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

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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
1½	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
7½	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	1½ 1
30	230 V 460 V	80 40	125 70	3 3	92 46	3 8	1½ 1
40	230 V 460 V	104 52	175 100	4 3	119.6 59.8	1 6	1½ 1
50	230 V 460 V	130 65	200 150	4 3	149.5 74.75	00 4	2 1½
60	230 V 460 V	154 77	250 200	5 4	177.1 88.55	000 3	2 1½
75	230 V 460 V	192 96	300 200	5 4	220.8 110.4	250kcmil 1	2½ 1½
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	3½ 2
150	230 V 460 V	360 180	600 300	6 5	414 207	700kcmil 0000	4 2½

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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