40 degree C Square D QO breaker

[lakee911]

I recently came across a 2P 20A Square D QO-style CB that was marked 40 degree C on the molded case. It actually had suffered from severe arcing (due to discoloration and residue on the breaker) and was also cracked. Other 2P 20A Square D breakers have no 40 degree C marking. Anyone seen this before? I thought 60 was as low as it goes.

Jason

 

[jim dungar]

I recently came across a 2P 20A Square D QO-style CB that was marked 40 degree C on the molded case. It actually had suffered from severe arcing (due to discoloration and residue on the breaker) and was also cracked. Other 2P 20A Square D breakers have no 40 degree C marking. Anyone seen this before? I thought 60 was as low as it goes.

Jason

The 40?C marking is the ambient air temperature rating of the breaker.
It has nothing at all to do with the lug/wiring temperature limitations.

 

[infinity]

I agree with Jim. I've had people ask me where the 40 degree column was in the code book to size conductors with these CB's.

 

[bphgravity]

Review the UL - circuit breaker and overcurrent device marking guide. This document will help you identify all the markings and labels on these devices and what they mean.