Wire Gauge and Voltage Drop Calculator

Sizes a copper conductor for both limits that matter: ampacity, so the cable does not overheat, and voltage drop, so the load actually works.

The current this circuit will actually carry.

Distance to the load, not there and back. The return conductor is counted for you.

3% is the usual target for a branch circuit.

Applies the NEC 690.8(B) 125% factor. Solar output circuits run at full load for hours, so this applies to them.

Recommended conductor

4 AWG

Set by voltage drop. A shorter run or a higher system voltage would allow thinner wire.

Voltage drop
0.25 V (2.1%)
Max overcurrent protection
85 A

This is a floor, not a specification. It does not account for ambient temperature above 30°C, bundling in conduit, or terminal temperature ratings, all of which make the real answer thicker.

How this works

Two independent limits decide what conductor a circuit needs, and they are not the same question.

Ampacity is how much current a conductor carries continuously without its insulation overheating. That is a safety limit. Voltage drop is how much voltage the wire's own resistance consumes on the way to the load. That is a performance limit. This calculator sizes for both and returns whichever is thicker.

It tells you which one decided the answer, because they are fixed by different changes. A voltage drop limit is relieved by shortening the run or raising system voltage. An ampacity limit is not; only reducing the current helps.

Where the numbers come from

  • Resistance: NEC Chapter 9, Table 8, DC resistance at 75°C, uncoated copper, stranded.
  • Ampacity: NEC Table 310.16, 75°C copper column.
  • Small conductor limits: NEC 240.4(D), which caps overcurrent protection at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG regardless of the higher figures in Table 310.16. A conductor you cannot legally protect at your required current is not a usable answer, so this bounds the result.
  • Continuous duty: NEC 690.8(B), the 125% factor for PV output circuits, applied only when you tick that box.

Voltage drop uses the round-trip conductor length. Enter the one-way distance to the load and the return path is counted for you, which is a common source of error when doing this by hand.

What this deliberately does not model

Several real factors make the correct conductor thicker than this returns. Treat the result as a floor rather than a specification:

  • Ambient temperature above 30°C. Tabulated ampacity assumes 30°C. A hot attic, a sealed battery box, or a sun-facing wall derates every conductor in it.
  • Bundling and conduit fill. More than three current-carrying conductors together shed heat less effectively and are adjusted down.
  • Terminal temperature limits (110.14(C)). Equipment terminals carry their own rating, often 60°C on smaller devices, and the circuit is limited to the lowest-rated component in it.

Marine and RV wiring

This calculator follows the NEC, which covers buildings in the United States. Boats and RVs fall under ABYC standards instead, which are more conservative in several relevant respects, particularly for bundled conductors and engine spaces. Using these numbers for a boat will in places give you an answer that is not adequate.

For the underlying principles, see off-grid wiring and safety. For why the fuse matters as much as the conductor, seebattery safety.