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BFL13's avatar
BFL13
Explorer II
Oct 04, 2013

Wire Gauge Required Question

For some reason I just can't understand the various wire gauge tables, and I have tried to, so I am hoping somebody will take pity and do all the work! :)

One table that I can follow says that for a 12v system, and a 16 amp solar controller, you need (to be within 1/2 volt drop) for a solar array connection to controller of 30 ft (round trip)-- #12, or for 40ft-- #10

I take that to mean either 15 ft away or 20 ft away.

This same source says to double the voltages for 24v in their example for calculating voltage drop, so does that mean with 24v, 1.0v drop is ok on that line? NOTE this is in a manual for a PWM controller. ISTR with MPPT you want a tighter control on voltage drop.

Anyway I am lost here. My question is, "What wire gauge of stranded copper would you need for 16 amps with a 24v system 30ft away (60 ft round trip) with an MPPT controller?"

EDIT--also, please with a 24v panel and MPPT, do you use the 12v figures for the controller battery, but the 24v figures for the controller array wires?

Thanks.

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  • ktmrfs's avatar
    ktmrfs
    Explorer III
    Oct 04, 2013
    Another bit of info on AWG (american wire gauge) wire gauges follow a logrithmic scale. So, changing a wire gauge by 3 doubles or halves the resistance. E.G. going from 13g (I know, doesn't exist) to 10 gauge will cut resistance in half. Or, going from 14 to 12 or 12 to 10, will not quite cut the resistance in half, probably cuts it by about 35-40%.

    Changing by 10 increases or decreases resistance by a factor of 10. So 1g has 1/10 the resistance of 10 gauge. and 20g. has 10 times the resistance of 10g.

    so if you don't have a handy dandy chart of ga. vs. ohms/ft, using the above will give you a good idea of what to expect with changes in AWG.

    Now, if it isn't AWG, I have no clue how resistance relates to ga.
  • BFL13's avatar
    BFL13
    Explorer II
    Oct 04, 2013
    To be sure, I mean the array is 30 ft away from the controller for this question, so 30 ft of pos wire and 30 ft of neg wire carrying 16 amps from array to controller. (unless it is 8 amps)

    IE the 24v (36v actual) panel has an Isc of 8.x amps, but with MPPT it will do about 16a. Or is that 16a only from the controller to battery and it is 8 amps array to controller? I know you can use thinner wire with 24v than with 12v, but is that because the amps are half on the array side of the controller? Ie 8a instead of the 16a seen at the battery?

    I have no idea what the appropriate amount of voltage drop would be to not "lose" any amps in MPPT. I don't understand percentages in term of voltage drop either. 2% of what? The voltage diff array to controller is always Vmp in MPPT but it varies controller to battery as battery voltage rises in 12v ??. Gads.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Oct 04, 2013
    I run a tabulation showing potential kWh loss over a one year period and let that decide what gauge wire I should use.
  • msiminoff's avatar
    msiminoff
    Explorer II
    Oct 04, 2013
    BFL13 wrote:
    "What wire gauge of stranded copper would you need for 16 amps with a 24v system 30ft away (60 ft round trip) with an MPPT controller?"

    In round numbers, assuming 60 feet of wire at 24V and 16A;
    4ga (42000 Circular Mills) wire will yield ~1% (0.24V) voltage drop,
    6ga (26600 CM) wire will have a ~2% (0.48V) voltage drop
    8ga (16800 CM) wire will have ~3% (0.72V) voltage drop.

    All of these three wire gauges are more than enough in terms of ampacity, from here your budget, installation constraints, and personal preference are the real deciding factors. In terms of safety, you could go as small as 16ga , but personally I wouldn't choose to install a solar charging system (MPPT or PWM) with any more than 3% voltage drop.

    Hope this helps
    Cheers,
    -Mark
  • time2roll's avatar
    time2roll
    Nomad
    Oct 04, 2013
    16 amps sounds like two big panels in parallel.
    Put them in series and use #10.. or does the MPPT not allow the series Voc?

    Max I would go is #8 so I could stay with MC4 connectors.
  • 2oldman's avatar
    2oldman
    Explorer II
    Oct 04, 2013
    BFL13 wrote:
    "What wire gauge of stranded copper would you need for 16 amps with a 24v system 30ft away (60 ft round trip) with an MPPT controller?"
    #12 would be fine, #10 to be sure.

    Honestly, I never pay much attention to voltage drop calcs, let alone specific to a certain voltage. Perhaps I'm naive in thinking that going that far is splitting hairs.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Oct 04, 2013
    I like to limit voltage drop for a low-voltage solar system at a maximum of three-tenths of a volt*. Don't give a damn what gauge wire I need to accomplish this. This means a three-volt drop for a high voltage system. 120vdc. Call it between one and two percent.

    *As measured between the manifold junction point "buss" for the panels and the entrance of the wire to the controller.

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