Motor Wire Size Guide: NEC 430 Requirements

Understanding NEC 430.22 — Motor Conductor Sizing

NEC Article 430 covers motors, motor circuits, and controllers. Section 430.22 (Sizing of Motor Branch-Circuit Conductors) is one of the most frequently referenced sections for electricians and engineers working with motor installations.

The core requirement is straightforward: motor branch-circuit conductors must have an ampacity of not less than 125% of the motor full-load current (FLC) rating as given in NEC Tables 430.248 (single-phase) and 430.250 (three-phase).

This 125% requirement exists because motors are continuous loads that can operate at full rated current for extended periods. The extra 25% provides a safety margin for:

• Voltage fluctuations and power quality issues
• Ambient temperature variations affecting wire ampacity
• Harmonic currents in VFD-driven motor circuits
• Starting surges and intermittent overload conditions
• Conductor aging and insulation degradation over time

The Motor Wire Sizing Formula

Example: A 3HP, 230V single-phase motor has a NEC FLC of 17A (from Table 430.248).

• Minimum ampacity = 17A × 1.25 = 21.25A
• Using 75°C copper THHN: #12 AWG = 25A ✓ (sufficient)
• Using 60°C copper TW: #10 AWG = 30A ✓ (sufficient, #12=20A insufficient)

Motor Wire Size Table — Single-Phase (230V)

This table provides recommended copper wire sizes for common single-phase motors at 230V. Wire sizes are calculated using 125% of FLC and selected from NEC Table 310.16 using the 75°C column:

Breaker values are maximum per NEC 430.52 for time-delay fuses (175% FLC). Verify with actual breaker trip curves. Wire sizes assume copper conductors in conduit at 30°C ambient.

Motor Wire Size Table — Three-Phase (460V)

Three-phase motors at 460V draw less current, allowing smaller wire sizes. This table covers common commercial and industrial motor sizes:

NEC FLC values from Table 430.250. Breaker values are maximum time-delay fuse ratings per NEC 430.52. Always verify with local codes.

Step-by-Step Motor Wire Sizing Procedure

Follow this systematic approach to correctly size motor branch-circuit conductors:

Voltage Drop Considerations for Motor Circuits

While NEC does not mandate maximum voltage drop, it is critical for motor performance. Voltage drop causes motors to draw more current, run hotter, and lose efficiency.

Impact of voltage drop on motors:

Conduit Fill for Motor Circuits

Motor circuits typically have 3 or 4 conductors (2 or 3 phase conductors plus equipment grounding conductor) in a conduit. Conduit fill requirements per NEC Chapter 9, Table 1:

• 1 conductor: 53% of conduit area
• 2 conductors: 31% of conduit area
• 3+ conductors: 40% of conduit area

For motor circuits with VFD (variable frequency drive) connections, you may need:

• Shielded cables or VFD-rated cable (typically larger OD)
• Separate conduits for power and control wiring
• Reduced conduit fill to allow for heat dissipation
• EMI/RFI filtering considerations

Common Motor Wire Sizing Mistakes

Avoid these frequently encountered errors when sizing motor branch-circuit conductors:

• Using nameplate FLA instead of NEC FLC — NEC 430.22 requires using the FLC from NEC tables (430.248, 430.250), not the motor nameplate. The nameplate value is used for overload protection only.
• Forgetting the 125% multiplier — This is not optional. Every motor conductor must be at minimum 125% of FLC.
• Using the wrong temperature column — Use the 75°C column unless equipment is rated for 90°C. Mixing temperature ratings creates a safety hazard.
• Ignoring ambient temperature correction — In hot environments (attics, rooftops, near furnaces), apply NEC 310.15(B)(1) correction factors which reduce wire ampacity.
• Not accounting for voltage drop on long runs — NEC minimum wire sizes don't account for voltage drop on runs exceeding 100 feet.
• Using NM-B (Romex) for motors — While technically permitted in some residential applications, NM cable has temperature limitations and is not ideal for motor circuits with high inrush currents.
• Sizing ground wire by FLC — The equipment grounding conductor is sized by the overcurrent protection device (breaker/fuse) rating per NEC Table 250.122, not the motor FLC.