I'm wiring a 10 HP 230V single-phase motor for a shop compressor.

Conductor sizing (NEC 2023):

• Motor nameplate: 42 FLA.
• Minimum conductor ampacity = 42 × 1.25 = 52.5 A (NEC 210.19(A)(1)).
• 6 AWG copper (60°C column, 55–75°C terminations) = 65 A ✅.
• 4 AWG aluminum = 55 A ✅.

Overcurrent device (NEC 210.20(A)):

• Max OCPD = 42 × 250% = 105 A.
• Standard 2-pole breaker: 100 A ✅.

Why they don't match:

• Conductors are sized at 125% of FLA to prevent insulation damage from continuous current.
• Breakers are allowed at 250% of FLA because motors draw high inrush current (6–8× FLA) at startup. A 100 A breaker won't nuisance-trip during startup, but 6 AWG conductors still carry the full 42 A safely.

Summary:

• Conductors: 6 AWG copper or 4 AWG aluminum.
• Breaker: 100 A 2-pole.
• Disconnect: Within sight of motor (NEC 210.7).

Conductors: 6 AWG THHN (65 A ampacity). Breaker: 100 A inverse-time (thermal-magnetic).

Why they don’t match Motors draw 6–8× FLA at startup. If the breaker were sized to the conductor ampacity (65 A), it would nuisance-trip on every start. The NEC permits the OCPD to be oversized only for motor-starting inrush; the conductors are still protected against overload by the motor’s own overload device (starter heater or electronic overload).

Step-by-step (2023 NEC)

1. FLA basis – Use nameplate 42 A (not Table 430.248).
2. Conductor ampacity – 430.22(A): 125 % × 42 A = 52.5 A. Table 310.16 (75 °C column): 6 AWG CU = 65 A → OK.
3. Short-circuit/ground-fault OCPD max – 430.52(C)(1) Ex. 2(a): 250 % × 42 A = 105 A. Next standard size down (240.6) = 100 A.
4. Overload protection – Separate device at 115–125 % FLA (430.32).
5. Disconnect – 430.110: ≥ 115 % FLA = 48.3 A → 60 A motor-rated switch.
6. Equipment grounding conductor – 250.122: 100 A OCPD → 8 AWG CU.

Practical notes

• Torque lugs to manufacturer spec; use 75 °C rated terminals.
• If run exceeds 100 ft, upsize conductors for voltage drop (3 % max recommended).
• Verify starter overload dial is set to 42 A.
• All work per NEC 110.14(C), 300.4, 430.6, 430.22, 430.52, 250.122.