How is a motor disconnect sized?

NEC 430.110(A) requires the disconnecting means for a single motor to have an ampere rating of at least 115% of the motor full-load current. You then round that value up to the next standard safety-switch frame, typically 30, 60, 100, or 200 amps.

What type of disconnect does NEC 430.109 require?

For a stationary motor larger than 2 HP, NEC 430.109(A)(1) requires a listed horsepower-rated motor-circuit switch, a molded-case switch, or a circuit breaker. The horsepower rating of the switch must equal or exceed the motor horsepower, not just its ampere rating.

Why does the switch need a horsepower rating, not just amps?

A motor disconnect must be able to interrupt the motor's locked-rotor current safely, which is far higher than the running current. Horsepower-rated switches are tested to break that inrush, so a plain ampere rating alone is not sufficient for motors over 2 HP.

Can I use a smaller disconnect for a tiny motor?

For motors of 2 HP or less, NEC 430.109(C) permits a general-use switch rated at not less than twice the motor full-load current. The tool flags this case so you do not over-specify a horsepower-rated switch for a small fractional or 2 HP motor.

Does this rule change for very large motors?

Yes. For motors over 100 HP, NEC 430.110 allows the disconnect to be rated by ampere only, since horsepower-rated switches above that size are uncommon. This tool targets the common motor range; verify the large-motor exception separately.

Motor Disconnect Switch Sizing Calculator

Every motor needs a disconnecting means that can be opened to isolate it for service, and the NEC sets both an ampere rating and a device type. This calculator applies NEC 430.110(A), which requires at least 115% of the motor full-load current, and NEC 430.109, which selects the switch type by horsepower, to recommend a code-compliant fusible or non-fusible safety switch.

How it works

Two rules combine, one for ampere rating and one for device type:

minimum rating = FLC × 1.15            (NEC 430.110(A))
switch frame   = next standard frame ≥ minimum rating   (30/60/100/200/400/600 A)
type (> 2 HP)  = HP-rated motor-circuit switch, molded-case switch, or breaker
                 with a horsepower rating ≥ the motor HP   (NEC 430.109(A)(1))
type (≤ 2 HP)  = general-use switch rated ≥ 2 × FLC       (NEC 430.109(C))

The horsepower requirement matters because the disconnect must interrupt locked-rotor inrush, not just running current.

Example and tips

A 10 HP motor drawing 28 A FLC needs at least 28 × 1.15 = 32.2 A, which rounds up to a 60 A frame safety switch, and because it exceeds 2 HP the switch must be horsepower-rated for at least 10 HP. A small 1 HP motor at 16 A could instead use a general-use switch rated for at least 32 A. Tips: match the horsepower rating of the switch to the motor, not only the ampere frame; choose fusible if the disconnect also provides branch-circuit protection, non-fusible if a separate breaker does; and check the short-circuit current rating of the switch against the available fault current.