Just looking for some confirmation on some cable sizing that I'm doing for my solar array. It's going to be 2 strings of 7 panels 400w each 9.2 imp... operating voltage of 43.2. I would like to combine them and run to my batteries which is around 200 feet away. Curious what everyone's calculation of wire size would be. And do I try for a 2% loss or is 5% fine for solar?
Help with cable sizing
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OffGridInTheCity
Solar Wizard
7 panels x 43v each = 301v @ 9a. 2 of these would be 301v @ 18a. The typical 10awg (30a) 'panel wiring' will be OK for 200ft - I would not expect much loss at this moderate distance. You don't say if its an outdoor / in-conduit / underground run. In addition to gauge you need wire (coating) that supports the physical situation 
So to be clear, you are talking about 2 parallel strings of 7 series connected panels for a total of: 18.4A @ 302.4V over a 400ft round trip distance?
Yes wiring will be in the house for a short distance in conduit and the rest will be Underground. Everything I'm coming up with says I can use 12 gauge but wanting someone to confirm what's acceptable loss7 panels x 43v each = 301v @ 9a. 2 of these would be 301v @ 18a. The typical 10awg (30a) 'panel wiring' will be OK for 200ft - I would not expect much loss at this moderate distance. You don't say if its an outdoor / in-conduit / underground run. In addition to gauge you need wire (coating) that supports the physical situation
OffGridInTheCity
Solar Wizard
I'm reading that you have a 200ft run. @Dzl is thinking 400ft run. 12awg will carry 18amps over 200ft - it 'might' get a little warm / loss of power. For example, I run #6awg with 5 parallel strings of 9.x amps = 45a (or so) over 150ft and it get's 'a little warm' in conduit... so there's some loss - but this is negligible to the 15% loss I get with my inverter. I think we're talking 3% vs 2% kind of thing.
Unless you use a calculator that specifically uses one way distance (some do, some don't) and multiplies by two behind the scenes, you need to use round trip distance (total amount of wire in the circuit).
So if there is 200ft between Array and Controller that is 400ft of wire, and 400ft of resistance. That is what matters in a technical sense, you just need to know the calculator you are using expects as the input.
Diysolar123
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20amps over a 400ft (2*200...when it comes to electrical drop you need to include the TOTAL electrical path, it is always measured this way)
that is a very serious distance even for a 20amp circuit and would result in a voltage drop of over 6% with 10awg wire.
You should use at least 4awg wire...but its your call.
here is a voltage drop calculator link for you to play with ;-)
www.jcalc.net
that is a very serious distance even for a 20amp circuit and would result in a voltage drop of over 6% with 10awg wire.
You should use at least 4awg wire...but its your call.
here is a voltage drop calculator link for you to play with ;-)
AC and DC Voltage Drop Calculator NEC
The Voltage Drop Calculator implements the USA NEC Code. It includes Voltage Drop Formulas and Examples of How To Calculate Voltage Drop.
www.jcalc.net
Using the values I think are accurate for you, I get just under 3% Vdrop @ 10AWG and just under 5% Vdrop @ 12AWG
(the calculator I am using and the calculator @Diysolar123 is using return substantially different results for reasons I haven't quite pinned down (possibly different assumptions about the wire). And doing the math by hand (using this wire resistance chart) returns results closer to my calculator, but still a little bit different.
(the calculator I am using and the calculator @Diysolar123 is using return substantially different results for reasons I haven't quite pinned down (possibly different assumptions about the wire). And doing the math by hand (using this wire resistance chart) returns results closer to my calculator, but still a little bit different.
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Definitely where I have concerns. Were you using four hundred feet or two hundred feet as your input?
400ft (total length) / 200ft 'one way', if you click the link, you can see the assumptions I made (302.4 VDC, 18.4A, 400ft total copper wire -- most of the other fields don't matter for this calculation)
My guess is that that other calculator assumed aluminum wire which has higher resistance but I cant say for sure.
Here is how the math works:
[voltage drop] = [current] * [resistance]
(where resistance is Ohms (resistance) per foot * number of feet in the circuit)
Then:
[voltage drop] / [voltage] * 100 = Voltage Drop %
-----------------------------------------------------------------
So for instance, using 10AWG copper wire:
Vdrop = 18.4 A * (0.0009989 Ω/ft * 400 ft)
Vdrop = 7.352V
(7.352V / 302.4V) * 100 = 2.43% Voltage Drop
edit: one caveat, the NEC ('national electrical code' uses slightly more conservative values, for instance the same variables (10AWG copper, 18.4A, 302.4V) result in 2.89% voltage drop, not sure where the discrepancy lies, but its fairly small).
edit 2: the discrepency between the calculators was due to the second calculator not making it clear it used one-way distance. Adjusting for that, both calculators (set to NEC standards) return 2.9% voltage drop for 10AWG copper wire, 4.9% for 12AWG copper wire (pretty convenient results if you want to stay below either 3% or 5% :D )
Here is how the math works:
[voltage drop] = [current] * [resistance]
(where resistance is Ohms (resistance) per foot * number of feet in the circuit)
Then:
[voltage drop] / [voltage] * 100 = Voltage Drop %
-----------------------------------------------------------------
So for instance, using 10AWG copper wire:
Vdrop = 18.4 A * (0.0009989 Ω/ft * 400 ft)
Vdrop = 7.352V
(7.352V / 302.4V) * 100 = 2.43% Voltage Drop
edit: one caveat, the NEC ('national electrical code' uses slightly more conservative values, for instance the same variables (10AWG copper, 18.4A, 302.4V) result in 2.89% voltage drop, not sure where the discrepancy lies, but its fairly small).
edit 2: the discrepency between the calculators was due to the second calculator not making it clear it used one-way distance. Adjusting for that, both calculators (set to NEC standards) return 2.9% voltage drop for 10AWG copper wire, 4.9% for 12AWG copper wire (pretty convenient results if you want to stay below either 3% or 5% :D )
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I figured out the discrepancy (most of it), see my later edits in the previous comment:
Both calculators and my own math now show 10AWG is between 2.43% and 2.9%
Diysolar123
Solar Addict
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none of them are "wrong" as it is not possible to get an exact answer unless you have more information about the exact cable and environment.
calculators all seem to pick slightly different values for copper wire electrical resistance... the local temperature alone can make a significant difference for long runs.
for example, here are the values used for 1000ft of copper 10awg wire from different sites:
Copper Wire - Electrical Resistance vs. Gauge
Gauge, weight, circular mils and electrical resistance in copper wire.
www.engineeringtoolbox.com
American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength
AWG Wire size chart and ampacity table for design engineers including skin depth frequencies and tensile strength data; electrical cable size
www.powerstream.com
Marine Wire Size and Ampacity | West Marine
West Marine is committed to outfitting your life on the water. With over 250 store locations, 100,000 products in stock, and knowledgeable Associates, trust West Marine for your boating, sailing, fishing, or paddling needs. Shop with confidence - get free shipping to home or stores + price match...
www.westmarine.com
AC and DC Voltage Drop Calculator NEC
The Voltage Drop Calculator implements the USA NEC Code. It includes Voltage Drop Formulas and Examples of How To Calculate Voltage Drop.
www.jcalc.net
so west marine uses the LOWEST resistance/1000ft, jcalc uses the highest resistance...
if it is colder the resistance will be even lower, if the wire warms up it could be higher (well, probably not higher than 149F unless something has gone wrong hehe)
It is just a simple calculation, but it is dependent on range of resistance values based on environment (and of course wire construction)...
Okay, but there has to be an acceptable answer. First 75 feet of wire will be in a house in conduit, second part will be buried underground in conduit. The difference between 4 awg and10awg is pretty extreme for calculators to vary that amount. And cost is substantially different as well. So what is the answer other than just going with bigger wire
I used the first value 0.9989 in my calcs (edit: but just to be clear that is the baseline value, there could be (and likely are) other variables or multipliers beyond that)
As to temperature, good point. I believe but am not 100% positive both the NEC (US electrical code) and the ABYC (Marine industry electrical code), assume 30C/86F as the baseline value for ambient temperature in many cases (I know this is true of Ampacity tables, not sure about resistance)
As to temperature, good point. I believe but am not 100% positive both the NEC (US electrical code) and the ABYC (Marine industry electrical code), assume 30C/86F as the baseline value for ambient temperature in many cases (I know this is true of Ampacity tables, not sure about resistance)
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IF this is for a structure/house and you are in the US, and you don't want to obsess over the details to the extent that we do here. Find a calculator that is based on NEC solar wiring guidelines/values and use that. Or oversize and spent the extra $$.
I believe the value I used 0.9989 is what the NEC uses for 10AWG (or at least did in 2008)
edit: also I should note, I am not an expert in any of this.
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