I have a customer who has a sub-panel fed by a 6/3 piece of AC. It's overcurrent protection is a double-pole 60AMP Homeline Square 'D' circuit breaker. One phase is operating at 53AMPS, the other at 43. On Tuesday, the circuit breaker tripped. I went there today and the breaker was RED-HOT. My question is, without running new #4 feeders to the sub-panel, can I upgrade to a double-pole 70 according to 310.16 using the 90º column?
First of the wire must be of a type listed in the 90 degree column. Second the terminals of the breaker must be rated for 90 degrees. Also make sure that ambient tempature doesn't play a role. IE: does the cable run through a hot attic?
[This message has been edited by bot540 (edited 06-22-2006).]
Jesus may have been a capenter,but God was an electrician.Genesis1:3
#98923 - 06/22/0610:58 PMRe: Ampacity for 6/3 Armored Cable
A 60A circuit breaker carrying 53A should not be 'red hot'. Quite warm, perhaps, but not red hot.
I would check very carefully for damage to the bus stabs, or damage to the end of the conductor, both of which could cause a high impedance connection and localized heating. Was the breaker correct for the panel, or one that 'just happens to fit'?
If you have a high impedance connection, then it will heat and trip a new 70A breaker just as it did the 60A breaker.
#98927 - 06/24/0605:55 PMRe: Ampacity for 6/3 Armored Cable
I was there today farting around during business hours. I balanced the loads (50AMPS and 48AMPS), but the new 70 AMP circuit breaker still got warm. Not red hot, but warm. The terminal connections to the breaker are solid, but I'll have to check for damage on the buss when I go back there later tonight (it's a bar so I do most of my work there when it is closed). And yes, the circuit breaker is the same as the manufacturer of the panelboard. 110.3 (B) was not violated.
Thanks to everyone for all of your help, and timely advice.
It's very much appreciated.
#98928 - 06/24/0606:34 PMRe: Ampacity for 6/3 Armored Cable