Motor Circuit Calculator
Size 3-phase motor branch circuit conductors and OCPD per NEC 430.22 and 430.52. FLC values from Table 430.250. Free, no account required.
Motor Details
From NEC Table 430.52. Inverse-time breaker (250%) is the most common.
10 AWG Cu · 70 A OCPD
Recommended conductor and short-circuit / ground-fault protection.
NEC Motor Sizing Reference
- 430.6(A)(1): Use Table 430.250 FLC (not nameplate) for conductor and OCPD sizing.
- 430.22: Branch-circuit conductors must have ampacity of at least 125% of motor FLC.
- 430.52(C)(1): Maximum branch-circuit short-circuit and ground-fault protection per Table 430.52.
- 430.52(C)(1) Ex. 1: When the computed value does not match a standard rating, the next higher standard size is permitted.
- 430.32: Separate overload protection (set to nameplate FLA, typically 115% to 125%) is required and is NOT what this calculator sizes.
Quick examples (230V, inverse-time breaker)
| HP | FLC | Min Wire | OCPD |
|---|---|---|---|
| 3 HP | 9.6 A | 12 AWG | 25 A |
| 5 HP | 15.2 A | 12 AWG | 40 A |
| 10 HP | 28 A | 10 AWG | 70 A |
| 20 HP | 54 A | 4 AWG | 150 A |
| 50 HP | 130 A | 2/0 AWG | 350 A |
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This calculator uses NEC 2023 Article 430 Tables 430.52 and 430.250 for general squirrel-cage induction motors. Wound-rotor, Design B energy-efficient, synchronous, and single-phase motors use different percentages. Always verify with your AHJ and the NEC edition adopted in your jurisdiction.
How the motor circuit calculation works
Motor circuits break the ordinary rule that conductor ampacity and overcurrent protection track each other. A motor draws several times its running current during starting, so the code sizes the branch-circuit short-circuit and ground-fault device large enough to ride through the inrush, and then handles sustained overload with a separate overload device sized much closer to the actual current.
The starting point is 430.6(A)(1), which says that for general motor applications you size conductors and overcurrent devices from the table values in Table 430.247 through 430.250, not from the motor nameplate. For the conductor ampacity and branch-circuit protective-device determinations addressed by 430.6(A)(1), use the applicable Article 430 table value. Apply 430.32 for overload protection as applicable.
Conductors come from 430.22: not less than 125 percent of the table full-load current for a continuous-duty motor. The branch-circuit protective device comes from Table 430.52, where the maximum percentage depends on the device type and motor type, commonly 250 percent for an inverse time breaker on a squirrel-cage motor, and 430.52(C)(1) Exception No. 1 permits going up to the next standard size when the calculated value does not correspond to one.
Step by step
- 1
Look up the full-load current
Table 430.250 for three-phase, 430.248 for single-phase. Use the table value, not the nameplate, per 430.6(A)(1).
- 2
Size the conductors
430.22, at 125 percent of the table FLC for continuous duty. Then find that ampacity in Table 310.16.
- 3
Size the branch-circuit device
Table 430.52 by device type. Inverse time breaker on a squirrel-cage motor is 250 percent of table FLC.
- 4
Round the device
430.52(C)(1) Exception No. 1 permits the next higher standard size from 240.6(A) when the calculation does not land on one.
- 5
Size the overload
430.32, from the motor nameplate FLA, generally 125 percent for a service factor of 1.15 or greater or a temperature rise of 40 C, otherwise 115 percent.
- 6
Size the grounding conductor
Table 250.122, based on the rating of the branch-circuit overcurrent device.
Worked example
A 25 hp, 460 V, three-phase squirrel-cage induction motor, continuous duty, protected by an inverse time circuit breaker. Size the conductors and the breaker.
- Table 430.250: 25 hp at 460 V three-phase = 34 A full-load current.
- Conductors, 430.22: 34 x 1.25 = 42.5 A minimum ampacity.
- Table 310.16, 75 C copper: 8 AWG at 50 A covers 42.5 A.
- Breaker, Table 430.52: inverse time breaker, squirrel-cage, 250 percent. 34 x 2.50 = 85 A.
- 85 A is not a standard size. 240.6(A) next standard size up is 90 A, permitted by 430.52(C)(1) Exception No. 1.
Answer: 8 AWG copper conductors on a 90 A inverse time breaker. Size the overload device separately from the nameplate current under the applicable provisions of 430.32.
How this is tested on the exam
A motor-circuit calculation can require the applicable Article 430 full-load-current table, Table 430.52, 240.6(A), and Table 310.16. Verify the tested scope and permitted reference materials in current official exam information.
Frequently asked questions
Why do I use the table value instead of the motor nameplate?
For the conductor ampacity, switch rating, and branch-circuit protection determinations listed in 430.6(A)(1), use the applicable Article 430 table value. Apply 430.32 for overload protection.
How can a 90 A breaker protect an 8 AWG conductor?
In a motor circuit the branch-circuit device is not the conductor's overload protection. It exists to clear short circuits and ground faults and must be large enough to survive starting inrush. Sustained overload is handled by the separate overload device sized at 115 to 125 percent of nameplate under 430.32.
What percentage do I use for the branch-circuit device?
It comes from Table 430.52 and depends on both the device type and the motor type. Common values for a squirrel-cage motor are 300 percent for a nontime delay fuse, 175 percent for a dual element time delay fuse, 800 percent for an instantaneous trip breaker, and 250 percent for an inverse time breaker.
What if the calculated device rating is not a standard size?
430.52(C)(1) Exception No. 1 permits the next higher standard rating from 240.6(A). If the motor still will not start at that rating, 430.52(C)(1) Exception No. 2 allows further increases up to defined ceilings.
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