up on wiring diagrams and reading questions, I've been doing that for a while and eventually got to the point where I just decided to start buying things and trying it out. I should have started this thread first. I have a habit of measuring once and cutting twice.

 

[benhasavan]

What I said was "incorrect" was the claim that a return wire was "good practice". It is not.
It is true that if the wire is big enough, the two approaches are electrically equivalent (or close enough that it doesn't matter). However, "best practice" evolves not simply as a result of functionality, but also considers financial cost, space, weight, understandability and other pragmatic factors. When these factors are taken into consideration, separate ground returns are simply not best practice. You would be hard-pressed to find a professionally-engineered product that is designed this way. If it were really a toss of the coin, this would not be true.

My "vastly more ampacity" comment was meant to establish that there was no electrical disadvantage to a chassis return (which many people do not seem to understand). We seem to agree on this point.

It is not my goal to attack you. But, as you say, this claim is repeated over and over. It is just wrong.

I've

This is incorrect. A proper chassis ground provides vastly more ampacity than any practical ground wire. The common advice to use separate ground wires is a "feel good" technique that is neither based in science nor consistent with professional best practice. For example, your starter motor (which is rather an extreme case) almost certainly uses a chassis return.

I've seen people say this quite a bit and not sure I understand it. If a proper chassis ground provides vastly more ampacity, why is the sterling manual so insistent that a common ground be made between the starter and house batteries? In the manual it shows a direct connection between the negative terminals, which is why I was freaked out by not connecting them directly and instead attaching the house negative close to the starter where it grounds to the chassis.

 

[avanti]

I've seen people say this quite a bit and not sure I understand it. If a proper chassis ground provides vastly more ampacity, why is the sterling manual so insistent that a common ground be made between the starter and house batteries? In the manual it shows a direct connection between the negative terminals, which is why I was freaked out by not connecting them directly and instead attaching the house negative close to the starter where it grounds to the chassis.

Could you send a link to the manual you are looking at and provide a pointer to the worrisome text? I couldn't find what you are describing in the manuals I looked at.

My guess is that the direct connection between the negative terminals was meant to be interpreted schematically, not physically. The chassis DOES provide a "common ground" between the batteries. There is no question that the chassis (if properly used) provides a return path greatly in excess of any plausible need.

The thing that does sometimes trip up such installations is the ground path from the engine to the chassis. There is always a copper strap that bridges the rubber engine mounts, and its capacity is limited. Sometimes it needs to be augmented when connecting to a second alternator (for example). I don't see where this applies in your case, however.

 

[Zundfolge]

My logic on the breaker size for the B2B was that it should be larger than the device draw but less than the ampacity of the wire. Is this incorrect thinking?

It's sound thinking, something like 80A would suffice.

I've been using the Blue Sea Circuit Wizard and it shows 1 awg as having a 245A capacity, so would the breaker not just need to be lower than that? Or should it just match the current being drawn?

Where the heck are you seeing 1AWG able to handle 245A??? Pretty sure the smallest wire that'll handle that safely is 4/0?

Also, you have to keep in mind that while a given AWG will carry x amps, it is always necessary to factor in voltage drop which is a function of distance for van-building purposes, hence the need to reference something like the ampacity chart above.

I indeed do not need 200A from the starter battery. I was under the vague impression that a bigger wire for the ground was safer, but it seems I have misunderstood the ABYC recs. How should I size my ground then? I've seen people match the wire size to the input on the sterling (1 awg in my case). But if I want to add solar, shore and potentially more loads at some point, would the ground need to match the total current of the whole system? The desire to beef things up later is why I used 2/0 wires to connect the house battery to the buss bars. What I'm asking is, is overkill on the ground wire size a bad thing or just unnecessary? Side note: I already bought and crimped the 2/0 wires.

You don't just add up all amperage from the whole system. You're specifically referencing your chassis ground tie in to your house system right? As we've just discussed above, using the chassis as ground to run - back to the battery is sufficient. I have 2/0 for my main battery connections, to the busbars/shunt etc. because I have a 2000W inverter hooked up to them. However that inverter does not need to pull power from the starter battery, thus my neg busbar to chassis ground cable is like 1' of 4AWG.

Your neg. busbar to chassis ground can be (as far as I can tell right now) 1AWG. That is btw common ground. Both batteries share the chassis as a ground.

My guess is that the direct connection between the negative terminals was meant to be interpreted schematically, not physically. The chassis DOES provide a "common ground" between the batteries. There is no question that the chassis (if properly used) provides a return path greatly in excess of any plausible need.

My thought as well, since they are largely catering to the boating application in that manual, there is not the assumption that there is a metal chassis, thus they diagrammed a wire.

I should have started this thread first. I have a habit of measuring once and cutting twice.

Welcome to the club.

 

[benhasavan]

Could you send a link to the manual you are looking at and provide a pointer to the worrisome text? I couldn't find what you are describing in the manuals I looked at.

My guess is that the direct connection between the negative terminals was meant to be interpreted schematically, not physically. The chassis DOES provide a "common ground" between the batteries. There is no question that the chassis (if properly used) provides a return path greatly in excess of any plausible need.

The thing that does sometimes trip up such installations is the ground path from the engine to the chassis. There is always a copper strap that bridges the rubber engine mounts, and its capacity is limited. Sometimes it needs to be augmented when connecting to a second alternator (for example). I don't see where this applies in your case, however.

Link and photo of the specific page below. I think I was reading it too literally with my lack of familiarity and because it does seem to cater to marine applications. The black box is specifically what was giving me pause, "Important NEGATIVES should all be common. DON'T USE CHASSIS. We recommend joining the BB's neg. to the starter battery negative. For improved efficiency."

https://cdn.shopify.com/s/files/1/0658/7343/files/2020_GREEN_V71_Rev_Instructions.pdf?v=1616500897

 

[benhasavan]

It's sound thinking, something like 80A would suffice.



Where the heck are you seeing 1AWG able to handle 245A??? Pretty sure the smallest wire that'll handle that safely is 4/0?

Also, you have to keep in mind that while a given AWG will carry x amps, it is always necessary to factor in voltage drop which is a function of distance for van-building purposes, hence the need to reference something like the ampacity chart above.



You don't just add up all amperage from the whole system. You're specifically referencing your chassis ground tie in to your house system right? As we've just discussed above, using the chassis as ground to run - back to the battery is sufficient. I have 2/0 for my main battery connections, to the busbars/shunt etc. because I have a 2000W inverter hooked up to them. However that inverter does not need to pull power from the starter battery, thus my neg busbar to chassis ground cable is like 1' of 4AWG.

Your neg. busbar to chassis ground can be (as far as I can tell right now) 1AWG. That is btw common ground. Both batteries share the chassis as a ground.



My thought as well, since they are largely catering to the boating application in that manual, there is not the assumption that there is a metal chassis, thus they diagrammed a wire.



Welcome to the club.

Thank you both, this is all starting to make much more sense to me. Yes, I was specifically referencing the chassis ground tie into my house system. So I basically need to change my diagram to a 1 awg chassis ground (probably like 1' and closer to the house battery right? Instead of going 20' back to the engine compartment?).

And I'll change my breakers for the sterling to 60 or 80 being equal to or slightly greater than the sterling but under the wire's capacity (which as I'm looking at a different chart is something more like 110).

The 245A number came from that blue sea calculator I mentioned, but I'm sure I read it wrong like the sterling manual: http://circuitwizard.bluesea.com/

 

[Zundfolge]

I don't know how it comes up with that number. I typed in the same values and it gives me 1AWG as well, even for 10'. That AWG size doesn't even agree with their own chart that I posted above, so either I'm missing something or....I'm probably missing something?

Edit: wire insulation temp ratings, whether its sheathed or not, in conduit or not, will all affect ampacities, though I'm still confused as to why it specs 1AWG...

 

[avanti]

The black box is specifically what was giving me pause, "Important NEGATIVES should all be common. DON'T USE CHASSIS.

I have no idea why they would say such a thing. (Maybe their British heritage of "Lucas, Prince of Darkness" ). The manuals of several of their other products have that black box, but without the "Don't Use Chassis" text).

 

[benhasavan]

I don't know how it comes up with that number. I typed in the same values and it gives me 1AWG as well, even for 10'. That AWG size doesn't even agree with their own chart that I posted above, so either I'm missing something or....I'm probably missing something?

Edit: wire insulation temp ratings, whether its sheathed or not, in conduit or not, will all affect ampacities, though I'm still confused as to why it specs 1AWG...

I'm not sure where that number comes from either. It's definitely far off of the ampacity charts I'm looking at now (NEC, cerrowire, et. al.). All are showing much lower ampacities for 1 awg. I'm seeing up to 145 depending on the type. Thank you for the correction! Could have been a big mistake.

I have no idea why they would say such a thing. (Maybe their British heritage of "Lucas, Prince of Darkness" ). The manuals of several of their other products have that black box, but without the "Don't Use Chassis" text).

Thanks Avanti, now I'm curious and might call them for an explanation of the misleading text. Will post if I do. Sounds like grounding to the chassis is the way to go anyway.

 

[VanGoSki]

I have no idea why they would say such a thing. (Maybe their British heritage of "Lucas, Prince of Darkness" ). The manuals of several of their other products have that black box, but without the "Don't Use Chassis" text).

They did explain it in the bubble note; they say it's unreliable. So basically they're just eliminating a potential support call, not saying it's electrically wrong. I am running a chassis ground only on my VS30 and I've pumped 200 amps though it. So I know it's solid.

 

[avanti]

They did explain it in the bubble note; they say it's unreliable.

Ah, I missed that. Specifically, they claim that "the conductivity is poor".
This is simply false, at least if you use one of the properly engineered Sprinter ground points. (not to say that you can't create your own if you are careful).

 

[benhasavan]

They did explain it in the bubble note; they say it's unreliable. So basically they're just eliminating a potential support call, not saying it's electrically wrong. I am running a chassis ground only on my VS30 and I've pumped 200 amps though it. So I know it's solid.

All good to know. I'll go with an existing engineered Sprinter ground point.

 

[RVBarry]

Speaking of, those breakers in the diagram are 100A. They should be 60A.

Hi, the Sterling will need MORE than 60A input to produce 60A output.
A 60A input breaker would trip constantly.
The manual should provide the recommended current.

60A on the output seems tight as well.

 

[Zundfolge]

Hi, the Sterling will need MORE than 60A input to produce 60A output.
A 60A input breaker would trip constantly.
The manual should provide the recommended current.

60A on the output seems tight as well.

What do you suggest?

Sorry if I gave bad info on that part, I'm not very familiar with the sterling.

 

[RVBarry]

What do you suggest

I'd read the manual. The Sterlings are well known for running hot, which means not very efficient, so I'd guess at least 80A in.
(but any DCDC needs more input than the rated output)

75A out should be safe and leave enough headroom to avoid accidental tripping under full load on a hot day.
Different types of breakers have different trip thresholds. A fast-blow 60A would be especially bad, as an example.