I've been having a bit of debate with a colleague on sub-panel grounding on buildings wired with BX/AC as it relates to NEC 250.32(B)
Typically, sub-panels are wired with a separate ground and floating neutral, but what about with systems wired with BX/AC? Can that fourth wire be eliminated in this case, since the grounding equipment conductor is the cable sheathing or conduit? Can the cable sheathing or conduit technically be classified as a "conductor" from a code standpoint?
The reason I'm asking is we got a request to add a sub-panel in a small warehouse (for 240V loads) in NYC, previously powered by only a single 20A breaker. The cable is run in 3/4" rigid (galvanized) conduit, under concrete among other things, approximately 45' from the main building, so you can see why I'd want to try and get by without that additional conductor.
My colleague says all he does is simply adds a grounding bushing at the sub, and runs the ground from the bushing to the grounding bus... and has never been tagged, nor should he be. He argues there is absolutely no reason to run a separate ground, since the wiring system designates the cable sheathing (or conduit) as the ground - unless the conduit is in bad shape in which case it should be dug up anyway.
He further argues since this is not a new service, inspectors won't require a ground to be pulled, as we would be required with new services... and not "open their eyes" on this... unless I want to break out the jackhammer. Needless to say, there will be quite a difference quoting this job with and without breaking up concrete.
-The cable is run in 3/4" rigid (galvanized) conduit, under concrete among other things, approximately 45' from the main building, so you can see why I'd want to try and get by without that additional conductor.-
Is the rigid run from panel to panel? If so good.
Or are you pulling an AC feeder thru it as a chase
Re: Sub-Panel Grounding and BX/AC 250.32(B)
Thanks for setting the record straight. I owe my colleague an apology.
I guess I was too rigid (no pun intended) in interpreting the term conductor in 250.32(B), and am sure glad we won't need to break out the jackhammers on this one. Doesn't matter if you're getting paid or not, it's never fun.
The rigid is in good shape, installed about a decade ago. The galvanized doesn't run straight to the panel - runs up to an LB, then nipplies into a J-box - so we'll just upsize the BX to the J-box and bush it... exactly as my "old timer" colleague originally suggested.