Information - Thermal Unit Selection

Thermal Unit Selection

General

All tables are based on the operation of the motor and controller in the same ambient temperature, 40oC (104oF) or less. Always be certain the correct thermal units are installed in the starter before operating the motor. Each thermal unit shall be installed such that its catalog number is visible. See page 16-133, Figure 1 for complete thermal unit installation instructions. On melting alloy thermal units the ratchet wheel must engage the pawl assembly.

Selection Procedure


1.	Determine motor data:
	a.	Full load current rating
	b.	Service factor
		Note: If motor full load current (FLC) is not known, a tentative thermal unit selection could be made, based on horsepower and voltage. Refer to page 16-133.
2.	Motor and controller in same ambient temperature:
	a.	All starter classes, except Class 8198:
		1.	For 1.15 to 1.25 service factor motors use 100% of motor FLC for thermal unit selection.
		2.	For 1.0 service factor motors use 90% of motor FLC for thermal unit selection.
	b.	Class 8198 only:
		1.	For 1.0 service factor motors use 100% of motor FLC for thermal unit selection.
		2.	For 1.15 to 1.25 service factor motors use 110% of motor FLC for thermal unit selection.
3.	Motor and controller in different ambient temperatures:
	a.	Multiply motor FLC by the multiplier in Table A. Use the resultant full load current for thermal unit selection.
4.	Locate proper selection table from index, pages 16-130 and 16-131.
	a.	The proper thermal unit number will be found adjacent, to the right of the range of full load currents in which the motor FLC or resultant full load current falls.
5.	See page 16-132 for calculation of trip current rating.

Slow Trip Thermal Unit Selection

To select Type SB slow trip thermal units, the selection table for a standard Type B thermal unit may be used with the following modifications: For continuous rated motors having service factors of 1.15 to 1.25, select thermal units from the standard Type B table using 93% (102% for Class 8198) of the full load current shown on the motor nameplate and then substitute an SB for the B in the thermal unit type number.

Example: A motor with a full load current of 12 amps controlled by an 8536SCG3 would require B22 thermal units for standard trip applications and SB19.5 thermal units for slow trip applications. The SB is selected by multiplying 12 amps times 93% for 11.16 amps and using this value to select B19.5. Then add the S prefix to arrive at SB19.5.

For continuous rated motors having a service factor of 1.0, select thermal units in the same manner using 84% (93% for Class 8198) of full load current shown on the motor nameplate.

SB thermal units are used on Size 0, 1, 2 and only some Size 3 applications. Check thermal unit tables for current ranges.

Table A: Selection of Thermal Units for Special Applications
Class of Controller	Continuous Duty Motor Service Factor	Melting Alloy and Non-Compensated Bimetallic Relays			Ambient Temp.- Comp. Relays
		Ambient Temperature of Motor
		Same as Controller Ambient	Constant 10oC (18oF) Higher Than Controller Ambient	Constant 10o C (18oF) Lower Than Controller Ambient	Constant 40oC (104oF) or less, for Any Controller Ambient
		Full Load Current Multiplier
All Classes, Except 8198	1.15 to 1.25	1.0	0.9	1.05	1.0
All Classes, Except 8198	1.0	0.9	0.8	.95	0.9
Class 8198	1.15 to 1.25	1.1	1.0	1.15	1.1
Class 8198	1.0	1.0	0.9	1.05	1.0