A standard 1/2 HP single-phase AC induction motor draws 9.8 amps at 120V and 4.9 amps at 230V under full mechanical load. If you are running a 3-phase 1/2 HP motor at 230V, the current draw drops significantly to roughly 1.6 amps. These are the National Electrical Code (NEC) Table 430.248 Full-Load Current (FLC) values, which you must use for sizing wire and breakers, regardless of what the motor's specific nameplate says.
The Direct Answer: 1/2 HP Motor Amp Draw by Voltage
When planning a circuit, you cannot rely on a single universal number. The amp draw shifts dramatically based on the supply voltage and the phase configuration. Here is the baseline data for a standard 1/2 HP (373 Watts output) AC induction motor:
- 115V / 120V Single-Phase: 9.8 Amps (FLC)
- 230V / 240V Single-Phase: 4.9 Amps (FLC)
- 230V Three-Phase: 1.6 Amps (FLC)
- 460V Three-Phase: 0.8 Amps (FLC)
The Conversion Formula and the 'Hidden' Variables
To understand why a 1/2 HP motor draws 9.8 amps at 120V, we have to look at the electrical power formula. Horsepower converts to watts at a rate of 746 watts per HP. The theoretical formula for single-phase AC motor current is:
Amps = (HP × 746) / (Volts × Efficiency × Power Factor)
Let us substitute real-world values for a fractional horsepower motor. Small motors are inherently inefficient. We will assume 65% efficiency (0.65) and a power factor (PF) of 0.75, which is typical for a 1/2 HP capacitor-start motor:
Amps = (0.5 × 746) / (120 × 0.65 × 0.75)
Amps = 373 / 58.5 = 6.37 Amps
Wait—6.37A is much lower than the 9.8A NEC standard. Why the discrepancy? The theoretical formula assumes perfect operating conditions. The NEC tables bake in assumptions for voltage sag, aging insulation, and starting surges. Furthermore, if you do not know the exact power factor and efficiency of your specific motor, the theoretical conversion is meaningless. This is why electrical inspectors and engineers discard the formula in favor of the standardized NEC tables when sizing branch circuits.
For a deep dive on reading motor data plates and understanding these hidden variables, Fluke's guide to motor nameplate basics is an excellent reference for bridging the gap between theory and field measurements.
Neighboring Motor Sizes: Amp Draw Reference Table
If you are swapping out a motor or upsizing a circuit, you need to know the neighboring values. The table below shows the NEC Table 430.248 FLC values for single-phase motors in the ±20% range around the 1/2 HP mark.
| Motor Size (HP) | Watts Output | 115V Single-Phase (Amps) | 230V Single-Phase (Amps) |
|---|---|---|---|
| 1/3 HP | 248W | 7.2 A | 3.6 A |
| 1/2 HP | 373W | 9.8 A | 4.9 A |
| 3/4 HP | 559W | 13.8 A | 6.9 A |
| 1 HP | 746W | 16.0 A | 8.0 A |
Decision Tree: Sizing Wire and Breakers for Your 1/2 HP Motor
Knowing the amp draw is only half the battle. You need to terminate this into a concrete wire gauge and breaker size. The NEC has strict rules for motor circuits (Article 430) that differ from standard lighting or receptacle circuits. Follow this decision path to pick your parts:
| IF Your Supply Is... | THEN Base FLC Is... | Wire Size (125% of FLC) | Max Breaker (250% Inverse Time) | Concrete Pick (Buy This) |
|---|---|---|---|---|
| 120V Single-Phase | 9.8 Amps | 12.25A minimum | 24.5A max | 12 AWG THHN wire, 20A standard breaker |
| 230V Single-Phase | 4.9 Amps | 6.12A minimum | 12.25A max | 14 AWG THHN wire, 15A standard breaker |
| 230V Three-Phase | 1.6 Amps | 2.0A minimum | 4.0A max | 14 AWG THHN wire, 15A 3-pole breaker |
Why these concrete picks? For the 120V setup, 9.8A × 125% (NEC 430.22) equals 12.25A. While 14 AWG wire is technically rated for 15A, using NEC guidelines and practical voltage drop limits, 12 AWG is the correct choice to pair with a 20A breaker. The 20A breaker handles the locked-rotor starting surge without nuisance tripping, which a 15A breaker would likely fail to do on a 1/2 HP motor starting under load.
Frequently Asked Questions
What assumption fixes the amp draw answer?
The answer is fixed by three assumptions: voltage, phase, and motor type. The 9.8A figure assumes a standard 115V/120V single-phase AC induction motor. If you are using a brushed universal motor (like in a router or vacuum), this AC induction math is meaningless; you must rely solely on the manufacturer's nameplate, as universal motors have vastly different power factors and speed-torque curves.
How does the answer shift for 120V vs 230V vs 3-phase?
Doubling the voltage from 120V to 230V exactly halves the current draw (from 9.8A to 4.9A) because Power = Volts × Amps. Moving to 3-phase at 230V drops the current even further to 1.6A. This happens because 3-phase power delivers constant torque and utilizes the √3 (1.732) multiplier in the denominator of the power formula, drastically reducing the amperage required per leg.
Can I use a 15A breaker for a 120V 1/2 HP motor?
Technically, NEC 430.52 allows an inverse-time breaker up to 250% of the FLC (24.5A). While a 15A breaker exceeds the 9.8A running load, a 1/2 HP motor can draw 300% to 600% of its FLC for a few seconds during startup (Locked Rotor Amps). A 15A breaker will likely trip instantly upon startup if the motor is under any mechanical load. Stick to the 20A breaker and 12 AWG wire concrete pick for reliable operation.
Where can I find standard motor current tables?
The definitive source is the National Electrical Code (NFPA 70), specifically Table 430.248 for single-phase motors and Table 430.250 for three-phase motors. For quick field reference, the Engineering Toolbox single-phase motor charts provide excellent cross-references that align with NEC standards.
