Voltage Drop Calculator
Size conductors right the first time. Instant NEC-based voltage drop for 12/24/48 V DC solar and battery systems, plus 120/240 V AC branch circuits.
System & Conductor
Voltage drop FAQ
What voltage drop is acceptable for a 12V or 24V solar system?
The NEC recommends a maximum of 3% on branch circuits. On a 12 V system that's only 0.36 V, so low-voltage DC wiring almost always needs larger conductors than ampacity alone would require — this is exactly why 10 AWG or larger is common even for modest loads.
Does voltage drop use one-way or round-trip wire length?
This calculator takes your one-way distance and applies the k=2 multiplier to account for the full out-and-back conductor path. Enter the distance from source to load only — don't double it yourself.
Why does aluminum need a bigger wire than copper for the same drop?
Aluminum has about 68% higher resistivity than copper, so for the same current and distance you typically need to upsize by roughly two AWG sizes (e.g., 1/0 aluminum ≈ 2 AWG copper) to achieve the same voltage drop.
Is the NEC 3% voltage drop rule a code requirement?
No — NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 present 3% (branch) and 5% (total feeder + branch) as recommendations for reasonable efficiency, not enforceable minimums. Still, inspectors and engineers treat them as standard practice, and on 12/24 V DC systems exceeding them causes real equipment problems.
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We stock PV wire, THHN/THWN-2, USE-2, and battery cable in every size this calculator recommends — copper and aluminum, cut to length.
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