Motor calculations for the journeyman exam

For sizing motor conductors, switches and short-circuit protection, the code wants a standard current for the motor's horsepower and voltage, not the nameplate amps.

Where to look
430.6(A)(1) says to use the table values instead of the nameplate. The 2026 edition puts the tables under new sections 430.247 to 430.250 at the end of Article 430: Table 430.248 for single-phase AC and Table 430.250 for 3-phase AC. Columns are rated motor voltages; the 230-V column is used for 220–240-V systems and the 460-V column for 440–480 V. Use the induction-type columns of Table 430.250 unless told the motor is synchronous.

Procedure

1. Phase? Single-phase → Table 430.248; 3-phase → Table 430.250.
2. Find the horsepower row (convert kW to hp at 746 W/hp if needed).
3. Read the column for the voltage (208 V has its own column).

Worked example
A 20-hp, 3-phase motor on a 480-V system.

• Table 430.250, 20 hp, 460-V column: 27 A.

Most common mistake
Using the nameplate current, or calculating amps from hp, efficiency and power factor. Nameplate current is for overload sizing only. Reading the 230-V column for a 208-V motor is the other common slip.

A single continuous-duty motor's branch-circuit conductors must carry 125 percent of the motor's full-load current.

Where to look
Article 430 Part II, 430.22. The current comes from the tables, not the nameplate (430.6(A)(1)): Table 430.248 for single-phase, Table 430.250 for 3-phase. Annex D Example D8 works the same steps.

Procedure

1. Find the table FLC for the horsepower and voltage (208 V has its own column; 480 V uses the 460-V column).
2. Multiply by 1.25.
3. Go down the terminal temperature column of Table 310.16 to the first ampacity at least that large.

Worked example
A 50-hp, 3-phase, 460-V motor, copper THWN, 75°C terminals.

• Table 430.250: 65 A.
• 65 × 1.25 = 81.25 A.
• 75°C copper: 6 AWG is 65 A (too small), 4 AWG is 85 A. Answer: 4 AWG.

Most common mistake
Using the nameplate current, or forgetting the 125 percent and stopping at the FLC. Nameplate amps are for overload relays only. Another slip is reading the 230-V column for a 208-V motor.

The motor's branch breaker or fuse only has to clear short circuits and ground faults (overloads are handled separately), so it may be rated well above the motor current.

Where to look
Article 430 Part IV, 430.52 and Table 430.52(C)(1). Read the row for the motor type and the column for the device. For ordinary squirrel-cage and single-phase motors: nontime-delay fuse 300 percent, dual-element (time-delay) fuse 175 percent, instantaneous-trip breaker 800 percent, inverse time breaker 250 percent. Design B energy-efficient motors allow 1100 percent instantaneous; wound-rotor motors are 150 percent for fuses and inverse time breakers. Always use the table FLC.
430.52(C)(1)(a) permits the next higher standard rating when the percentage does not land on one. 430.52(C)(1)(b) allows higher values only if the motor will not start.

Procedure

1. Table FLC.
2. Multiply by the percentage for that motor and device.
3. If allowed to apply (C)(1)(a) and the result is not standard, go to the next higher standard size.

Worked example
20-hp, 460-V, 3-phase squirrel-cage motor, inverse time breaker.

• FLC 27 A × 2.5 = 67.5 A.
• Not standard; apply (C)(1)(a) → 70 A.

Most common mistake
Reading the wrong column, or using the nameplate current.

Overloads protect the motor from running too hot under too much mechanical load. Unlike conductors and short-circuit devices, they are set from the motor's nameplate current.

Where to look
Article 430 Part III, 430.32(A)(1) for continuous-duty motors over 1 hp: 125 percent of nameplate full-load current for motors with a marked service factor of 1.15 or more, or a marked temperature rise of 40°C or less; 115 percent for all other motors. 430.6(A)(2) confirms overloads use the nameplate. 430.32(C) permits going up to 140 percent (or 130 percent for "all other") only if the first setting will not start the motor or carry the load; use it only when the question says so.

Procedure

1. Read the nameplate full-load current (not the table value).
2. Check service factor and temperature rise.
3. Multiply by 1.25 or 1.15.
4. That is the maximum trip rating or setting.

Worked example
Nameplate 32 A, service factor 1.15.

• Service factor 1.15 or more: 125 percent.
• 32 × 1.25 = 40 A maximum.

Most common mistake
Using the Table 430.250 current instead of the nameplate, or using 125 percent for a motor with a 1.0 service factor and no temperature-rise marking.