Generally, depending on the manufacturer, a thermal circuit breaker for equipment (CBE) approved to UL 1077, CSA 22.2 No. 235, CCC and IEC 60934 is calibrated at 77°F (give or take a few degrees).

This means that the “Hold” and “Trip” characteristics defined on manufacturers’ data sheet (also known as the time/current curve) indicate the performance of the supplementary protector when operating at an ambient temperature of 77°F.

For example, at 77°F a 10 Ampere rated breaker will “Hold” 10 amps indefinitely and “Trip” within the specified time windows for given percentages of overload.  At an operating ambient of 77°F, a typical data sheet might indicate:

100%	135%	200%	400%	600%	1,000%
Hold	Trip within 1 hr.	4 -40 sec.	0.6 – 5 sec.	.3 – 1.8 sec.	0.12 – .4 sec.

Thermal circuit breakers react to the effects of heating.  Therefore, it is important to keep in mind that performance characteristics will differ with changes in ambient temperature.

To achieve the desired “Hold” and “Trip” performance (to avoid nuisance tripping on a circuit at elevated ambient temperatures for example) the ampere rating of the thermal circuit breaker may need to differ from what would be used on the circuit at a nominal 77°F.   This adjustment to the selected CBE’s rating is typically determined by multiplying the rating of the circuit breaker that would be required at the nominal 77°F by an Ambient Temperature Correction Factor (aka Derating Factor) to arrive at the correct circuit breaker rating for the given ambient temperature.

For the 10 ampere circuit protector in our example above, a manufacturer’s typical Ambient Temperature Correction Factor table may indicate the following multipliers:

C°	0	20	25	40	50	60
F°	32	68	77	104	122	140
Correction Factor	0.85	0.95	1.00	1.15	1.25	1.40

 

From the table, for an ambient temperature of 122°F, the 10 ampere nominal circuit breaker in our example would be replaced by a 12.5 – 13 ampere rated supplementary protector to avoid nuisance trips (10 x 1.25).

The selection of the correct circuit breaker rating for a given ambient condition is very simple.  Giving this a few minutes of consideration will help assure your circuit is adequately protected.