When you size a contactor, a motor starter, a breaker or the wires for a motor in North America, you need the motor's full-load current. Two sources give it: the motor nameplate, and the horsepower-to-full-load-amps (HP to FLA) tables found in the electrical code and in product standards such as UL 508. They are not used for the same purpose. This article explains how to read an HP to FLA table, which value to use for what, and how to avoid the most common sizing mistakes.
The HP to FLA table
The currents in this table are for standard three-phase, four-pole squirrel-cage induction motors (about 1,800 rpm at 60 Hz, 1,500 rpm at 50 Hz). They are values for guidance: the actual current of a given motor depends on its manufacturer, its design, its efficiency and its number of poles. Slower motors with more poles usually draw more current for the same horsepower.
How to read the table
- Find the motor horsepower in the first column. Use the nameplate HP, not the load you think the machine needs.
- Select the column for the motor voltage. North American tables are organized by motor nameplate voltage, which is slightly lower than the system voltage: for example, 200 V motors on 208 V systems, 230 V motors on 240 V systems, 460 V motors on 480 V systems and 575 V motors on 600 V systems.
- Read the current where the row and the column meet. This is the full-load current used for sizing.
- Check the motor type. These tables cover standard induction motors. Single-phase motors, DC motors, multispeed motors and special designs have their own tables or require the nameplate data.
Current is roughly inversely proportional to voltage: a motor reconnected from 230 V to 460 V draws about half the current at the higher voltage. That is why dual-voltage motors list two currents on the nameplate.
Nameplate FLA vs code table: which one to use
This is the point that causes the most confusion. In North American practice the two values have different jobs:
| Sizing task | Value normally used |
|---|---|
| Branch-circuit conductors (wire size) | Full-load current from the code table |
| Branch-circuit short-circuit and ground-fault protection (breaker or fuses) | Full-load current from the code table |
| Disconnecting means and controller ampere rating | Full-load current from the code table, together with the horsepower rating |
| Motor overload protection (overload relay setting or heater selection) | Nameplate full-load current (FLA) of the actual motor |
For guidance only. In the United States this approach is set out in NEC Article 430, which includes the full-load current tables for single-phase, three-phase and DC motors; the Canadian Electrical Code has equivalent rules and tables. Check the applicable electrical code and its current edition.
The reasoning is simple. The table values are deliberately conservative and do not depend on which motor is installed today, so the wiring and the short-circuit protection stay valid if the motor is later replaced by another one of the same horsepower. Overload protection, on the other hand, must follow the real motor closely, so it is set from the nameplate.
Nameplate FLA or code-table current: which value is used for what.
Setting the overload relay
Start from the nameplate FLA and follow the overload relay manufacturer's instructions. The code sets a maximum overload trip setting as a percentage of the nameplate current, and that percentage depends on the motor's service factor and temperature rise. In the NEC, the usual limit for a separate overload device is 125 % of nameplate FLA for motors with a service factor of 1.15 or more, or a marked temperature rise of 40 °C or less, and 115 % for other motors; check the current edition of the code. Many adjustable overload relays are simply dialed to the nameplate FLA because their trip point already includes a margin above the setting. See Overload Relays: Thermal vs Electronic and Trip Classes 10, 20, 30 for trip classes.
A quick check with the power formula
If you have no table at hand, the three-phase current can be estimated from the motor output power:
I = (HP x 746) / (1.732 x V x efficiency x power factor)
For example, a 10 HP motor at 460 V with an assumed efficiency of 0.90 and a power factor of 0.85 gives about 12.2 A. Code tables usually show somewhat higher values because they are conservative. Use the formula only as a sanity check, never in place of the code table or the nameplate.
Common mistakes
- Setting the overload relay from the code table instead of the motor nameplate.
- Sizing wires or breakers from a low nameplate current, then finding them undersized when the motor is replaced.
- Reading the 480 V column of a catalog when the table is organized by motor voltage (460 V), or the opposite.
- Forgetting that a contactor or starter must also be rated for the motor horsepower at the actual voltage, not only for its current.
- Using a four-pole table for a low-speed or special motor without checking its nameplate.
To choose the contactor for the motor, read Contactor Rating: IEC Utilization Categories. For motors rated in kilowatts, see the companion article on motor nominal current in kW according to IEC.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Key takeaways
- HP to FLA tables give typical full-load currents of standard four-pole three-phase motors.
- Tables are organized by motor voltage (200, 230, 460, 575 V), not system voltage.
- In North America, code-table currents size the conductors, short-circuit protection and disconnects; the nameplate FLA sets the overload protection.
- Check the applicable electrical code and the manufacturer's data before final sizing.
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