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lazylnm
03-07-2017, 03:34 PM
As a result of several comments concerning solar - or lack thereof - to my thread https://sprinter-source.com/forum/showthread.php?t=54173
we revisited solar and decided now was the time to add it. Not wanting to totally reinvent the wheel, I relied on several installed, up and running systems as a basis for design. The particulars are:

(4) 2'x4' panels covers my available space
The panels will run east west supported by a continuous aluminum upturned angle that may also serve as a mounting point for a future awning
The panels will be connected in a Series/Parallel configuration
I have included the panel - Renogy RDG-100D - specifications for reference and have listed the watts, amps and voltage for each of the three legs - two Series and the combined Parallel. My questions are these:
Are my calculations correct? Never done solar or S/P circuits before.
The 8AWG connector cable is - I believe - way over sized. In running the numbers - assuming they are correct - a wire sizing calculator showed 14 ga - even with a 2% loss. I will probably use a 10ga cable with MC4 connectors molded on.
The isolation switch between the panels and Controller - will something as simple as a garden variety 120v/15A wall switch work. If not, what should be used?
There needs to be a breaker/fuse between the Controller and the 12v bus. What kind of device is used here? I've come up empty on 45v/15A breakers/fuses.
I know the panels are connected with premade cables with molded MC4 connectors and that the panels have a box with pigtails attached. What about cable management under the panels? Seems like there are lots of cables subjected to wind and water. Cable trays/ducts, bottom panels added to each panel or lots of duct tape - what works? Perhaps a duct system mounted to the inside of the support angle under the panel. Access is an issue. What have you done?

I'd like to thank sprint2freedom for his input and schooling.
Take a look and look forward to your comments.[/LIST]

sprinterPaul
03-07-2017, 05:17 PM
45V isn't a multiple of 12 and would be very rare indeed. Why 15A?

https://www.renogy.com/renogy-20a-30a-40a-60a-100a-anl-fuse-set-w-fuse/




Sent from my iPhone using Tapatalk

sprint2freedom
03-07-2017, 06:40 PM
A few comments after taking a quick look:

- Voltage and current from the panels will vary over a large range depending on the sun brightness, sun angle, time of day, shading, etc. so those listed on your drawing should be thought of only as 'ideal' values. Panels can actually put out slightly over their rated output in certain conditions but that's rarely an issue for a typical installation.

- Breakers have a current rating which is the main thing to look for. They also specify a voltage range and type (AC or DC) for which they can be used- you don't need to look for an exact voltage value as long as your use is within the acceptable range.

- The FM60 can output up to 60 amps, but 400 watts of panels would give you a maximum battery charge current of 400 / 12~14V = about 28.5-33.5 amps in the best of circumstances. A 35A or 40A breaker would probably be most appropriate on the battery side of the charge controller.

- Automotive or marine stores are a good source for DC breakers. It's probably a bit overkill, but something like this (rated for 6-48VDC) would certainly work in your application:
40A: https://www.amazon.com/Bussmann-CB285-40-Surface-Mount-Circuit-Breakers/dp/B01G5WHRPC
35A: https://www.amazon.com/Bussmann-CB285-35-Surface-Mount-Circuit-Breakers/dp/B01G5WHP2W
30A: https://www.amazon.com/Bussmann-CB285-30-Surface-Mount-Circuit-Breakers/dp/B01FWMM260
You can also buy breakers marketed specifically for solar.

- As you've calculated, the most current you'd expect to see on the panel side would be around 5.75A x 2 = 11.5A. You could fit a 15A breaker here in lieu of a switch, or yes- even a wall switch. You might want to check the physical lug size for that 8 gauge wire and whether the breaker or switch you use has the clearance for it. Also- a minor point but wall switch ratings are typically for AC and may be lower for DC which is harder to interrupt because the voltage never crosses through zero. Circuit breakers and DC rated switches are designed for this, although in fairness it probably won't matter at the low currents in your system and with how infrequently you'll open that switch when the system is energized.

- Don't forget to order a battery temperature sensor (https://www.amazon.com/OutBack-Power-RTS-Temperature-Charging/dp/B007ZU2NE4) for the FM60. You should be able to buy it from the same place you get the controller (it's been a while but I found a good deal on an FM60 at thesolarbiz). The BTS is required if you don't want to ruin your expensive batteries by charging them at a too high/too low voltage for their current temperature.

- Renogy RNG-100D

- Be careful with wire size calculators as they are often dealing with ampacity ratings, not voltage drop. You can figure your wire losses with something like this:
http://www.calculator.net/voltage-drop-calculator.html

For 18 feet (36 feet roundtrip) of 14 AWG @ 35VDC and 11.5A, I get a 3% drop of 1.05V.
For 18 feet (36 feet roundtrip) of 10 AWG @ 35VDC and 11.5A, I get a 1.17% drop of 0.41V.
For 18 feet (36 feet roundtrip) of 8 AWG @ 35VDC and 11.5A, I get a 0.74% drop of 0.26V.

These losses are pretty small but wire is cheap and I see no reason to push the envelope with smaller wire. I certainly agree that there is little point in running 4 AWG to your roof.

- In general, your wire size should be the same or larger on the battery side of the controller vs. the panel side because the battery is at a lower voltage, so the current will be higher vs. the panel side. Your plan has this backwards- it shows a smaller wire on the battery side. Using 8 gauge for both is probably fine but you might want to move up to 4 gauge on the battery side.

- Place the breaker as physically near the bus bar as practical to protect the wire downstream.

- MC4 cables are engineered for outdoor use with rain and UV exposure. They should be fine but you probably want to zip tie the wires to the solar panel frames so they don't bang around on your roof.

lazylnm
03-08-2017, 12:05 AM
45V isn't a multiple of 12 and would be very rare indeed. Why 15A?

https://www.renogy.com/renogy-20a-30a-40a-60a-100a-anl-fuse-set-w-fuse/




Sent from my iPhone using Tapatalk

The voltages are what are listed for that particular panel. I compared these with panels that AM Solar sells and the overall specs are similar. I targeted these because I saw several Sprinter installs using them. Didn't find any bad reviews.
I used an on-line solar calculator. I first computed Series legs then used those numbers to calculate the parallel figures. Not having experience with this type of calculation nor ever having designed/installed solar, my first concern was "Are these numbers even realistic?". I have no basis. I also know that there are nuances to electrical design, particularly low voltage, that I simply don't know. Hence the request for review.
Thanks for the lead on fuses - I had thought about something like that, but the manufacturer I was looking at only offered really big ones.

lazylnm
03-08-2017, 12:23 AM
A few comments after taking a quick look:

- Voltage and current from the panels will vary over a large range depending on the sun brightness, sun angle, time of day, shading, etc. so those listed on your drawing should be thought of only as 'ideal' values. Panels can actually put out slightly over their rated output in certain conditions but that's rarely an issue for a typical installation.

- Breakers have a current rating which is the main thing to look for. They also specify a voltage range and type (AC or DC) for which they can be used- you don't need to look for an exact voltage value as long as your use is within the acceptable range.

- The FM60 can output up to 60 amps, but 400 watts of panels would give you a maximum battery charge current of 400 / 12~14V = about 28.5-33.5 amps in the best of circumstances. A 35A or 40A breaker would probably be most appropriate on the battery side of the charge controller.

- Automotive or marine stores are a good source for DC breakers. It's probably a bit overkill, but something like this (rated for 6-48VDC) would certainly work in your application:
40A: https://www.amazon.com/Bussmann-CB285-40-Surface-Mount-Circuit-Breakers/dp/B01G5WHRPC
35A: https://www.amazon.com/Bussmann-CB285-35-Surface-Mount-Circuit-Breakers/dp/B01G5WHP2W
30A: https://www.amazon.com/Bussmann-CB285-30-Surface-Mount-Circuit-Breakers/dp/B01FWMM260
You can also buy breakers marketed specifically for solar.

- As you've calculated, the most current you'd expect to see on the panel side would be around 5.75A x 2 = 11.5A. You could fit a 15A breaker here in lieu of a switch, or yes- even a wall switch. You might want to check the physical lug size for that 8 gauge wire and whether the breaker or switch you use has the clearance for it. Also- a minor point but wall switch ratings are typically for AC and may be lower for DC which is harder to interrupt because the voltage never crosses through zero. Circuit breakers and DC rated switches are designed for this, although in fairness it probably won't matter at the low currents in your system and with how infrequently you'll open that switch when the system is energized.

- Don't forget to order a battery temperature sensor (https://www.amazon.com/OutBack-Power-RTS-Temperature-Charging/dp/B007ZU2NE4) for the FM60. You should be able to buy it from the same place you get the controller (it's been a while but I found a good deal on an FM60 at thesolarbiz). The BTS is required if you don't want to ruin your expensive batteries by charging them at a too high/too low voltage for their current temperature.

- Renogy RNG-100D

- Be careful with wire size calculators as they are often dealing with ampacity ratings, not voltage drop. You can figure your wire losses with something like this:
http://www.calculator.net/voltage-drop-calculator.html

For 18 feet (36 feet roundtrip) of 14 AWG @ 35VDC and 11.5A, I get a 3% drop of 1.05V.
For 18 feet (36 feet roundtrip) of 10 AWG @ 35VDC and 11.5A, I get a 1.17% drop of 0.41V.
For 18 feet (36 feet roundtrip) of 8 AWG @ 35VDC and 11.5A, I get a 0.74% drop of 0.26V.

These losses are pretty small but wire is cheap and I see no reason to push the envelope with smaller wire. I certainly agree that there is little point in running 4 AWG to your roof.

- In general, your wire size should be the same or larger on the battery side of the controller vs. the panel side because the battery is at a lower voltage, so the current will be higher vs. the panel side. Your plan has this backwards- it shows a smaller wire on the battery side. Using 8 gauge for both is probably fine but you might want to move up to 4 gauge on the battery side.

- Place the breaker as physically near the bus bar as practical to protect the wire downstream.

- MC4 cables are engineered for outdoor use with rain and UV exposure. They should be fine but you probably want to zip tie the wires to the solar panel frames so they don't bang around on your roof.

The DC breakers you linked to - I had looked at similar ones, but, again, they were really big. Thanks for the link.
I'll search out DC switches - I guess I got excited with the thought of 15A parts vs the 300A that I've been working with on the battery/alternator side and forgot about AC vs DC.
Wire size - I probably underestimated the TOTAL wire length vs the physical distance - hence the #14 wire. Will review. I think I forgot to change the battery side wire size.
Not ANOTHER battery temp sensor. I'll get it added in.
Thanks for your comments and critique.