Solar Voltage Drop and Wire Size Calculator
Find the smallest wire that keeps voltage drop under your limit and carries the current, using the standard ampacity tables.
A long wire run wastes power as heat and starves the equipment at the far end. This calculator finds the smallest copper or aluminum wire that keeps voltage drop under your limit and still carries the current.
It works for DC circuits like battery cables and panel strings, and for single-phase and three-phase AC. Results show in AWG with the nearest metric size, and you can switch the length to meters.
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Planning estimates only. Check the result against your local code and the equipment data sheets. An inspector or licensed electrician has the final say. Data as of October 2026.
How this calculator works
Voltage drop is the current times the resistance of the wire, out and back. The tool uses the circular-mil area of each wire size and the resistance of copper or aluminum at the temperature rating you pick. It shows the drop in volts and as a share of the source voltage, and the watts lost as heat.
Current-carrying capacity comes from NEC Table 310.16, for up to three conductors in a raceway in 30 °C air. The tool compares it with 125% of your current, the usual margin for solar circuits. Small wires are held to the limits in NEC 240.4(D): 15, 20 and 30 A for 14, 12 and 10 AWG copper, and 15 and 25 A for 12 and 10 AWG aluminum.
The NEC’s informational notes suggest keeping voltage drop to 3% on a branch circuit and 5% overall. Many solar designers aim for 2% or less on DC runs, because the loss is paid for with energy you generated.
What it doesn’t do
- It isn’t a code check. Rooftop heat, bundles in conduit and your local rules can call for a bigger wire. Your inspector and a licensed electrician have the final say.
- AC results ignore reactance and power factor. That is close enough for short runs and wires up to about 1/0 AWG.
Where the numbers come from
- NEC (NFPA 70) Table 310.16 for allowable ampacity, section 240.4(D) for small conductors, and the informational note to section 210.19(A) for the 3% and 5% guidance.
- Conductor resistance from circular-mil area, with copper at 12.9 and aluminum at 21.2 ohm-circular-mils per foot at 75 °C reference values, corrected for temperature.
Questions
What voltage drop is acceptable for solar?
The NEC’s informational notes suggest 3% at most on a branch circuit and 5% for feeder and branch together. Many solar designers aim for 2% on the DC side, since every lost watt is energy you paid to make.
Why does a 12 V system need such thick wire?
The same power draws four times the current at 12 V as at 48 V, and a drop of a volt is a much bigger share of 12 V. A 20 A load 15 feet from a 12 V battery needs 6 AWG for a 3% drop. Five amps at 48 V over the same distance needs only 14 AWG.
Copper or aluminum?
Copper carries more current in a smaller size and is easier to connect. Aluminum is cheaper on long, large runs, but it needs lugs rated for it and an anti-oxidant compound.
How do I measure the length of the run?
Measure the one-way route of the actual wire, including climbs and turns. Enter that one-way length. The tool doubles it for DC and single-phase circuits.
Which temperature rating should I pick?
Pick 75 °C unless the equipment says otherwise. Most terminals are rated for 75 °C, so a 90 °C wire is still limited to its 75 °C ampacity at the connection.
Can I use this for panel strings?
Yes. Use the string’s operating voltage and current, and the one-way distance to the inverter. Panel strings run at high voltage, so even long runs need only a small wire.
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