Calculator 08 / Electrical

Electrical Calculator

Check voltage drop on a circuit run, or get a starting-point wire gauge and breaker size for a residential branch circuit.

These calculators use standard published copper-conductor reference values for estimating purposes. They do not replace a licensed electrician's load calculation and do not account for ambient temperature, conduit fill, bundling derates, or local code amendments. All electrical work should be performed by a licensed electrician and inspected per local code.

Wire gauge (AWG)

Conductor material

Circuit type

One-way distance (ft)

Load current (amps)

System voltage (V)

How this calculator works

Voltage drop: VD (V) = (K × Multiplier × I × D) ÷ CM VD (%) = (VD ÷ System voltage) × 100 K = 12.9 (copper) or 21.2 (aluminum), ohm·cmil/ft Multiplier = 2 (single-phase, round trip) or 1.732 (three-phase) CM = circular mils of the chosen wire gauge Wire/breaker: Required ampacity = Load current × 1.25 (if continuous load), else Load current Wire gauge = smallest standard gauge with ampacity ≥ required ampacity Breaker size = smallest standard breaker size ≥ required ampacity

Voltage drop matters most on long runs or heavy loads — the NEC recommends keeping voltage drop to 3% or less on a branch circuit (5% total including the feeder) for reasonable equipment performance. Wire sizing here uses the standard 60°C copper ampacity column, which matches common residential NM-B (Romex) at typical 15–200A branch circuit sizes. The continuous-load adjustment follows NEC 210.19(A)/210.20(A) — circuits that run 3 hours or more at full load need 125% of the wire and breaker capacity.

What counts as a "continuous load"?

Anything expected to run at full load for 3 hours or more without interruption — most lighting circuits, some HVAC and water heater circuits. Standard receptacle and general-use circuits are usually treated as non-continuous.

Why does this only cover copper?

Aluminum wiring uses different ampacity tables and requires different terminations and anti-oxidant compound — sizing it correctly needs a dedicated table this tool doesn't include. If you're running aluminum conductors, consult a licensed electrician.

Does the breaker size account for wire ampacity limits?

This tool recommends a breaker based on the load; it's still on you to confirm the recommended breaker doesn't exceed the ampacity of the wire gauge shown — the two are displayed together so you can check.

Estimates only, for planning purposes. Always verify against the current NEC, your local code amendments, and a licensed electrician before any installation.