Motor circuits do not follow the same sizing rules as general branch circuits. Under NEC Article 430, motors need three separate levels of protection: branch circuit overcurrent protection (OCPD) sized generously enough to handle starting inrush, separate overload protection sized close to the motor's full-load current, and a disconnecting means. Each is sized differently, and confusing them is one of the most common code violations on commercial and industrial jobs.
Why motors need different protection
When an induction motor starts, it draws five to eight times its full-load current for a brief period — typically one to three seconds for a direct-on-line start. A protective device sized to the motor's running current would trip every time the motor started. Article 430 accounts for this by allowing the branch circuit OCPD to be sized well above the running current, with separate overload protection to catch sustained overloads that would damage the windings.
Full-load current (FLC)
All motor circuit calculations start with the full-load current from NEC Tables 430.247, 430.248, or 430.250 — not from the nameplate. The NEC specifies this because motor nameplates often show rated current at a specific voltage and power factor that may differ from design conditions. The tables give standardised values for the protection calculations.
- Table 430.247 — DC motors
- Table 430.248 — single-phase AC motors
- Table 430.250 — three-phase AC motors
Example values from Table 430.250 (three-phase, 230 V):
- 1 HP: 4.2 A
- 2 HP: 6.8 A
- 5 HP: 15.2 A
- 10 HP: 28 A
- 20 HP: 54 A
- 50 HP: 130 A
Branch circuit conductor sizing (NEC 430.22)
The branch circuit conductor from the panel to the motor controller must be rated at a minimum of 125% of the motor's full-load current. This accounts for the continuous nature of most motor loads (motors are typically running for more than three hours — the NEC definition of a continuous load).
For a 10 HP, 230 V three-phase motor (FLC = 28 A from Table 430.250):
- Minimum conductor ampacity = 28 × 1.25 = 35 A
- From Table 310.16, 8 AWG copper (40 A at 60 °C) is the minimum size
Branch circuit OCPD sizing (NEC 430.52)
The branch circuit overcurrent protective device protects the conductor from short circuit and ground fault — not from overload (overload is handled separately). Because the OCPD must not trip on motor starting inrush, Article 430.52 allows much higher ratings than for general branch circuits. The maximum ratings from Table 430.52 are:
- Non-time delay fuse: 300% of FLC
- Dual-element (time-delay) fuse: 175% of FLC
- Instantaneous trip breaker: 800% of FLC (for certain motor types)
- Inverse time breaker: 250% of FLC
These are maximums — you can always use a smaller rating if the motor starts reliably without tripping. If the maximum allowed rating does not prevent tripping on starting, the NEC permits increasing to the next standard size up.
For the same 10 HP, 230 V motor (FLC = 28 A), using an inverse time breaker:
- Maximum OCPD = 28 × 2.50 = 70 A
- Select the next standard breaker at or below 70 A = 70 A breaker
Overload protection (NEC 430.32)
Overload protection is the device that protects the motor windings from sustained overcurrent due to mechanical overload, stalled rotor, or single phasing. It is typically a thermal overload relay in the motor starter, not the branch circuit breaker.
The maximum overload device rating under 430.32(A)(1) is:
- 115% of the motor nameplate current for motors with a service factor of 1.15 or greater, or a temperature rise of 40 °C or less
- 125% of the motor nameplate current for all other motors
If this setting causes nuisance tripping (on a hard-starting load, for example), 430.32(C) allows increasing to a maximum of 140% or 130% respectively. This is the last resort — the first step is to verify that the motor starting conditions and voltage are correct.
Note that overload protection uses the nameplatecurrent, not the NEC table value. For the 10 HP motor with a nameplate of 27 A and service factor 1.15:
- Maximum overload setting = 27 × 1.15 = 31.1 A
Disconnecting means (NEC 430.102)
Every motor must have a disconnecting means in sight of the motor and its driven machinery — within 50 feet and visible from the motor. The disconnect must be rated at a minimum of 115% of the motor's full-load current:
- Minimum disconnect rating = 28 × 1.15 = 32.2 A
Select the next standard motor disconnect switch size above 32.2 A. Common sizes are 30, 60, 100, 200, 400 A. For this motor, a 60 A fusible disconnect is typical (the 30 A rating is insufficient).
Full worked example
A 5 HP, 460 V three-phase induction motor. Service factor 1.15, nameplate current 7.6 A. From Table 430.250, FLC at 460 V = 7.6 A.
- Branch circuit conductor: 7.6 × 1.25 = 9.5 A minimum → 14 AWG (15 A) copper
- Branch circuit OCPD (inverse time breaker): 7.6 × 2.50 = 19 A → 20 A breaker
- Overload relay (SF ≥ 1.15): 7.6 × 1.15 = 8.7 A setting
- Disconnecting means: 7.6 × 1.15 = 8.7 A → 30 A motor disconnect switch (minimum)
Variable-frequency drives
Where a VFD (variable-frequency drive) is installed, the drive effectively acts as a current-limiting device during starting — there is no inrush. However, the wiring from the panel to the drive input must still be sized for 125% of the drive's input current. Between the drive output and the motor, the conductors are sized to the motor's FLC at 125%, as usual. The drive typically incorporates its own overload protection for the motor.
Related calculators
Use the breaker size calculator to find the maximum OCPD for any motor under NEC 430.52. For conductor sizing, the wire size calculator gives NEC-compliant AWG sizing from any ampacity value. For three-phase motor current calculations, start with the formulas in our three-phase power guide.