Wire Size Calculator

Two independent constraints

Sizing a conductor is not one calculation. It is two, run separately, and the larger answer wins.

Ampacity keeps the wire from overheating. Current through resistance makes heat, and the insulation has a temperature rating it must not exceed. NEC Table 310.16 lists how much current each size can carry — 50 A for 8 AWG copper in the 75 °C column, 65 A for 6 AWG, and so on. Exceed it and the insulation cooks, slowly, inside a wall.

Voltage drop keeps the load working. Every foot of conductor has resistance, and current through it consumes voltage that never reaches the equipment. Nothing overheats; the motor just runs hot and slow, the lights dim, the heater underperforms, and the electronics reset. It is a performance problem, not a fire problem, which is why the NEC treats it as an informational note (210.19(A) and 215.2(A)) rather than a hard rule in most cases.

The formulas

Ampacity check: table ampacity × ambient factor × conductor-count factor ≥ load current.

Voltage drop, single phase: VD = 2 × K × I × L ÷ circular mils

Voltage drop, three phase: VD = 1.732 × K × I × L ÷ circular mils

Where K is 12.9 for copper and 21.2 for aluminium, I is amps, and L is the one-way length in feet. The 2 in the single-phase version accounts for the return path — the current has to come back. Circular mils come from Chapter 9, Table 8: 16,510 for 8 AWG, 26,240 for 6 AWG, 41,740 for 4 AWG.

A worked example

A 40 A load at 240 V single phase, copper, 120 ft one way, 30 °C, three conductors in the raceway, 3% drop allowed.

By ampacity: derating is 1.0 × 1.0 = 1.0, so we need 40 A of table ampacity. 8 AWG copper carries 50 A at 75 °C. 8 AWG passes.

By voltage drop: allowed drop is 3% of 240 = 7.2 V. - 8 AWG: (2 × 12.9 × 40 × 120) ÷ 16,510 = 123,840 ÷ 16,510 = 7.50 V, which is 3.13%. Fails, barely. - 6 AWG: 123,840 ÷ 26,240 = 4.72 V, which is 1.97%. Passes.

Governing size: 6 AWG, one size larger than ampacity alone would have told you. This is the entire point of the exercise, and it is what people forget.

On long runs, voltage drop always wins

Ampacity does not care how long the run is. Voltage drop is directly proportional to it. Double the distance and you double the drop, so beyond roughly 100 feet the drop calculation starts overtaking ampacity, and past 150 feet it governs almost every time.

This is why a detached garage, a well pump, a dock, a barn or a landscape lighting run needs a conductor that looks absurdly oversized for its breaker. A 20 A circuit 250 feet out to a shed on 12 AWG loses over 9% of its voltage at full load. The breaker will never trip and the wire will never get warm — the tools out there will just run badly and the motors will die early.

The working guideline, from the informational notes: 3% on a branch circuit, and 5% total from the service to the furthest outlet, feeder and branch combined. Budget it as two halves — 2% on the feeder, 3% on the branch — rather than spending it all in one place.

Derating, which cuts the other way

The table ampacity assumes a 30 °C ambient and no more than three current-carrying conductors in the raceway. Break either assumption and you multiply the ampacity down:

• Ambient: 0.88 at 40 °C, 0.75 at 50 °C, 0.67 at 55 °C on the 75 °C column. Attics, rooftops and boiler rooms routinely hit these, and rooftop conduit in the sun gets an additional adder.
• Conductor count: 0.8 for four to six current-carrying conductors, 0.7 for seven to nine, 0.5 for ten to twenty. Grounding conductors never count; the neutral of a balanced multiwire branch circuit does not count either, but the neutral of a circuit serving significant nonlinear load does.

The two factors multiply. Six conductors in a 45 °C attic gives 0.8 × 0.82 = 0.66 — a 6 AWG copper conductor's usable 65 A becomes 43 A.

What this does not decide

The breaker. Overcurrent protection is a separate selection, and NEC 240.4(D) caps 14 AWG at 15 A, 12 AWG at 20 A and 10 AWG at 30 A no matter what the ampacity table says. Continuous loads — anything running three hours or more — get sized at 125 percent. Motors, welders, air conditioners and services all have their own articles. Use this to understand the trade-off, then have the design confirmed by someone licensed to sign it.