Panel main/feeder coordination

[chuckd83]

If a panel has molded case circuit breakers for both the main and feeders, they are technically not coordinated for a short-circuit fault (looking at a TCC). However, feeders always trip first. This happens at my house panel. Never once has my main tripped for a short-circuit even though it does not have an intentional "delay." Is there a technical reason for this?

 

[GPTech]

Yes - the breakers are series rated. A technical explanation is the feeder breaker will draw an arc introducing a large(r) impedance into the circuit which may prevent the upstream breaker from un-latching.

 

[waross]

Does the TCC show Amps or PU values?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[GroovyGuy]

As usual, it depends.
1) If your feeder CBs are 15A, and your main is (say) 100A; you will have good coordination with both overloads and short-circuits.
2) If your feeder CBs are 15A, and your main is (say) 30A; you may have decent coordination with overloads but short-circuit coordination will be poor (ie likely non-existent).

Fuses are generally easier to coordinate, even down to a ratio of 2:1. Circuit-breakers are a bit tricky, and you really need to review the respective TCCs.

GPTech is correct, in that most short-circuits are downstream from the feeder CB, and therefore the added impedance limits the fault-current to less than the instantaneous pickup of the main CB. Therefore, the feeder CB usually trips first.

 

[unclebob]

Most of the time, small molded case circuit breakers (<250 A) have a set instantaneous setting at 10X the amp rating.
If your house has a 200 A Main breaker, then a >2000 A fault would trip it.

 

[waross]

And the 15 Amp breaker will trip at about 150 Amps. The conductor impedance of the branch circuit limits most branch circuit fault currents to less than the instantaneous trip level of the main breaker. And occasionally, for a close in short, both will trip.

Bill
--------------------
"Why not the best?"
Jimmy Carter