CalcStack — free, instant, no signup. The interactive calculator loads automatically.

Wire Size Calculator

Load amps in, wire gauge and breaker out.

Wire sizing has two rules that collide: the conductor must carry the load (ampacity), and continuous loads must be multiplied by 125% before sizing anything. This calculator applies both, using NEC Table 310.16 75°C ampacities for copper and aluminum, and outputs the minimum gauge and standard breaker size.

Why people use this

Anyone adding an EV charger, hot tub, or shop heater asks "what wire and what breaker?" — and the 125% continuous-load rule is exactly what casual answers miss. This applies the NEC table and the rule together, copper or aluminum.

How it works

1. Enter the load in amps.
2. Say whether it runs 3+ hours at a stretch (EV chargers, heaters, lighting) — that triggers the 125% rule.
3. Pick copper or aluminum and the system voltage.
4. Read the minimum wire size, breaker, design amps, and load wattage.

Common questions

What size wire for a 50-amp circuit?

Intermittent 50 A load: 8 AWG copper (50 A at 75°C). Continuous: 50 × 1.25 = 62.5 A design, so 6 AWG copper and a 70 A breaker. The continuous rule is why EV chargers upsize wire.

Can I use aluminum instead of copper?

Yes for feeders and large branch circuits — it is roughly two gauges larger and much cheaper per foot. Use AL-rated terminals, antioxidant compound, and proper torque; the 1970s fire problems were termination failures, not the metal.

Browse all 500+ free calculators →