Brochure
Table Of Contents
- Introduction
- Motor Failure and Protection
- Trip Class Designation
- Withstand Ratings
- Thermal Overload Relay
- Melting Alloy
- Non-Compensated Bimetallic
- Automatic Reset
- Ambient Temperature Compensated Bimetallic
- Thermal Overload Relay Trip Characteristics
- Solid State Overload Relay
- Additional Standard Features
- Optional Features
- Additional Standard Features
- Optional Features
- General
- Nameplate Versus NEC Full-load Current
- Service Factor
- Motor Branch Circuit Design
- Overload Relays
- Hand Reset Melting Alloy
- Accessories
- Isolated Auxiliary Contacts for Motor Logic Overload Relays
- DIN Adapter
- Lug-Lug/Lug-Extender Kits
- Remote Reset Module
- 4 – 20 ma DC Communication Module
- Contact Units for Melting Alloy Type Overload Relays
- Melting Alloy Overload Relay Jumper Strap Kits
- Bimetallic Overload Relays
- Motor Logic™ – Solid State Overload Relay
© 1998 Square D All Rights Reserved
24
5/98
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 18.
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.
Only Use This Table When Motor Full Load Current Is Not Known
Motor
Horsepower
Motor Full Load Current
Three PH Single PH
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. – –
NOTE: 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.
Figure 1