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Short Circuit Protection #87295 02/06/04 11:55 PM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline OP
Member
There are cases when conductors are exempt from overload protection and only require short circuit protection. What is the method for sizing short circuit protection for these conductors?


George Little
2017 / 2014 NEC & Related Books and Study Guides
Re: Short Circuit Protection #87296 02/07/04 02:34 AM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
I don't know of any case where the conductors are exempt from overload protection. There are cases, like motors, where the branch circuit OCPD provides only short circuit and ground fault protection, but the motor overload device provides overload protection for both the motor and the branch circuit conductors. For motor circuits see Table 430.52.
Don


Don(resqcapt19)
Re: Short Circuit Protection #87297 02/07/04 09:33 AM
Joined: Dec 2003
Posts: 751
E
earlydean Offline
Member
He may be referring to fire pumps.


Earl
Re: Short Circuit Protection #87298 02/07/04 09:45 AM
Joined: Oct 2000
Posts: 2,749
Joe Tedesco Offline
Member
Quote
240.4 Protection of Conductors.

(A) Power Loss Hazard.

Conductor overload protection shall not be required where the interruption of the circuit would create a hazard, such as in a material-handling magnet circuit or fire pump circuit.

Short-circuit protection shall be provided.


Joe Tedesco, NEC Consultant
Re: Short Circuit Protection #87299 02/07/04 10:17 AM
Joined: Nov 2000
Posts: 2,148
R
resqcapt19 Offline
Member
Good point. I had forgotten about fire pumps.
Don


Don(resqcapt19)
Re: Short Circuit Protection #87300 02/07/04 06:25 PM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline OP
Member
You are so right gentlemen, it is a fire pump I am talking about. Sorry to be so vague. There seems to be another case where you only need short circuit protection and I seem to remember seeing this exemption for wiring associated with metering. It only specifies short circuit protection and I am still looking for the method of calculating short circuit protection for say # 12 wire or in the case of my fire pump, #1 copper wire. In the case of our fire pump we are asked to size the fuse or CB to carry locked rotor current. Does this automaticly provide short circuit protection for the conductor?


George Little
Re: Short Circuit Protection #87301 02/07/04 10:55 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline
Member
George,

I would think (mainly speculation) that the sizing of the OCPD would have to be done on a case by case basis, depending on what the load and application is. In your fire pump example, locked rotor current makes sense because that is the maximum current that could be drawn by the motor in a pre-failure condition. After failure, whether phase to phase or phase to ground, the available fault current would come into play which could be anywhere from 5000 amps to 25,000 amps or even (4) times that depending on many many variables. That should be enough to open the circuit (provided the OCPD is rated for the available fault current).

It might not be a bad idea to check available fault current to make sure you use the proper class breaker or fuse, but I guess that's the case for all of us no matter what the application.

Re: Short Circuit Protection #87302 02/08/04 12:36 AM
Joined: Jan 2004
Posts: 1,507
G
George Little Offline OP
Member
JPS- The problem also involves a xformer and the issue that now comes up is the protection of the xformer and associated conductors. Looking in the Handbook (I know it isn't official) in Article 695 they show sizing the xformer and the conductors using motor(s) FLA and sizing the overcurrent based on LRC which only allows the system to operate at LRC till it melts down. No overload protection is provided for the xformer or the conductors. I'm okay with this except the contractors/inspectors wrestle with protecting the components of the system and I want to make sure I am correct. :-)) So I go back to my original question, "Is protecting the conductors using LRC for the motors(s) giving me short circuit protection as called out in 240.4(A)?" Thanks for your response.


George Little
Re: Short Circuit Protection #87303 02/09/04 10:28 PM
Joined: Jan 2004
Posts: 615
J
Jps1006 Offline
Member
George,

Are you looking for a formula or chart that has ampere ratings that coincide with loads that say "given 'x' load 'y' is overcurrent size and 'z' is short circuit size"?

I don't know of such a way. sorry I'm of little use there.

but in answer to the question "Is protecting the conductors using LRC for the motors(s) giving me short circuit protection as called out in 240.4(A)?" I would say yes.

I don't have the NEC definitions handy for "overcurrent" vs. "short circuit", but I think logically and practically one could argue that between 0-FLA would be normal condition. FLA-LRC would be overcurrent, and anything beyond LRC would be short circuit. Also picture circuit conditions for each.

Under normal conditions - fine no sweat

Overcurrent - either a bearing has failed, or the motor is being asked to do more than it is designed for or any other senario. But if the water is still flowing, list the new pump and xfrmer on the insurance claim with everything else.

Short circuit - serious failure or something else has entered the circuit that shouldn't be there. Safer to kill it.

I think you're right. Tell those contractors/inspectors not to worry about the equipment and think about the guy that needs water to put out a fire. Protect him/ (her?) and the people he's protecting.


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