To size a branch-circuit breaker for a standard AC motor, multiply the motor's Full-Load Amps (FLA) by 250% for a standard inverse-time thermal-magnetic breaker, per NEC Article 430.52. For example, a 230V single-phase motor with a nameplate FLA of 10A requires a maximum breaker size of 25A (10A × 2.5). If the exact calculation does not match a standard breaker size listed in NEC 240.6, you are permitted to round up to the next standard size.
The NEC-Compliant Motor Circuit Breaker Sizing Chart
This reference table is based on NEC Article 430.52 (Maximum Rating or Setting of Motor Branch-Circuit Short-Circuit and Ground-Fault Protective Devices) and uses baseline FLA values from NEC Table 430.248 for single-phase AC motors.
How to read this table: The 'FLA' column represents the NEC baseline full-load current, which is often slightly higher than the motor's actual physical nameplate to ensure conservative safety margins. The 'Wire Ampacity' column applies the NEC 430.22 rule (125% of FLA) for sizing the branch circuit conductors. The 'Max Breaker' column applies the 250% multiplier for inverse-time breakers, and the final column shows the actual standard breaker size you will purchase at the supply house.
| Motor HP | 230V 1-Phase FLA (NEC 430.248) | Min Wire Ampacity (125% FLA) | Max Breaker Calc (250% FLA) | Standard Breaker Size (NEC 240.6) |
|---|---|---|---|---|
| 1/2 HP | 4.9A | 6.1A | 12.25A | 15A |
| 3/4 HP | 6.9A | 8.6A | 17.25A | 20A |
| 1 HP | 8.0A | 10.0A | 20.0A | 20A |
| 1.5 HP | 10.0A | 12.5A | 25.0A | 25A |
| 2 HP | 12.0A | 15.0A | 30.0A | 30A |
| 3 HP | 17.0A | 21.25A | 42.5A | 45A |
| 5 HP | 28.0A | 35.0A | 70.0A | 70A |
| 7.5 HP | 40.0A | 50.0A | 100.0A | 100A |
| 10 HP | 50.0A | 62.5A | 125.0A | 125A |
Source: NFPA 70 (National Electrical Code), Articles 430.22, 430.52, and 240.6. For a deeper dive into motor circuit calculations, refer to EC&M's motor circuit guide.
Applying Derating and Installation Variables
A common point of failure on the bench or jobsite is misunderstanding which ampacity column applies to your specific installation and how environmental derating modifies the base values.
Which Temperature Column Applies?
When selecting your wire gauge from NEC Table 310.16, you must match the lowest temperature rating of any connected termination. Most modern Square D, Eaton, and Siemens breakers feature 75°C rated lugs. However, many fractional-horsepower motor terminal blocks, older disconnect switches, and budget contactors are only rated for 60°C. Per NEC 110.14(C), if the motor termination is 60°C, you must pull your wire ampacity from the 60°C column, even if the breaker and the wire insulation (like THHN) are rated for 75°C or 90°C. Using the 90°C column for a 60°C motor lug is a frequent code violation that risks melting the terminal block under continuous load.
How Derating Modifies the Base Value
Derating factors for ambient temperature and conduit fill (NEC 310.15) apply strictly to the conductor ampacity, not the breaker trip setting.
- The Breaker: Remains sized at 250% of the motor FLA to handle the massive inrush current during startup without nuisance tripping.
- The Wire: Must be sized at 125% of FLA, and then upsized if derating applies. For example, if you are pulling wire through an attic with an ambient temperature of 110°F (43°C), you must multiply the base ampacity by 0.82. If your 125% calculation calls for 20A wire, the derated wire must still safely carry 20A, meaning you must install wire with a base ampacity of at least 24.4A (20 / 0.82), pushing you from 12 AWG up to 10 AWG copper.
What This Chart Cannot Tell You
While the motor circuit breaker sizing chart is essential for preventing short circuits and ground faults, it does not cover the complete motor protection scheme. Relying solely on the branch-circuit breaker will result in burned-out motor windings.
- Motor Overload Protection: The branch breaker is too large to protect the motor from a mechanical jam or slow overloading. A 3 HP motor drawing 17A is protected by a 45A breaker. If the motor seizes and draws 35A continuously, the 45A breaker will not trip, and the motor will catch fire. You must install a separate motor overload relay (or rely on the internal thermal overload button on fractional HP motors) sized precisely to the motor's actual nameplate FLA, per NEC 430.32.
- Locked Rotor Current (LRC): The chart accounts for standard inverse-time magnetic trip thresholds, but it does not tell you the exact starting inrush. According to Fluke's motor testing guidelines, inrush can be 6 to 10 times the FLA for a fraction of a second. If your breaker still nuisance-trips on startup despite being sized to 250%, you may need to switch to a Motor Circuit Protector (MCP) or a D-curve breaker.
- Voltage Drop: If your motor is located 150 feet from the subpanel, the wire size dictated by the 125% rule may result in a voltage drop exceeding the recommended 3% at full load. You must calculate voltage drop separately and upsize the conductors accordingly, which may also require upsizing the equipment grounding conductor per NEC 250.122.
Frequently Asked Questions
How do I size a breaker for a motor with a missing or faded nameplate?
If the physical nameplate FLA is illegible, you are legally permitted by the NEC to use the baseline values provided in Tables 430.248 (single-phase) or 430.250 (three-phase). Find the motor's horsepower and voltage rating on the casing, locate the corresponding row in the NEC table, and apply the 250% multiplier. Never guess the FLA based on the physical size of the motor, as efficiency variations between older and modern motors can drastically alter the actual current draw.
Why does my standard breaker trip instantly on startup, and should I use a D-curve breaker?
Standard thermal-magnetic breakers (B or C curve) have a magnetic instantaneous trip threshold that can be triggered by the extreme inrush current of a high-inertia motor starting under load. If your correctly sized 250% breaker trips instantly upon startup (not after a few seconds of thermal heating, but instantly with a loud snap), the magnetic core is saturating. The fix is to install a breaker with a higher magnetic trip threshold, such as a D-curve breaker or a dedicated Motor Circuit Protector (MCP), which tolerates brief inrush spikes up to 15-20 times the rated current without tripping.
Does this chart account for voltage drop on long wire runs to the motor?
No. NEC Article 430 dictates minimum safety thresholds for heat and short-circuit protection, but it does not enforce strict voltage drop limits for branch circuits (it only provides informational recommendations in Fine Print Notes). If you are running a 5 HP motor 200 feet away from the panel, the 8 AWG wire required by the ampacity chart will suffer severe voltage drop, causing the motor to draw higher amps to compensate, overheat, and eventually fail. Always run a separate voltage drop calculation for runs exceeding 75 feet and upsize your wire gauge to maintain a maximum 3% drop at full load.
