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Thermal Unit Selection

 

 

Approximate Thermal Unit Selection Based On Horsepower and Voltage

General—Thermal units selected using approximate full load currents from the table below will provide a trip current between 101% and 125% of full load current for many 4-pole, single speed, normal torque, 60 Hz motors. Since full load current rating of different makes and types of motors vary so widely, these selections may not be suitable.

Thermal units should be selected on the basis of motor nameplate full load current and service factor. Thermal unit sizes originally selected on an approximate basis should always be rechecked and corrected at the time of installation if required.

Instructions:

1.	Locate motor horsepower and voltage.

2.	Determine approximate full load current from the table below.

3.	Use approximate full load current in place of actual nameplate full load current and follow instructions on page 16-129.

Only Use This Table When Motor Full Load Current Is Not Known

 

Table 16.385:

Motor Horsepower	Motor Full Load Current
	Three Ø				Single Ø
	200 V	230 V	460 V	575 V	115 V	230 V
1/20	0.39	0.34	0.17	0.14	1.30	0.65
1/12	0.55	0.48	0.24	0.19	1.90	0.95
1/8	0.74	0.64	0.32	0.26	2.60	1.30
1/6	0.90	0.78	0.39	0.31	3.24	1.62
1/4	1.22	1.06	0.53	0.42	4.40	2.20
1/3	1.52	1.32	0.66	0.53	5.47	2.74
1/2	2.07	1.80	0.90	0.72	7.45	3.73
3/4	2.88	2.50	1.25	1.00	10.1	5.07
1	3.68	3.20	1.60	1.28	12.6	6.31
1-1/2	5.18	4.50	2.25	1.80	17.2	8.59
2	6.67	5.80	2.90	2.32	21.4	10.7
3	9.66	8.40	4.20	3.36	29.1	14.5
5	15.4	13.4	6.68	5.35	42.9	21.4
7-1/2	22.6	19.6	9.82	7.86	58	29.2
10	29.7	25.8	12.9	10.3	—	36.3
15	43.6	38.0	19.0	15.2	—	49.9
20	57.4	49.9	24.9	20.0	—	—
25	70.9	61.7	30.8	24.7	—	—
30	84.3	73.3	36.7	29.3	—	—
40	111.	96.4	48.2	38.5	—	—
50	137.	119.	59.6	47.6	—	—
60	163.	142.	70.8	56.6	—	—
75	201.	175.	87.6	70.0	—	—
100	265.	230.	115.	92.0	—	—
125	327.	284.	142.	114.	—	—
150	389.	338.	169.	135.	—	—
200	511.	445.	222.	178.	—	—

These currents should not be used for selection of fuses, circuit breakers or wire sizes. See NEC tables 430-148 through 430-150. For motors rated 208-220 volts, use 230 V column. For motors rated 440 to 550 volts, use 460 and 575 V columns, respectively.

Mounting Thermal Units

Always be certain the correct thermal units are installed in the starter before operating the motor. Thermal units should always be mounted so that their type designation can be read from the front of the starter (see Figure 1). Melting alloy thermal units should be mounted so that the tooth of the pawl assembly can engage the teeth of the ratchet wheel when the reset button is pushed.

Mounting surfaces of starter and thermal units should be clean and care should be taken to insure that thermal unit mounting screws are fastened securely.