Newbie seeks feedback on off-grid system plan

ericbakuladavis

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Hi Folks

I'm new to electrical / solar stuff, but I've been reading up online. A friend of mine is moving to an off-grid 5th wheel and I'm trying to help her get a system set up. I'm hoping to get some feedback on my plan.

Here's a sketch of it:
IMG_20210725_225858740.jpg

Here are some specs on purchased components (all returnable except the battery): Component Specs

The charge controller has 2 positive and 2 negative output terminals. Could I just divide the strands of each 8 AWG wire and feed half into teach terminal? What if there aren't an even number of strands? Is there a better way to do this?

Lugs and wire will be copper.

How does this look? Suggestions welcome.
 
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mopat

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Page 4, item 2 of charge controller instructions requires circuit breaker between charge controller input from PV panels.
 

MisterSandals

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Yea, a way to turn off power from panels is a must. Breaker adds current protection over a switch.

Your trailer DC loads need to be fused of course.

Looks pretty well sized and setup. Not familiar with that SCC but it sounds odd that there are 2 identical outputs from the SCC. If both are battery out then splitting strands should be fine. Looks like they spec 2x 9awg?
 

ericbakuladavis

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Page 4, item 2 of charge controller instructions requires circuit breaker between charge controller input from PV panels.
Thank you! I forgot about this. This got me thinking more about the breakers...

The charge controller instructions specify that the input and output breakers should be 2-pole and should have positive and negative wires connected to them. In my drawing, I have breakers only on the positive wires. I'm inclined to do what the instructions say, but does anyone have an idea as to why one way might be be better?

Yea, a way to turn off power from panels is a must. Breaker adds current protection over a switch.

Your trailer DC loads need to be fused of course.

Looks pretty well sized and setup. Not familiar with that SCC but it sounds odd that there are 2 identical outputs from the SCC. If both are battery out then splitting strands should be fine. Looks like they spec 2x 9awg?
I will add that breaker between panels and controller (y)

The DC loads will be fused. I'll replace the trailer's stock batteries with the new battery. To the new battery, I'll connect the same wires that were connected to the old batteries. These wires lead to a fuse panel inside the trailer.

Yeah, there are 2 negatives and 2 positives for both the PV in and the battery out. I see they spec 2x 9 AWG. I'm not sure exactly what that means but I suppose the safest interpretation would be to have a total of 4x 9 AWG wires on the battery out side wired to a 2-pole breaker as shown in the instructions. I'll see if I can find a suitable breaker for that setup.

Thanks for your help!
 

MisterSandals

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Looking at voltage drop (sorry i did not do this earlier), it looks like for 18" run and 60A, even 1x 10awg is sufficient at only 1.67% drop (under 3% is the standard goal).
Screen Shot 2021-07-26 at 2.57.33 PM.png
 

ericbakuladavis

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@MisterSandals OK - Thanks for checking that and providing the link to the tool. So are voltage drop and wire heating two sides of the same coin? In other words, if voltage drop is under 3%, will wire heating be at a safe level? On that calculator, do you know what Power Factor is?
 

ericbakuladavis

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if voltage drop is under 3%, will wire heating be at a safe level?
@MisterSandals I think the answer to this is "not necessarily"

Decreasing wire length decreases voltage drop. But I don't think wire length affects wire temperature while carrying current.

If we had an 18 awg wire carrying 200A, we could get the approval of a voltage drop calculator by putting in a very short length. But I think the wire would still get too hot.

I believe a wire should pass a voltage drop check AND should conform to an NEC ampacity chart.
 
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MisterSandals

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If we had an 18 awg wire carrying 200A, we could get the approval of a voltage drop calculator
The voltage drop calculator only goes down to 14awg. And with 14awg, it does not even come close to "getting approval" of the voltage drop calculator. Over 14% voltage drop.

Am i missing something? I am very willing to learn something new.
Screen Shot 2021-08-06 at 2.45.06 PM.png
 

ericbakuladavis

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I was referring to a different calculator that goes down to 18awg. I can make my point using the one at calculator.net though. Here's the same set of values you used, but with 0.02 feet instead of 1.5 feet:

Screen Shot 2021-08-06 at 8.27.10 PM.png

I understand this is may not be a very common example. But there are realistic examples where choosing wire gauge based on 2% voltage drop will result in a wire gauge that is smaller than what's recommended by an NEC ampacity chart. I encountered such a case when sizing the cable between my battery and inverter.
 

Hedges

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The charge controller instructions specify that the input and output breakers should be 2-pole and should have positive and negative wires connected to them. In my drawing, I have breakers only on the positive wires. I'm inclined to do what the instructions say, but does anyone have an idea as to why one way might be be better?

Maybe because the NEC used to prohibit putting a breaker or switch in the grounded negative PV lead.
Then they changed their mind and decided to require it.

I like to completely disconnect PV array, both ends of the string, before touching wires. Two pole can do that.
Your sketch on corrugated cardboard shows three "12V" panels in series, so not particularly high voltage.
Some breakers are rated 150V and some systems are 250V, so two poles in series lets them be used (so long as the poles are ganged together, trip at same time.)

Not applicable to your setup, but transformerless grid-tie inverters have continuity from utility grid to PV strings (through transistors), and that's a case where both positive and negative need fuses (even if just one or two parallel PV strings, which normally don't require OCP)
 

Hedges

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Decreasing wire length decreases voltage drop. But I don't think wire length affects wire temperature while carrying current.

Pretty much always correct. Obey ampacity limits and also design for acceptable voltage drop (which could be quite high on the PV string side.)

But a short wire can get rid of its heat by conducting lengthwise, so runs cooler. Code requires derating in bundle/conduit of many current carrying conductors, but has an exception for up to 2' length. Wire going through conduit between two boxes close together wouldn't need derating for the larger number of wires in it. Code doesn't say so, but I take that to be so long as they have enough length in small bundles or individual wires to get rid of the heat.
 

ericbakuladavis

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@Hedges Thanks for your input. Some questions...

Maybe because the NEC used to prohibit putting a breaker or switch in the grounded negative PV lead.
Then they changed their mind and decided to require it.

I like to completely disconnect PV array, both ends of the string, before touching wires. Two pole can do that.
Why do you suppose the NEC requires it? Why do you like to do it? I'm guessing the answer is "redundancy".

Your sketch on corrugated cardboard shows three "12V" panels in series, so not particularly high voltage.
Some breakers are rated 150V and some systems are 250V, so two poles in series lets them be used (so long as the poles are ganged together, trip at same time.)
What does "two poles in series" mean? The breaker I have is like this: DC Circuit Breaker

But a short wire can get rid of its heat by conducting lengthwise, so runs cooler. Code requires derating in bundle/conduit of many current carrying conductors, but has an exception for up to 2' length.
Short wire conducting heat out of the ends makes sense. What is derating?
 

Hedges

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NEC previously prohibited disconnecting grounded wires, which are supposed to be zero volts.
No good reason to prohibit that when isolating power sources, such as PV or battery. I like it completely disconnected so the circuit I'm working on is completely dead. NEC may have decided it was safer that way. If there was a short to ground in the array (which won't blow the fuse) the negative would become hot. That is a safety problem. The new transformerless grid-tie inverters have the potential to drive DC onto the power grid, which could easily cause failure of transformers. Fuse makes it easier for current from the grid to cause the failed circuit to disconnect.

Schneider breakers of that style appear to be rated 250VDC per pole.
If you wired PV+ to one pole and PV- to the other, it could open a 500Voc circuit. That would interrupt both positive and negative.
If you wired PV+ in one pole, over to the other, and back out, it could also open 500Voc circuit. That would only interrupt the positive.


NEC allows a certain amount of current through conductors, depending on insulation temperature rating. The table is good up to 30 degrees C ambient temperature and up to 3 current-carrying wires in a bundle.
Notes 4 and 5 on the second page say to reduce allowed current to a percentage of what the table says, if higher ambient temperature or more wires in a bundle. That is "derating", applying a reduction to the rating.

 

ericbakuladavis

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If you wired PV+ in one pole, over to the other, and back out, it could also open 500Voc circuit. That would only interrupt the positive.
Interesting idea. Thanks for sharing that.

Notes 4 and 5 on the second page say to reduce allowed current to a percentage of what the table says, if higher ambient temperature or more wires in a bundle. That is "derating", applying a reduction to the rating.
Gotcha. Thanks.

I'm afraid the hot negative scenario you described is a bit beyond me as I believe I am lacking in some basic electrical understanding. But disconnecting 2 wires instead of 1 for safety makes sense intuitively. Anyhow, I appreciate your effort. .
 
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