Wire Gauge Advice

[TimC]

I am designing a 1.3KW system. I can run the wires from the array 35' into my garage, then I would like to use NM (or other interior wire) an additional 35' to the SCC inside my basement. I am seeking info about how to wire this 2S2P system.

Combined I'm looking at 81v and 20.5a. Disregarding the SCC for now should I combine the 2S2P at the array with 10awg solar wire (MC4)? Then run to the inside of the garage with 8 or 6awg solar wire to a breaker box? Then run similar gauge with NM wire to the SCC?

I'm seeing threads which say to just use 10awg all the way to the SCC. Seems that will result in undesirable voltage drop. I'm not trying to go the cheapest route. I want to run the correct gauges.

VOC = 40.49
ISC = 10.25a
Four 330w panels
Panels to SCC about 70'

 

[MisterSandals]

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Shoot for less than 3% loss/drop.

 

[Diysolar123]

loses due to wires/connectors is just wasted heat which you need to trade-off with your budget and goals.
long wire runs are especially tricky with how much power you end up losing, and can really hurt your pocket book if you just need to add another solar panel or two and discover your "old" wire just cannot deal with anotehr 5amps.
If you go to this site ,https://www.powerstream.com/Wire_Size.htm, you will find a table that gives good recommendations on the cable size to use for power transmission(do NOT look at the chassis wiring column, you are doing power transmission!) and it also has a voltage drop calculator.

Think of it this way:
10awg wire carrying 11amps to SCC 70' away will mean you are heating up the wires with 16watts of power.
Same scenario with 8awg wire makes the wires a 10watt heater...or for the same voltage drop as the 10awg wire you could increase your current flow from 11amps to 17amps in the future!

If it is not to great a burden on the budget get better/larger wires always.
fyi, when it comes to connectors do not exceed more that 50% of their rated capacity, they get warm and waste power as well!
For example, as a ham radio operator I love anderson power connectors. I have lots of cables and devices with 45amp connectors on them (my large battery packs use anderson SB175 connectors).
here are my connector observations with SHORT cables as this is about connectors:
an anderson powerpole 45 amp connector will get very warm at 15amps, even with 10awg wire, the connector itself has "limits".
XT60 connectors with 12awg do well at 20amps and get just warm to the touch.
XT90 connector with 10awg wire is cool to the touch with 30amps
an anderson SB175 is ok with 100amps, but its not a small connector, one SB175 housing is about the same size as 5 XT90's pairs hehe

watch out for cheap knock-off SB175 connector pins. They should be silver plated copper and pretty thick. I had a few ebay buys that results in very thin walled connectors that were tin plated).

 

[Forbisher]

You use Vmp not Voc so you are not 81V for 2S

I am designing a 1.3KW system. I can run the wires from the array 35' into my garage, then I would like to use NM (or other interior wire) an additional 35' to the SCC inside my basement. I am seeking info about how to wire this 2S2P system.

Combined I'm looking at 81v and 20.5a. Disregarding the SCC for now should I combine the 2S2P at the array with 10awg solar wire (MC4)? Then run to the inside of the garage with 8 or 6awg solar wire to a breaker box? Then run similar gauge with NM wire to the SCC?

I'm seeing threads which say to just use 10awg all the way to the SCC. Seems that will result in undesirable voltage drop. I'm not trying to go the cheapest route. I want to run the correct gauges.

VOC = 40.49
ISC = 10.25a
Four 330w panels
Panels to SCC about 70'

 

[Positronium]

I think running parallel sets of 10AWG wires to a combiner box in garage is feasible. The advantages with parallel sets of PV grade 10AWG wires is that it is easy to source and because of economy of scale not overly expensive. The rest of the wire run from combiner box to SCC can be done with plain old 6AWG or 8AWG wires you suggested.

Using the calculator mentioned above and assuming a 40ft one way length (80 ft round trip) with your parameters of 81V and 11A through 10 AWG wire the results are:
Voltage drop: 0.88V
Voltage drop percentage: 1.09%
Voltage at the end: 80.12V
The caveat is that price of 160ft 10 AWG PV wire must be less then 80ft of 8 AWG PV wire.

Combining at the output of the panels and running a single set of 6AWG or 8AWG wires from panels to SCC would be great however sourcing PV grade UV resistant wires of that gauge may be difficult or overpriced even if only 80ft of the circuit is with PV grade wires.

 

[TimC]

Thanks guys. MrSandals' link is easier for me to understand. In the powerstream's Voltage Drop Calculator I don't know what to pick for voltage. It'd be interesting if I could throw in 81v and see if both calculators come up with similar results. Unless I am entering the data points incorrectly. Do you use array VOC for that?

Anyway, it looks like I should just go with 8awg solar wire with MC4s on the array end - 2.28% drop. I could just put a short pigtail on the SCC with an MC4 but if I understand it correctly going bare wire on the SCC's terminal would be less voltage drop (probably insignificant tho). Where would I put a breaker/fuse between the array and the SCC? Or doesn't the SCC require one?

 

[TimC]

You use Vmp not Voc so you are not 81V for 2S

OK. Not sure what I am doing. Thought I read that I should use the potential max voltage so I picked Voc.

Is Vmp the same as Umpp? Umpp is 33.74v, so I would expect 67.48 as array voltage?

 

[Positronium]

Where would I put a breaker/fuse between the array and the SCC? Or doesn't the SCC require one?

I would suggest in a combiner box in your garage. You would connect lightning protector to combiner box too; lightning protector prevents damage to SCC, battery, and inverter in event of lightning strike. PV wire is more expensive than ordinary wire so the wires from combiner box to SCC can be ordinary wires.