Motor & HVAC Disconnect Sizing Calculator

What the disconnect rating covers

The disconnecting means is the switch that isolates a motor or piece of equipment from its supply so it can be serviced safely. NEC 430.110 and 440.12 both fix a minimum ampere rating for that switch. This is a separate question from the branch-circuit breaker or fuse (which protects against short circuits and ground faults), the running overload (which protects the motor), and the conductor size. A motor circuit needs all of them, sized independently.

A disconnect is not an overcurrent device, so its rating is not chosen from the 240.6(A) standard ratings and there is no next-standard-size rule. The code sets a floor; any listed disconnect rated at or above it is acceptable. Enclosed safety switches happen to be built in fixed sizes, so the tool also shows the smallest available switch that clears the minimum.

Motor disconnect: NEC 430.110

NEC 430.110(A) requires the disconnecting means to have an ampere rating of at least 115 percent of the motor full-load current:

minimum disconnect rating = table FLC x 115%

The full-load current is the Table 430.248 (single-phase) or Table 430.250 (three-phase) value for the horsepower and voltage, used per NEC 430.6(A)(1), the same table value used for conductor and branch-circuit sizing. A horsepower-rated motor-circuit switch must also carry a horsepower rating at least equal to the motor horsepower.

A/C compressor disconnect: NEC 440.12

A hermetic refrigerant motor-compressor is a sealed unit with no horsepower-based table current, so NEC 440.12 sizes its disconnect from the nameplate values:

minimum disconnect rating = 115% x greater of (RLA, BCSC)

The branch-circuit selection current (BCSC) is a nameplate value manufacturers mark when it is higher than the rated-load current (RLA); when marked, it governs. For combination equipment (a compressor plus a condenser-fan motor on the same disconnect), NEC 440.12(B) uses 115 percent of the sum of all rated-load currents.

Rating a horsepower-rated switch for a compressor involves an additional step, NEC 440.12(A)(2), which compares the horsepower for the running current against the horsepower for the locked-rotor current (Tables 430.251(A)/(B)) and takes the larger. That method is outside this version; the 115 percent ampere rating shown here is a complete requirement on its own under 440.12(A)(1).

Worked examples

Motor. A 10 hp, 460 V three-phase motor has a Table 430.250 full-load current of 14 A:

14 A x 115% = 16.1 A minimum, so a 30 A switch

A/C compressor. A condensing unit with a 24 A rated-load current and no marked branch-circuit selection current:

24 A x 115% = 27.6 A minimum, so a 30 A switch

If that same unit also carried a 1.5 A condenser-fan motor on the disconnect, the basis becomes 24 A + 1.5 A = 25.5 A, and 25.5 A x 115% = 29.33 A minimum, still a 30 A switch.

How this differs from the breaker and overload tools

The disconnect, the branch-circuit protective device, and the overload are three separate devices sized by three separate rules. This page sizes the disconnect ampere rating (430.110 / 440.12). The Motor Breaker Sizing Calculator sizes the branch-circuit short-circuit and ground-fault device (Table 430.52, up to 250 or 300 percent) and the conductor (430.22). The Motor Overload Protection Calculator sizes the running overload (430.32, 115 to 125 percent of nameplate). For HVAC, the MCA & MOCP Calculator handles the minimum circuit ampacity and maximum overcurrent protection.

Frequently asked questions

How do you size a motor disconnect under NEC 430.110?

The disconnecting means must be rated at least 115 percent of the motor full-load current, using the Table 430.248/430.250 value for the horsepower and voltage (NEC 430.6(A)(1)). A 10 hp, 460 V three-phase motor (14 A table FLC) needs a disconnect rated at least 14 x 1.15 = 16.1 A.

How do you size an A/C compressor disconnect under NEC 440.12?

At least 115 percent of the greater of the nameplate rated-load current (RLA) or the branch-circuit selection current (BCSC). A 24 A RLA compressor needs at least 24 x 1.15 = 27.6 A. Combination equipment uses 115 percent of the sum of all rated-load currents (440.12(B)).

Nameplate current or table FLC?

Table FLC for a standard motor (430.6(A)(1)); nameplate RLA or marked BCSC for a hermetic compressor (440.4 and 440.12), which has no horsepower-based table current.

Does the disconnect have to be a standard size?

No. A disconnect is not an overcurrent device, so 240.6(A) does not apply. The NEC gives a minimum rating; any listed disconnect at or above it works. The tool shows the smallest common enclosed-switch size (30, 60, 100 A and up) as guidance only.

Does this cover the 440.12(A)(2) horsepower method?

Not in v1. Rating a horsepower-rated switch also compares the horsepower for the locked-rotor current (Tables 430.251(A)/(B)) and takes the larger. That needs the compressor locked-rotor amps and those tables. The 115 percent ampere rating shown here is complete on its own under 440.12(A)(1).

Related calculators

• Motor Breaker Sizing Calculator: the branch-circuit breaker or fuse and conductor per NEC Table 430.52 and 430.22.
• Motor Overload Protection Calculator: the running overload relay per NEC 430.32, on the motor nameplate current.
• MCA & MOCP Calculator: HVAC minimum circuit ampacity and maximum overcurrent protection per NEC Article 440.
• Wire Ampacity Calculator: conductor ampacity with the 110.14(C) termination and 310.15 derating rules.