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Wednesday, June 7, 2017

Question:

Ever Good morning!

I have a question on bonding... We have a 277/480V panel feeding a 112.5 KVA transformer a foot away with FMC and then FMC from the Xfmr to the 120/208V panel a foot away, both sides measure around 4' in length of FMC. We have the correct size EGC in the FMC's bonding the panel can to Xfmr case. Does the supply side of the Xfmr FMC connectors have to have bonding bushings (or just a plastic bushing) on them to satisfy the supply side bonding jumper requirement 250.30(A)(2) or for any other reason?

Thanks, Kelly Wofford

A

Answer:

Hey Kelly thanks for your question, it's good to hear from you again. The equipment grounding conductor (EGC) in the flexible metal conduit (FMC) between the 480Y/277 volt panelboard and the transformer (for the primary conductors) is sized using 250.122.  Bonding at those FMC fittings is required as stated in 250.97 but the FMC fitting with a single locknut is sufficient - if there are no concentric or eccentric knockouts  as stated in 250.97 Exception (3).

The secondary conductors in the FMC supplying the 208/Y120 volt panelboard  must have a supply side bonding jumper installed with them that is sized using 250.102(C) as stated in 250.30(A)(2). No additional bonding is required at those FMC fittings because the voltage is not over 250 volts to ground therefore 250.97 does not apply. Your question seemed to infer that the supply side bonding jumper was on the supply side of the transformer but that is not the case. It is a "supply side" bonding jumper because it is providing an effective ground fault current pat for ungrounded conductors on the supply side of an overcurrent protective device as defined in Article 100.

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