A motor circuit has three separate sizing jobs, and each one uses a different current. The conductor and the short-circuit device are sized from the full-load current in a code table. The overload protection is sized from the nameplate. Mixing these up is the most common error, so this calculator keeps them apart.
The three steps
Overload setting = 1.25 × nameplate current (1.15 × if the motor does not qualify)
Short-circuit device ≤ table percentage × table full-load current
Full-load current comes from Table 430.250 for three-phase motors and Table 430.248 for single-phase motors, by horsepower and voltage. The code directs you to use these tabulated values for conductor and short-circuit protection sizing, not the figure on the nameplate. The tables list the voltages 208, 230, 460 and 575 V for three-phase and 115 and 230 V for single-phase, which this calculator offers.
Conductors carry at least 125 % of the table current (430.22) for a single motor with continuous duty.
Overload protection uses the nameplate current: 125 % for a motor with a service factor of 1.15 or more or a marked temperature rise of 40 °C or less, and 115 % for other motors (430.32). The maximum setting is 140 % and 130 % respectively. If the motor still trips at those, check the load before raising anything.
The short-circuit and ground-fault device (430.52) has a maximum, which depends on its type: 250 % for an inverse-time circuit breaker, 175 % for a dual-element fuse, 300 % for a non-time-delay fuse and 800 % for an instantaneous-trip breaker. If the result is not a standard size, the next higher standard size is permitted.
Worked example
A 10 hp, 460 V, three-phase motor with a nameplate current of 13 A:
- Table current: 14 A.
- Conductors: 1.25 × 14 = 17.5 A or more.
- Overload: 1.25 × 13 = 16.25 A, with a maximum of 18.2 A.
- Inverse-time breaker: 250 % × 14 = 35 A, which is a standard size.
- Dual-element fuse: 175 % × 14 = 24.5 A, so 25 A.
- Non-time-delay fuse: 300 % × 14 = 42 A, so 45 A.
A 5 hp, 230 V motor has a table current of 15.2 A, so the conductor needs 19 A and the breaker maximum of 38 A rounds up to a 40 A standard size.
What this does not cover
- Several motors on one circuit, or a motor with other loads, which follow 430.24 and 430.53.
- Variable-speed drives and soft starters, which have their own requirements and manufacturer instructions.
- Motor-type rows in Table 430.52. The percentages here are the general case. The code table has other rows for particular motor types, for example a higher instantaneous-trip percentage for some energy-efficient Design B motors and lower percentages for wound-rotor and direct-current motors. Check the row for your motor.
- Higher device ratings that the code allows when the first choice cannot carry the motor’s starting current.
- Disconnecting means, controllers and the conductors from the controller to the motor, each with its own rule.
- Voltage drop, which matters for starting. Use the voltage drop calculator.
- Conductor size itself. The calculator gives the ampacity needed. Use the wire size calculator to choose the conductor, with derating.
Common mistakes
- Sizing the conductor from the nameplate. Use the table value.
- Setting the overload from the table. Use the nameplate.
- Reading 250 % as a target. It is a ceiling. A smaller device that still starts the motor is better.
- Using a table voltage the motor is not on. A 480 V supply is usually served by the 460 V column.
Questions
What is the full-load current of a 5 hp motor?
It depends on voltage and phase: 7.6 A at 460 V three-phase, 15.2 A at 230 V three-phase and 28 A at 230 V single-phase.
Why does the code use a table and not the nameplate?
Motors with the same horsepower can have different nameplate currents. The table gives a consistent basis for sizing circuits, while the overload device protects the actual motor.
What size breaker for a 3 hp motor?
At 230 V three-phase the table current is 9.6 A, so the inverse-time breaker maximum is 250 %, or 24 A, which is not a standard size. The next higher standard size is 25 A.
Does a motor need a separate overload device?
Generally yes. The short-circuit device protects the circuit and the overload device protects the motor.