The chart below displays information for motor wire size, breaker size, thermal overload ratings, and other data for three-phase AC motor circuit protection.

Full-Load Current and Circuit Protection Data for Three-Phase AC Motors

Motor Horsepower Motor Amperes Breaker Size Starter Size Heater Amperes Wire Size Conduit Size
½
230 V
460 V
2.2
1.1
15
15
00
00
2.53
1.265
12
12
¾
¾
¾
230 V
460 V
3.2
1.6
15
15
00
00
3.68
1.84
12
12
¾
¾
1
230 V
460 V
4.2
2.1
15
15
00
00
4.83
2.415
12
12
¾
¾

230 V
460 V
6
3
15
15
00
00
6.9
3.45
12
12
¾
¾
2
230 V
460 V
6.8
3.4
15
15
0
00
7.82
3.91
12
12
¾
¾
3
230 V
460 V
9.6
4.8
20
15
0
0
11.04
5.52
12
12
¾
¾
5
230 V
460 V
15.2
7.6
30
15
1
0
17.48
8.74
12
12
¾
¾

230 V
460 V
22
11
45
20
1
1
25.3
12.65
10
12
¾
¾
10
230 V
460 V
28
14
60
30
2
1
32.2
16.1
10
12
¾
¾
15
230 V
460 V
42
21
70
40
2
2
48.3
24.15
6
10
1
¾
20
230 V
460 V
54
27
100
50
3
2
62.1
31.05
4
10
1
¾
25
230 V
460 V
68
34
100
50
3
2
78.2
39.1
4
8

1
30
230 V
460 V
80
40
125
70
3
3
92
46
3
8

1
40
230 V
460 V
104
52
175
100
4
3
119.6
59.8
1
6

1
50
230 V
460 V
130
65
200
150
4
3
149.5
74.75
00
4
2
60
230 V
460 V
154
77
250
200
5
4
177.1
88.55
000
3
2
75
230 V
460 V
192
96
300
200
5
4
220.8
110.4
250kcmil
1

100
230 V
460 V
248
124
400
200
5
4
285.2
142.6
350kcmil
2/0
3
2
125
230 V
460 V
312
156
500
250
6
5
358.8
179.4
600kcmil
000

2
150
230 V
460 V
360
180
600
300
6
5
414
207
700kcmil
0000
4

Note: The wire size in this table is based on 75°C terminations and insulation. See Maximum Allowable Ampacities for Conductors in Raceway, Cable or Earth (30°C) for other insulation ratings.

Motor Wire Sizing

The NEC requires circuits supplying single motors to have an ampacity rating of greater than or equal to 125% of the full-load current rating of the motor. For circuits with multiple motors, the ampacity rating of the wire must be at least 125% of the full-load current of the largest motor, plus the sum of full-load currents for the rest of the motors. For example, if there are three 15-amp motors in the circuit, the ampacity rating of the wire feeding the circuit must exceed 15 + 15 + (15 * 1.25) = 48.75 Amps. Some exception occur to this rule, such as if the motors are interlocked and cannot operate at the same time. Typically, the nominal system voltage for a motor will be above the nameplate voltage to compensate for any voltage drop in the circuit.

Motor Nameplate Voltages versus Nominal System Voltages
Motor Nameplate Voltage
Nominal System Voltage
115 120
230 240
460 480
575 600
4,000 4,160
6,600 6,900
13,200 13,800

Motor Protection

There are typically two different sets of protections to design for when designing motor circuits. The first is to protect the cable supplying the motor, the power source, and any components upstream on the circuit from the motor. This is called circuit protection, or feeder protection. The second is to protect the motor windings due to thermal overloads from excess current. This is called motor winding protection. These two sets of protections must be coordinated with each other to properly protect your motor and motor circuits.

Breaker size must be rated for a minimum of 125% of the full-load current of the motor it is protecting, rounded up to the next standard breaker size. For example, the full-load current of a 5 HP motor at 460V is 7.6 Amps. The breaker must be sized for 7.6 * 1.25 = 10 Amps. Therefore, it is good practice to size the breaker at 15 Amps.

A heater is a device that protects a motor from thermal overloads due to excess current in the windings. According to the NEC, the overload rating must not exceed 115% of the full-load current of the motor. For example, the full-load current of a 10 HP motor at 230V is 28 Amps. Therefore, the heater must be sized for 28 * 1.15 = 32.2 Amps.

Conduit size in this table is conservatively based on rigid metal conduit with some spare capacity. Always abide by NEC when sizing conduit.


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