Breaker Size Calculator

Calculation steps

1. Load current
   40 A
2. Minimum required rating
   40 A
   Ref: NEC 210.20(A)
3. Standard rating (rounded up)
   40 A
   Ref: NEC 240.6(A)

Export

The PDF contains your inputs, the result, the verdict, the formula with the values substituted, and the code reference for every step. No signup, no watermark, nothing held back.

What the breaker is sized from

The overcurrent device is sized from the circuit load, not from the conductor size. The conductor is then chosen so that the device protects it. Working in the other direction is how a 20 A breaker ends up protecting 14 AWG copper, which is a code violation even though the conductor will carry the current for a while.

For a continuous load the device is sized at not less than 125% of the continuous load plus 100% of the noncontinuous load (NEC 210.20(A)). That factor is applied before the result is rounded to a standard rating.

The tool takes the continuous portion separately, so a mixed load is not multiplied by 1.25 as a whole. Choose Part of the load and enter the continuous amperes when only part of the circuit runs for three hours or more.

The continuous-load factor

A load is continuous when it operates for three hours or more. Water heaters, electric vehicle chargers, most commercial lighting and many motor loads fall into that category, which is why a circuit that looks like 40 A of load can need a 50 A device.

The same factor flows into conductor sizing and into feeder calculations, so it is worth deciding early whether the load is continuous rather than adjusting the answer afterwards.

Standard ratings and rounding up

NEC 240.6(A) lists the standard ampere ratings. When the calculated load falls between two standard sizes, the device is generally the next standard rating up, provided that rating still protects the conductor it is connected to.

This tool rounds to the next standard rating and reports the required current alongside it, so the headroom in the choice is visible rather than implied.

Where this usually goes wrong

• Ignoring the continuous-load factor because the nameplate current looks comfortable.
• Sizing the device from the conductor instead of from the load. The direction of the calculation matters.
• Assuming the next standard rating up is always acceptable, without checking it against the conductor ampacity.
• Confusing the device rating with the conductor ampacity. They are two separate numbers that have to be checked against each other.

Frequently asked

How do I size a breaker for a continuous load?

Multiply the continuous portion of the load by 1.25, add the noncontinuous portion, and round up to the next standard rating from NEC 240.6(A). Then confirm that the resulting rating still protects the conductor you intend to install after its own ampacity corrections.

What are the standard breaker sizes?

NEC 240.6(A) lists the standard ampere ratings, and the familiar values such as 15, 20, 30, 40, 50, 60, 100 and 200 A come from that list. A device that is not a standard rating is permitted only in the specific applications the code identifies, which is rare in ordinary branch-circuit work.

Can the breaker be larger than the wire ampacity?

Generally the device is not larger than the conductor ampacity after corrections, since the device exists to protect the conductor. NEC 240.4(B) conditionally permits the next higher standard rating, up to 800 A, only when all its conditions are met, including its restrictions on certain receptacle circuits and nonstandard conductor ampacities. Separate equipment-specific rules may also apply. This calculator does not automatically assume the next-higher exception; confirm the applicable code and installation.

Do I size the wire for the breaker or the load?

Size the conductor for the load after the ambient, conductor-count and termination corrections, then choose a device that protects that conductor. The load drives the conductor and the conductor constrains the device, so starting from the device is the step that usually produces the error.

Related guides and references

• The 80% rule
• Wire size for a 60 A water heater
• Quick reference (printable PDF)