sub panel

[Dennis Alwon]

081105-2144 EST

A 60 A breaker at the load end of the cable does not protect the cable. This 60 A breaker only protects the wiring or load following the 60 A breaker.

Gar-- if you install a 60 amp main breaker in a sub panel and wire the feeder conductors to the breaker then, IMO the wire is protected from overload. The 100 amp breaker will protect it from short circuit faults and the 60 amp will indeed protect the wire from overload. How can you have more than 60 amps on the wire if there is a 60 amp breaker ahead of the load?

 

[gar]

081106-0736 EST

Dennis:

By having a short that produces a 100 A load, but not a dead short, on the cable before the 60 A breaker. Maybe not likely but possible. You do not incorrectly design a system because the probability of a fault is low.

.

 

[yankj]

I agree totally with Gar. Why not change the 100A breaker to 60A and use smaller wire? Problem solved.

 

[infinity]

IMO, since the upstream OCPD is protecting the wire at more than its ampacity, unless you are following a tap rule this is a violation.

I agree also. Otherwise there would be no need for distance limitations on tap conductors.

 

[don_resqcapt19]

081106-0736 EST

Dennis:

By having a short that produces a 100 A load, but not a dead short, on the cable before the 60 A breaker. Maybe not likely but possible. You do not incorrectly design a system because the probability of a fault is low.

.

There are many cases in the NEC where this would be permitted, I am not sure if this is one of them as we do not have enough information. Tap conductors, motor circuit conductors, service conductors and air conditioning circuit conductors are all permitted to have the overload protection at the load end of the conductor. In all of the cases, except for the service conductors ground fault and short circuit protection is required at the supply end of the conductor.

 

[Dennis Alwon]

081106-0736 EST

Dennis:

By having a short that produces a 100 A load, but not a dead short, on the cable before the 60 A breaker. Maybe not likely but possible. You do not incorrectly design a system because the probability of a fault is low.

.

Okay, I have no choice but to trust you on this but I would like to know what kind of scenario could arise where there could be 100 amps of load on the wire thru a ground fault that would not trip the ground fault of the 100 amp breaker. Seems to me to be more than highly improbable.

 

[glene77is]

I would like to know what kind of scenario could arise where there could be 100 amps of load on the wire thru a ground fault that would not trip the ground fault of the 100 amp breaker.

How about a fault to ground
where the ground is through a poorly connected conduit
( as in a loose and corroded locknut ) .

Comments are welcome.

 

[Doug S.]

... but I would like to know what kind of scenario could arise where there could be 100 amps of load on the wire thru a ground fault that would not trip the ground fault of the 100 amp breaker. Seems to me to be more than highly improbable.

Faulty main at the sub-panel.

I'll agree you can't run your life based on non-realistic fault probabilities, much less a business, and the 100a breaker would probably never trip. However this appears to be a pretty straight forward situation, covered by pretty straight forward code language, regardless of probabilities