Sizing a Circuit Breaker for a Motor: 5HP 230V NEC Sizing Guide

For a standard 5 HP, 230V single-phase motor (28A Full Load Amps), use a 70A inverse-time circuit breaker and 10 AWG copper THHN wire (minimum). This specific combination handles the high locked-rotor inrush current without nuisance tripping while keeping conductors fully protected under standard 75°C terminal ratings.

The Baseline Pick: 5 HP, 230V Motor Sizing

Before pulling wire or buying breakers, we must establish the baseline assumptions. Sizing a circuit breaker for a motor is highly sensitive to ambient conditions and termination ratings. If your jobsite deviates from these parameters, the baseline pick changes.

The Math: Why a 70A Breaker and 10 AWG Wire?

The most common mistake DIYers make when sizing motor circuits is reading the nameplate Full Load Amps (FLA) and applying standard 125% branch circuit rules. NEC 430.6(A)(1) explicitly requires you to use the NEC tables (Table 430.248 for single-phase motors) for conductor and breaker sizing, not the nameplate, unless dealing with specific listed multispeed or part-winding motors.

Wire Sizing (NEC 430.22)

NEC Table 430.248 lists the FLA for a 5 HP, 230V single-phase motor as 28 Amps. (Your nameplate might say 24A, but we must use 28A for code compliance).

• Minimum Ampacity: 28A × 1.25 (125% rule) = 35 Amps.
• Table Lookup: Checking NEC Table 310.16 for Copper, THHN, in the 75°C column, 10 AWG is rated for exactly 35 Amps.

Therefore, 10 AWG copper is the absolute legal minimum. (Note: While THHN is rated 90°C, we must use the 75°C column because standard breakers and motor starter lugs are only rated for 75°C per NEC 110.14(C)).

Breaker Sizing (NEC 430.52)

Why use a 70A breaker on a wire rated for 35A? This violates standard Article 240 rules, but Article 430 allows it. Motors draw massive inrush current—often 6 to 8 times the FLA—when starting. This is called Locked Rotor Amps (LRA).

• Maximum Breaker Size: For an inverse-time breaker, NEC 430.52 allows up to 250% of the FLA.
• Calculation: 28A × 2.50 = 70 Amps.

Why this size and not one smaller? If you install a 40A breaker to match the wire closer, the magnetic trip mechanism will instantly latch during the motor's LRA inrush phase, causing nuisance tripping before the rotor even reaches operating speed. The 70A breaker allows the inrush to pass. The motor's internal thermal overload or the starter's overload relay (typically set to 115% of nameplate FLA) is what actually protects the motor windings from overheating.

Voltage Drop Verification at 75 Feet

While 10 AWG meets the minimum ampacity requirement, we must verify voltage drop. Motors are highly sensitive to low voltage; a 10% drop in voltage causes a 19% drop in starting torque, which can cause the motor to stall and burn out.

A 2.27% drop is well under the 3% maximum recommended by NEC 210.19(A) Informational Note. However, if your run exceeds 90 feet, the drop will cross the 3% threshold, and you must upsize to 8 AWG copper to maintain starting torque.

Decision Tree: When to Deviate from the Baseline

Use this decision path to finalize your exact material pick based on your specific jobsite conditions. Follow the logic down to terminate at your required wire and breaker.

What Changes the Answer: Edge Cases and Material Swaps

The baseline assumes a clean, simple installation. Here is what forces a deviation from the standard 10 AWG / 70A setup:

• Bundling and Derating: If you are pulling the motor feeders through a conduit that already contains other active circuits (4 to 6 current-carrying conductors total), NEC Table 310.15(C)(1) requires an 80% derating factor. Your 10 AWG wire (35A) derates to 28A, which is no longer sufficient for the 35A minimum requirement. You must jump to 8 AWG.
• Aluminum vs. Copper: Never present aluminum and copper interchangeably. Aluminum has higher resistance and expands/contracts more under thermal cycling. If you switch to aluminum to save money on a 150-foot well pump run, you cannot use 10 AWG (it doesn't exist in standard building wire). You must use a minimum of 6 AWG aluminum, and you must verify that both the breaker lugs and the motor starter lugs are explicitly rated for AL-CU connections. Apply an anti-oxidant compound (like Noalox) to prevent galvanic corrosion and high-resistance heating at the terminations.
• High Ambient Temperatures: If the conduit runs across a hot roof or through a boiler room where ambient temperatures exceed 30°C (86°F), you must apply the temperature correction factors from the bottom of NEC Table 310.16. At 40°C (104°F), the correction factor for 75°C insulation is 0.88. Your 10 AWG wire drops to 30.8A, forcing an upsize to 8 AWG.

When to Pull in an Engineer or the AHJ

While NEC Article 430 covers 95% of standard residential and light commercial motor installations, certain scenarios require stamped engineering drawings or explicit Authority Having Jurisdiction (AHJ) approval:

1. Variable Frequency Drives (VFDs): If you are feeding a motor through a VFD, the output side wiring rules change. VFDs can cause harmonic heating and require specific shielded VFD cables. The breaker sizing on the input side of the VFD is dictated by the VFD manufacturer's manual, not NEC 430.52.
2. Motors Over 50 HP: Large three-phase motors often require reduced-voltage starters (soft starters, wye-delta, or autotransformer). The inrush profiles are entirely different, and the utility company may require a formal inrush study to ensure you don't brown out the local grid.
3. Hazardous Locations: If the motor is in a Class I, Division 1 or 2 environment (e.g., a paint booth, grain elevator, or wastewater treatment plant), standard EMT and THHN are illegal. You will need rigid steel conduit, explosion-proof seals, and specific hazardous-location breakers.
4. Standard Size Limits: If your 250% calculation results in a number that does not match a standard breaker size listed in NEC 240.6 (e.g., you calculate 62A), you are permitted to round up to the next standard size (70A). However, if rounding up pushes you past the next standard size into a frame size that requires a panel upgrade, consult the AHJ to ensure the busbar rating isn't violated.

By anchoring your design to NEC Table 430.248 rather than the nameplate, respecting the 75°C termination limit, and verifying voltage drop at your specific distance, you eliminate the guesswork. For a standard 5 HP, 230V setup under 80 feet, pull your 10 AWG copper, terminate it on a 70A inverse-time breaker, and let the motor starter's overload relay do its job.