Solar Wire Size Calculator
Checked against both limits that matter — voltage drop and ampacity — because a cable that holds the drop can still be too small to carry the current safely.
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In short: Correct cable size is the larger of the gauge required by voltage drop and the gauge required by ampacity. Testing voltage drop alone, which most calculators do, returns a cable that can overheat on short high-current runs.
Cable size
4 AWG
limited by voltage drop
Design current (125%)
25.0 A
Smallest that passes drop
4 AWG
Smallest that passes ampacity
12 AWG
Copper, resistance from NEC Chapter 9 Table 8, ampacity from NEC 310.16 at 75°C. Length is one way; the return conductor is accounted for. This is a design aid, not a substitute for an inspection.
Two limits, not one
Ampacity is how much current a conductor can carry before it gets hot enough to damage its insulation. It is a safety limit and it does not care how long the run is — three feet or thirty, the same current produces the same heating per foot.
Voltage drop is how much you lose to resistance along the way. It is a performance limit and it scales with length, so it dominates on long runs and barely registers on short ones.
That is why one test is never enough. On a short, high-current run the drop is tiny and a drop-only calculator returns something far too small to carry the load. On a long, low-current run ampacity is satisfied by a cable that loses so much voltage the equipment at the far end underperforms. The right answer is always the larger of the two, and the figures above show both so you can see which one is doing the work.
Voltage is the lever
For the same power, drop as a percentage falls with the square of the system voltage. Doubling from 12V to 24V cuts it to a quarter. Going to 48V cuts it to a sixteenth.
The run above needs 4 AWG at 12V, and only 10 AWG at 48V — the same power, the same distance, dramatically less copper. Copper is priced by weight, so this is usually the difference between an affordable run and one that makes you rethink where the battery lives.
When there is no answer
If the result says no single cable works, that is a real result rather than an error: no gauge in the table satisfies both limits at once. Raise the system voltage, shorten the run, split the load across parallel circuits, or accept a larger voltage drop if the equipment genuinely tolerates it. What you should not do is pick the biggest cable listed and hope.
Find the parts
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Questions
Why does your answer come out bigger than other calculators?
Because most of them check voltage drop only. A cable has two separate limits: it must carry the current without overheating (ampacity, a fire-safety limit) and it must deliver the power without losing too much along the way (voltage drop, a performance limit). The correct answer is whichever limit demands more copper, never whichever one passes first. On short high-current runs ampacity governs and a drop-only calculator will hand you something dangerous.
Is the length one way or round trip?
Enter the one-way distance. The calculation doubles it for you, because current has to travel out and back. Forgetting that is the most common reason a hand calculation comes out half the size it should be.
Why 3% voltage drop?
It is the usual design target for a branch circuit and a sensible default. The NEC treats it as a recommendation rather than a requirement, but on low-voltage DC it matters far more than on household AC: 3% of 12V is 0.36 volts, and there is very little headroom before equipment starts misbehaving.
What is the 125% for?
NEC 690.8 treats solar output as a continuous load, so conductors are sized at 125% of it. We apply that to the ampacity check. Voltage drop is calculated on the actual current, since that is what really flows.
My run needs enormous cable. What now?
Raise the system voltage. Voltage drop as a percentage falls with the square of voltage for the same power, so the same 20A run that needs 4 AWG at 12V needs only 10 AWG at 48V. Shortening the run helps too, but moving the battery nearer the array is usually easier than paying for copper — and if the calculator returns no answer at all, it means no single cable in the table satisfies both limits and the design itself has to change.