Breaker / OCPD Size Calculator

Enter the load current and whether it is continuous (on for three hours or more). The tool applies the 125% continuous-load factor, rounds up to the next standard breaker size and shows the smallest 75°C copper conductor that pairs with that overcurrent device.

Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.

Conductor and overcurrent sizing must comply with the NEC edition adopted by your jurisdiction, including all temperature, bundling and termination-rating derating. Values reference NEC 310.16 / Chapter 9 (ed. 2020/2023) — confirm the current code with a licensed electrician.

Calculator

Required rating	20.0 A
Standard breaker	20 A
Minimum copper conductor	12 AWG

Formula

Required rating = load × 1.25 for a continuous load (three hours or more), or × 1.00 for a non-continuous load. That value is rounded up to the next standard OCPD size from NEC 240.6(A): 15, 20, 25, 30, 35, 40, 50, 60, 70, 80, 90, 100 A and up. The minimum conductor is the smallest 75°C copper wire whose ampacity covers the breaker, honoring the small-conductor limits of 240.4(D) (15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG).

Worked example

A 16 A continuous load needs 16 × 1.25 = 20 A, which is already a standard size, so the breaker is 20 A and the minimum conductor is 12 AWG copper. A 1,500 W load on 120 V draws 12.5 A; continuous, that is 12.5 × 1.25 = 15.6 A, rounded up to a 20 A breaker on 12 AWG copper.

A breaker protects the wire, not the appliance: its job is to open before the conductor overheats, so it is sized to the conductor and the load, never to whatever happens to fit. The rule this tool applies is the 125% continuous-load factor — any load expected to run three hours or more, such as lighting, an EV charger or an HVAC compressor, is sized at 125% of its steady current so the breaker and wire run cool. It also respects the cap that limits 14, 12 and 10 AWG to 15, 20 and 30 A. Size the wire first, then choose the device that protects it, and never up-size a breaker to stop nuisance trips without up-sizing the conductor too.

Frequently asked questions

What is the 125% continuous-load rule?

A continuous load is one expected to run for three hours or more. The NEC requires the overcurrent device and conductors to be rated for at least 125% of that load, which is the same as dividing the device rating by 0.8.

How do I size a breaker for watts?

First convert watts to amps by dividing by the voltage (amps = watts ÷ volts) using the appliance amp-draw tool, then enter those amps here. For example, 1,500 W on 120 V is 12.5 A before the continuous factor.

Why round up to a standard size?

Breakers are only manufactured in the standard ratings listed in NEC 240.6(A). When the calculated minimum falls between two sizes, you move up to the next standard size so the device fully protects the circuit.

Does the breaker protect the wire or the load?

The overcurrent device primarily protects the conductor from overheating, which is why the wire must be rated for the breaker. This tool returns the minimum copper conductor that matches the chosen OCPD, but always verify terminations and any derating.

Source: NEC 240.4 and 240.6 plus the 125% continuous-load rule (210.20 / 215.3) · All sources