Attachment 1 – Ambient Temperature Influence on Circuit Breakers

 (1)  E-T-A thermal and thermal-magnetic circuit breakers


To ensure optimum matching of circuit breaker performance to the system requirements, E-T-A thermal and thermal-magnetic circuit breakers are not normally compensated for fluctuations in ambient temperature. The circuit breaker is usually subjected to the same heat source as the system so will automatically track its protective requirements.

However, some applications require the circuit breaker to operate continuously in either high or low temperatures. The following table shows the correction factors that typically should be applied. The performance of magnetic circuit breakers and type 1410 is not affected significantly within this temperature range.

Ambient temperature °C -20 -10 0 23 40 50 60
Ambient temperature °F -4 14 32 73.4 104 122 140
Multiplication factor   0.76 0.84 0.92 1 1.08 1.16 1.24

Example: IN = 10 A at +50 °C. By applying t factor of 1.16 the current value obtained is 11.6 A. A 12 A CBE rating is recommended.


A 30A circuit breaker in a panel in the hot sun, could theoretically reach temperatures in the 140+ °F range. A 30A breaker could trip out at 30/1.24=24A

(2)Close mounting of CBEs

When several devices are mounted together, an air gap between each is recommended. If this is not possible, each device should carry only 80 % of its rating.

Horizontal installation is preferable.

Plug-in Mounted E-T-A Devices:

The continuous rating capability of E-T-A sockets for plug-in circuit breakers is a function of the total number of circuit breakers fitted and the individual ratings of each. Please enquire with details of your application.


(3)AMBIENT RERATING CURVES – Square D Circuit Breakers