Three-Phase Motor Starter and Overload Sizing per NEC 430

Why this matters

Article 430 of the NEC is its own dialect of code, with its own ampacity tables, its own multiplier system, and its own conductor sizing rules that ignore the ones in Article 240 most electricians use every day. A motor circuit sized by the conductor tables alone will burn through fuses, trip the starter overload on every startup, and on a long enough run starve the motor enough to fail an insulation-life test years early. A correctly sized three-phase motor circuit follows a deterministic sequence: pick conductor from FLC, pick branch-circuit protection from FLC and a multiplier, pick starter contactor by horsepower or FLA, set overload to a percentage of nameplate. This reference walks the sequence, the most-used multipliers, and the application notes that separate a clean motor install from a string of nuisance trips.

The starting point: FLC vs. nameplate FLA

NEC 430.6 makes a deliberate distinction:

• FLC (Full-Load Current): the table value from NEC Table 430.250 (three-phase) or 430.248 (single-phase). FLC is used to size conductors and short-circuit/ground-fault protection.
• FLA (Full-Load Amps): the actual nameplate value of the specific motor. FLA is used to size the overload device only.

The FLC values in Table 430.250 are deliberately a little higher than typical nameplate FLA. Using the table value for conductor sizing produces conductors that are sized for any commercially available motor of that horsepower, not just the one currently installed. This means a future motor replacement does not require a conductor change.

A 10 HP, 460V three-phase motor has FLC = 14 A per Table 430.250. The specific motor on the truck might be nameplate FLA = 13.0 A. Use 14 A for conductor and breaker sizing; use 13.0 A for overload sizing.

Step-by-step sizing sequence

Step 1: Conductor sizing per NEC 430.22

Branch-circuit conductors for a single motor are sized at 125% of FLC (NEC 430.22(A)). Example: a 10 HP, 460V, three-phase motor with FLC = 14 A.

• 14 A × 125% = 17.5 A required ampacity
• Per NEC Table 310.16 (75°C column for THHN/THWN-2): 14 AWG copper THHN is rated 20 A at 75°C, but NEC 240.4(D) limits 14 AWG to 15 A overcurrent protection in most general-use cases. The 14 AWG would be acceptable for the motor branch under the motor rules, but practice is to use 12 AWG to align with the protective device.
• Recheck after rounding: 12 AWG copper THHN, 75°C rating 25 A, exceeds 17.5 A. Use 12 AWG.

For multiple motors on a single branch (rare but allowed by NEC 430.24), the conductor must carry 125% of the largest motor's FLC plus 100% of all other motors' FLC.

Step 2: Branch-circuit short-circuit and ground-fault protection per NEC 430.52

The motor branch-circuit protective device (fuse or circuit breaker) is sized by FLC times a multiplier from NEC Table 430.52. The multiplier depends on the device type and the motor design code:

For our 10 HP, 460V motor (FLC = 14 A) with a thermal-magnetic circuit breaker (inverse-time, the standard catalog breaker for most motor installs):

• 14 A × 250% = 35 A
• Next standard breaker size: 35 A. Per NEC 240.6, 35 A is a standard rating. Use 35 A.

If 35 A trips on startup (a normal occurrence with some loaded motors), NEC 430.52(C)(1) Exception 2(b) allows stepping up to the next standard size without violating code, up to specific limits. For an inverse-time breaker, the upper limit is 400% of FLC. For our motor: 14 × 400% = 56 A, rounded down to next standard at or below = 50 A.

Step 3: Disconnect sizing per NEC 430.110

The motor disconnect must have an ampacity at least 115% of the motor FLC, with the appropriate horsepower rating. For our 10 HP motor:

• 14 A × 115% = 16.1 A
• Practical disconnect: a 30 A HP-rated safety switch with 10 HP rating at 460V is the standard catalog item

Step 4: Starter contactor sizing per NEMA or IEC ratings

Contactors are sized by horsepower for NEMA-rated devices or by AC-3 amp rating for IEC-rated devices:

• NEMA Size 1: typical 10 HP at 460V three-phase (verify manufacturer's specific rating chart)
• NEMA Size 2: typical 25 HP at 460V three-phase
• IEC equivalents: ABB AF-series, Siemens 3RT, Schneider LC1-D series; select by AC-3 rating that exceeds the motor's continuous current with appropriate utilization category

NEMA contactors are larger, more conservatively rated, and more tolerant of poor maintenance and dirty service entrances. IEC contactors are smaller, more cost-effective, and require accurate sizing because they have less inherent margin. Both are code-compliant; pick by the service environment and the customer's installed-base preference.

Step 5: Overload protection sizing per NEC 430.32

Overload is the device that protects the motor itself from extended overcurrent, distinct from the branch-circuit protection that handles short circuits. NEC 430.32(A) sets the maximum setting:

• Continuous-duty motor, service factor 1.15 or greater: 125% of nameplate FLA
• Continuous-duty motor, service factor less than 1.15 OR temperature rise marked at or below 40°C: 115% of nameplate FLA
• All other motors: 115% of nameplate FLA

For our 10 HP motor with nameplate FLA = 13.0 A and a service factor of 1.15 marked on the data plate:

• 13.0 A × 125% = 16.25 A maximum overload setting
• Select an overload relay with a range that includes 16.25 A: most thermal overload relays in the 9 to 18 A range cover this; set the dial to 16 A or the closest available value at or below 16.25 A

If the motor will not start with the overload set to the maximum allowed value (typical with high-inertia loads like fans or unloaded pumps), NEC 430.32(C) allows increasing the setting to a higher value up to specific upper limits:

• Service factor 1.15 or greater: maximum 140% of FLA
• Less than 1.15: maximum 130% of FLA

For our motor: 13.0 × 140% = 18.2 A maximum after step-up. The next dial position at or below 18.2 A is set.

Combination starter (motor controller) packages

References

• 2023 National Electrical Code, Article 430 (Motors, Motor Circuits, and Controllers)
• NEC Section 430.6 (FLC vs. nameplate FLA), 430.22 (conductor sizing), 430.32 (overload setting), 430.52 (branch-circuit protection), 430.110 (disconnect rating)
• NEC Table 430.250 (three-phase motor FLC) and Table 430.248 (single-phase motor FLC)
• NEMA ICS 2, Industrial Control and Systems Controllers, Contactors, and Overload Relays
• IEC 60947-4-1, Low-Voltage Switchgear and Controlgear, Electromechanical Contactors and Motor-Starters
• NEMA MG 1, Motors and Generators (motor design and service factor)
• IEEE Std 141 (Red Book), Recommended Practice for Electric Power Distribution for Industrial Plants
• UL 508A, Industrial Control Panels (SCCR determination)