JATC Forums - Code Discussions

This area should be used for questions dealing with the current NEC (2008), the previous NEC (2005), and any changes that are relevant to the industry.

Showing: 1-5 out of 5 messages

12/20/09 - 11:37 AM
Tarn
4/19/10 - 6:17 PM
Mike Towne
I know we went over this in code...but my memory isn't working right now...my mechanic told me that any time 480V passes through a concentric or eccentric knockout, a bond-bushing is needed...the way I read the code book is a bond-bushing is needed anytime any conductors with voltage over 250V passes thru a concentric or eccentric knockout...I know there are exceptions...but I'm just asking generally...I don't even know what I'm asking...maybe just some clarification?
4/19/10 - 8:28 PM
Sean Myers
Bonding for over 250V must be done to ensure an equipotential plane for all non-current carrying metal parts of the system. The compliant methods for bonding at or above 250V (which in the real world means 277V and higher) are the same as the methods listed for bonding at a service (Art 250.92(B))excluding the use of the grounded conductor which can only be used for bonding on the supply side of the service disconnecting means and one or two specific dedicated branch circuits (i.e. dryers).

Where concentric or eccentric kos are encountered, bonding jumpers must be used to maintain the equipotential plane because the ko itself lowers the reliability of the path to facilitate the operation of the OCPD in a grounded system.

These jumpers must be sized according to table 250.66 if they're on the line side of the service, or table 250.122 if they're on the load side. The goal is to make the groinding and bonding conductors large enough to handle "the maximum fault current likely to be imposed", so sizing the jumper based on the largest hot feeding the service (tbl 250.66) or alternately sizing them according to the rating of the feeder or branch circuit OCPD (tbl 250.122) makes sense.

So to answer you question... Yes, the mechanic is right.

I Hope that helps Mike.
4/19/10 - 8:32 PM
Sean Myers
after re-reading your question I realize that I may not have addressed the actual issue. You are correct; any circuit with a volatge over 250 requires bonding jumpers (which necessitates the use of bond bushings) around concentric and eccentric kos. The mechanics mention of 480V might be a simple oversight or maybe he assumed that you would understand he meant 277/480 3 phase systems. At any rate, happy bonding.
4/20/10 - 4:24 AM
Mike Towne
That's the way I understood it...but thanks for clearing it all up...I also saw the bond bushings or the like are required in hazardous locations...no matter what the voltage...but thanks again!

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