wire sizing question

ramarama Solar Expert Posts: 36
Hi all,

Quick question on sizing proper wire - the scenario is that the distance from combiner box to charge controller is 15m and max running amp is determined as 17A. At a system voltage of 24V and tolerance voltage drop of 3% - can I safely say that the wire size I need is AWG 6?

I looked into a number of references and the formula used was VDI = (amp*feet)/(%volt drop*sys volt).

Is this any good?

I've had a guy who has been a PV installer for 5 years telling me that I don't need such a big wire. He thinks that the ampacity of size 10 wire, which is 30A, is already sufficient to cover 17A so it's good enough.

Am I thinking too much?



  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: wire sizing question

    As you describe the problem--that is the correct answer. Approximately 0.65 volts drop... Also, if there is a fuse or circuit beaker, then you can have additional voltage drop. If you size the fuse for the 6 awg wire vs the 17a*1.25=21.25amps, you will have less voltage drop (larger value fuses/breakers have less resistance).

    If you have other issues (high frequencies, loads that cannot operate with a 0.7 volt drop).

    The 3% voltage drop comes from an NEC "requirement" (I think)--There is nothing "magic" about 3% and you can set your requirements higher or lower based on your specific needs.

    You can have larger voltage drops between the Array and the Charge controller (but, you do lose power). However, you should not have more than ~0.05 to 0.1 volts drop between the charge controller and a 12 volt battery bank (~0.1 to 0.2 volts for 24 volt battery bank). You want a the charge controller to be able to accurately measure the battery bank voltage during charging.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ramarama Solar Expert Posts: 36
    Re: wire sizing question

    Hi Bill,
    Thanks for the quick confirmation. Can I ask how you come to 0.65V drop?
    I calculated the voltage drop but only got about 0.33575V (over 15m). This is what I did:

    Size 6 wire (13.3mm2) has a resistance in ohms per foot of 0.000395, so my overall drop over 15m (or 50ft) would be 0.000395*50ft*17A = 0.33575V, and this is about 1.4% voltage drop.

    I'm confused here as I was taking a 3% voltage drop in my previous VDI calculation but this shows only 1.4% voltage drop will occur. Does that mean I can use a smaller size wire like 8 or 10?

    For size 8 I did the same calculation from above and got 2.2% drop, size 10 I got 3.5% drop. So does this mean if I'm comfortable with 3.5% drop I'll can use size 10 for cost saving?

    The resistance table I used are what I got from a book and data look like this:

    AWG size / resistance in ohms per foot
    14 / 0.002525
    12 / 0.001588
    10 / 0.000999
    8 / 0.000628
    6 / 0.000395
    4 / 0.000249

    Does the above look right to you?
  • ramarama Solar Expert Posts: 36
    Re: wire sizing question

    Oh wait... I need to multiply the distance by 2, isn't it? Feed & return?

    Did I just answer my own question? lol
  • ramarama Solar Expert Posts: 36
    Re: wire sizing question

    Bill - could you tell me if the size of grounding wire would matter? I've checked my combiner box output will host the size 6 wire just fine, but the grounding terminal only fits with max 6mm2 wire (about size 10?). Would this matter?

    Thank a lot!
  • BB.BB. Super Moderators, Administrators Posts: 32,006 admin
    Re: wire sizing question


    You make giving answers here easy! What we call a low maintenance poster. ;)

    I just used one of the many online voltage drop calculators... As the temperature of the wire rises, you can add factors for ambient (and/or self heating effects of wire bundles and in conduit/insulated walls/buried underground/etc.). To get higher resolution (0.7 vs 0.65), I just multiply the wire length by 10x to get the extra digit (and shift the decimal point back to the right place in the answer).

    For grounding--I believe in the US, #6 wire is the minimum accepted bare copper wire for a ground run to the rod/earth.

    There are several other ways of sizing the ground wire--same size as the rest of the power wires. Or based on the maximum surge current the wire will need to carry to ground if there is a short--and the current rating of the circuit breaker...

    In your case, you want low voltage drop to carry 17 amps, but are using cable rated for much more current. If the available power is only 20-25 amps, then the 6 awg ground run is fine (also, you need to make sure the power+ground cable is heavy enough to trip the circuit breaker--I.e., if you have a large circuit breaker and a very long wire run, the wire resistance may not allow enough current to trip the breaker--and the entire wire run would behave like a large resistance heater--But that issue was mostly with large equipment in specialized applications).

    If you have stranded #6 cable--You can cut off a couple strands to allow the rest of the cable into the existing cable clamp.

    When I did a little bit of code many decades ago (code may have changed--or simply not apply to your area)--We were able to use #8 insulated wire for the main ground run instead of #6 bare wire.

    Are you in a lightning prone area? Or is this just for fire and shock safety?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ramarama Solar Expert Posts: 36
    Re: wire sizing question

    Thanks BB - you always give such thorough and detailed answers I feel like I should do some homework before I ask... :)

    The combiner box I'm currently using will fit a size 10 ground wire at most... so it's different from the size 6 power wire. We're not in a stormy area so it's pretty much just for fire safety.

    Thanks for your help, as always. :)

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