Circuit Breaker and Fuse Sizing Calculator (NEC)

Find the breaker or fuse size for a motor circuit or a continuous load, the overload setting limit, and whether a copper conductor suits the device. Each result shows the NEC rule behind it.

A starting point, not the final design. This applies common NEC rules for standard cases. It does not cover derating for temperature or bundled conductors, voltage drop, terminal temperature limits, special motor types, listed equipment instructions, available fault current or local amendments. Check the NEC edition adopted where the work is done, and have the design reviewed by a qualified person.

Motor circuit (NEC Article 430)

Motor size

Voltage (three-phase)

Nameplate current (A, for the overload)

Service factor 1.15 or more, or temperature rise 40 °C or less?

The short-circuit and ground-fault device is sized from the NEC table current for the motor size. The overload is sized from the nameplate. If you leave the nameplate blank, the table current is used as a stand-in. Results assume a squirrel-cage motor (Design B).

An instantaneous-trip breaker (motor circuit protector) is only allowed as part of a listed combination motor controller.

Branch circuit or feeder with continuous load

Continuous load (A)

Other (non-continuous) load (A)

A continuous load is one expected to run for three hours or more. Assemblies listed for 100% continuous operation follow their own instructions.

Check a copper conductor against a device

Conductor size

Terminal temperature rating

Breaker or fuse rating (A)

Circuits rated 100 A or less generally use the 60 °C column unless the equipment is marked for 75 °C (NEC 110.14(C)). Motor circuits follow Article 430 for conductor and device sizing, so the small-conductor limits in this check may not apply to them.

How it works

Overcurrent protection has more than one job, and they use different rules:

Short-circuit and ground-fault protection (the breaker or fuse) must be large enough to let a motor start but small enough to clear a fault. For motors, Table 430.52 gives a maximum percentage of the full-load current for each device type.
Overload protection protects the motor from sustained overcurrent. It is sized from the nameplate current (NEC 430.32), not the table.
Conductors must carry the load, and the protection must not allow more current than they can safely carry.

Motor: device limit = table full-load current × percentage (fuse 175% or 300%, inverse-time breaker 250%, instantaneous-trip 800%)
Overload limit = nameplate current × 125% (or 115% if the service factor and temperature rise conditions are not met)
Continuous load: minimum = 1.25 × continuous load + other load

When the calculated limit is not a standard size, the NEC allows the next standard size up for motor circuits (430.52(C)(1) Exception 1). If a motor will not start on the base rating, the NEC allows larger devices up to a stated maximum. The "show the math" panel lists these.

Worked example

A 10 hp, 460 V three-phase motor has a table full-load current of 14 A.

Dual-element fuse: 14 × 175% = 24.5 A, so the next standard size is a 25 A fuse.
Inverse-time breaker: 14 × 250% = 35 A, a standard size, so a 35 A breaker.
Overload, with a nameplate current of 12.8 A: 12.8 × 125% = 16 A at most.
Minimum conductor ampacity: 14 × 125% = 17.5 A.

For a branch circuit with 16 A of continuous load and 4 A of other load, the minimum is 1.25 × 16 + 4 = 24 A, so the smallest standard device is a 25 A breaker.

Common questions

Why not just use the nameplate current for the fuse?

The NEC sizes branch-circuit protection from the table value so that every motor of a given size is treated the same. The nameplate is used for the overload, because it reflects that actual motor.

Why can a 14 AWG wire feed a 35 A breaker on a motor?

For motor circuits, Article 430 sets conductor and protection rules separately (conductors at 125% of the table current, protection per Table 430.52), and the general small-conductor limits in 240.4(D) do not apply. For ordinary circuits they do, which is what the conductor check shows.

Does this check the interrupting rating?

No. A breaker or fuse must also be rated for the fault current available at its location. Use the available fault current calculator and see the guide to fault current and interrupting ratings.

Calculations are provided for convenience and education and are not a substitute for the electrical code or a qualified engineer. Verify against the edition of the NEC adopted where the work is done.