7/27/2019 Continuous Current Carrying Capability of Low-Voltage Circuit Breakers.pdf

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Spring 2002 1

Various types of low-voltage circuit breakers have differing con-

tinuous duty capabilities. Some are rated to carry 100 percent of 

their trip rating continuously while others are rated to carry only

80 percent of their trip rating continuously. It is important that we un-

derstand the difference and apply these breakers properly.

The general run of molded-case circuit breakers in frame sizes of 400amperes and below are rated to carry only 80 percent of their rated tripcurrent on a continuous basis. Particularly, when these breakers aremounted close to each other in a panelboard, the extra heat generated bycarrying 100 percent of the trip rating will both lead to false tripping andcause long-term degradation of the insulating material of which these breakers are made.

On the other hand, all low-voltage, power circuit breakers and thegeneral run of insulated-case circuit breakers are capable of carrying 100percent of their trip rating on a continuous basis.

Some confusion can arise when using large molded-case circuit break-ers in frame sizes of 600 amperes and above. These breakers may berated either 80 percent or 100 percent, depending on the model and themanufacturer. As you would expect, the 100 percent breaker costs con-siderably more than the 80 percent breaker. Some models have both 80percent and 100 percent ratings available. The 100 percent rated breaker

may require a larger enclosure and/or more ventilation than the 80 per-cent rated breaker of the same model.

Please observe the following application rules:

• Apply MCCBs in 400 ampere frame size and smaller based on con-tinuous loads of not more than 80 percent of the circuit breaker’s triprating.

• Apply insulated-case breakers and low-voltage, power circuit break-ers based on continuous loads of not more than 100 percent of the breaker’s trip rating.

• Apply large, molded-case circuit

 breakers based on either the 80percent or the 100 percent rat-ing, making sure that the breaker selected fits the applica-tion and that adequate spaceand ventilation are provided forthe breaker chosen.

Reprinted with permission of PowellElectrical Manufacturing Co.

Baldwin Bridger, PE, is retired Techni-cal Director of Powell Electrical Manufac-turing Co., Houston, Texas. He has workedas an engineer and engineering managerin the design of low- and medium-voltageswitchgear since 1950, first at GE and since1973 at Powell. He is a Fellow of IEEE anda past president of the IEEE Industry Ap-plications Society

by Baldwin Bridger, PE Powell Electrical Manufactur-ing Co. (retired)

Technical Brief 

Continuous Current-Carrying Capabilityof Low-Voltage Circuit Breakers