Breaker Ratings 1

[steris]

When sizing a breaker, UL requires that the breaker be 25% greater than the rated load. Is this true just for inductive loads or for resistive loads as well? If it is different for resistive, does anyone know the required overage is? Thanks in advance!

Best,
Steris

 

[dpc]

Here's how it works:

For a standard molded case circuit breaker, UL (and manufacturer) lists the minimum conductor size that can be used for that breaker. Regardless of anything else. This is necessary because the conductors act as a heat sink and the design of the thermal element in the breaker assumes a certain wire size.

This generally corresponds to NEC conductor ampacity assuming a current that is 80% of the breaker trip rating. NEC requires the conductor be sized at 100% of the intermittent load plus 125% of the continuous load, with "continuous" defined as 3 hours or longer. If all of the load is considered continuous, the resulting conductor size and breaker size is 125% of the continuous load. The inverse of 125% is 80%.

 

[steris]

Hi DCP -

That really clears up the issue. Thanks a lot!