Is this "poor man's co-ordination" or wishful thinking?

[fmtjfw]

225.40 Access to Overcurrent Protective Devices.
Where a feeder overcurrent device is not readily accessible, branch-circuit overcurrent devices shall be installed on the load side, shall be mounted in a readily accessible location, and shall be of a lower ampere rating than the feeder overcurrent device.

Is the idea here that the OCPDs will somehow co-ordinate and the branch circuit will open before the feeder does?

I'm under the impression that in the real world, without some study of trip curves and careful selection of OCPD's, this is not likely to work.

 

[iwire]

Is the idea here that the OCPDs will somehow co-ordinate and the branch circuit will open before the feeder does?

I'm under the impression that in the real world, without some study of trip curves and careful selection of OCPD's, this is not likely to work.

I think as far as a short circuit / ground fault event you are absolutely correct. That without a study its a crap shoot which OCPD will open first.

On the other hand for overload event, lets say too many space heaters plugged in on one circuit, this will work. The smaller device should trip first in most cases.

 

[charlie b]

I don't think the issue here is selective coordination. But you are right that achieving selective coordination does require a formal calculation.

I am not sure what the intent of this article would be. It is worth noting that since it addresses a feeder, then there necessarily much be one or more branch circuit overcurrent devices further downstream. Also, earlier in 225 it states that the feeder's disconnecting means must be readily accessible, and then gives some exceptions. So for 225.40 to come into play at all, we must be invoking one of those exceptions. That is as far as my thinking takes me.

 

[texie]

I've often wondered about this as well. As iwire said, it might work in an overload situation but even that may be dicey depending on the size and rate of the overload and the types of OCPDs involved. And if this rule is so important why is it not in Art 215 Feeders as well? After all, there are millions of feeders in apts. and other buildings where the OCPD is remote. And then you get into the issue of accessible VS readily accessible.

 

[charlie b]

I have had to think this one through for a while. I suspect this rule is analogous to that contained in 240.24(B). That rule requires overcurrent devices to be readily accessible to the occupants of the spaces. But for 225.40, we are dealing with a building that is fed from some other building (else we would not be in 225). Specifically, the overcurrent device that serves this feeder is located in the other building. So if that overcurrent device is not readily accessible, then the occupants of the sub-fed building will not have the rights granted to others under 240.24(B). I think the purpose of 225.40 is to give them back those rights. It requires the occupants of the sub-fed building to have ready access to the local branch circuit overcurrent devices.

Bottom line: if the occupant of the sub-fed building overloads a circuit, the local (and readily accessible) branch circuit breaker should trip. I agree with Bob?s statement that for overload conditions, the breaker closest to the point of overload should trip before a feeder breaker further upstream. The occupant can?t easily get to the upstream breaker to reset it, but they can get to the local branch circuit breaker. I think the reason the local branch circuit breaker is required to have a lower setting than the upstream feeder breaker is to give a higher probability that it will trip first on overload.