What is the NEC 80% rule?

NEC 310.15(B) and 210.19(A)(1) require that a circuit breaker used for continuous loads (on for 3 hours or more) must not be loaded beyond 80% of its rating. So a 20 A breaker can only supply 16 A continuously. The calculator divides the required amperage by 0.8 to find the minimum breaker size.

How do I find total load wattage?

Add the wattage of every device that will run on the circuit simultaneously. Check device nameplates or spec sheets. For motors, use the nameplate FLA (full-load amps) multiplied by voltage.

What are standard residential breaker sizes?

Standard single-pole ANSI/UL 489 sizes (amps): 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200. The calculator automatically rounds up to the next available size.

Why does voltage affect breaker size?

Amperage = Watts / Volts. A 2,400 W load at 120 V draws 20 A, but at 240 V the same load draws only 10 A. Higher voltage circuits carry the same power at lower current, so smaller breakers suffice.

Does this account for motor starting current?

No. Motors draw 5–7× their running current at startup. For motor circuits, NEC Article 430 permits oversizing the breaker to handle inrush. This tool covers general-purpose branch circuits; consult an electrician for motor branch circuits.

When should I use three-phase?

Three-phase power is common in commercial and industrial buildings. If your panel is three-phase, the line current is Watts / (√3 × Volts × Power-Factor). This tool assumes PF = 1.0 for simplicity; derate manually for motors or inductive loads.

Circuit Breaker Size Calculator

Name: Circuit Breaker Size Calculator
Availability: InStock
Author: Nham Vu

Enter load wattage and voltage to find the minimum breaker amperage required under the NEC 80% continuous load rule.

Circuit Parameters

Total Load (watts)

Supply Voltage

Power System

Single-phase Three-phase

Load Type

Continuous Non-continuous

Continuous = on for 3 hours or more (NEC 100). Applies the 80% derating rule.

Enter load wattage above to see the required breaker size.

Summary

Enter load wattage and voltage to find the minimum breaker amperage required under the NEC 80% continuous load rule.

How it works

Enter the total load wattage (or watts per circuit).
Select the supply voltage: 120 V, 240 V, or 208 V.
Select load type: continuous (3+ hours) or non-continuous.
Choose single-phase or three-phase for commercial circuits.
The calculator shows the minimum breaker amps and the nearest standard size.

Use cases

Size a breaker for a new kitchen appliance circuit.
Verify an existing breaker is not undersized for added loads.
Plan sub-panel branch circuits for a home addition or workshop.
Size breakers for commercial HVAC, lighting, or motor loads.
Check NEC 80% compliance for continuous-duty circuits.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu