chase nipples used on concentric k.o. for 277/480

[DJE]

A 3phase 60 amp 277/480 disconnect mounted on a rooptop a/c unit has concentric K.O. and is mounted back to back on ac/panel that also has concentric K.O.. Is it ok to use 1 locknut like on a typical back to back location or should you use a standard nipple with locknuts inside and out with bonding bushings to comply with 250.97?

 

[infinity]

Unless the enclosures are listed for bonding over 250 volts, only the second method you've described can be use to satisfy the requirement since you have concentric KO's. Are these in a wet location?

250.97 Bonding for Over 250 Volts
For circuits of over 250 volts to ground, the electrical conti-
nuity of metal raceways and cables with metal sheaths that
contain any conductor other than service conductors shall be
ensured by one or more of the methods specified for services
in 250.92(B), except for (B)(1).
Exception: Where oversized, concentric, or eccentric
knockouts are not encountered, or where a box or enclosure
with concentric or eccentric knockouts is listed to provide a
reliable bonding connection, the following methods shall be
permitted:
(1) Threadless couplings and connectors for cables with
metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate
metal conduit, one inside and one outside of boxes and
cabinets
(3) Fittings with shoulders that seat firmly against the box
or cabinet, such as electrical metallic tubing connec-
tors, flexible metal conduit connectors, and cable con-
nectors, with one locknut on the inside of boxes and
cabinets
(4) Listed fittings

 

[travish]

Please clarify for me, I understand the KO's can brake out and you would lose you grd/bonding if you didn't pull a ground wire, and the conduit was providing the ground path. So are you bonding the bushings on the conduit because you are not pulling a groundwire thru the conduit, or because the conduit itself has to be bonded? A chase nipple does not provide a satifactory ground path?

The reason for asking is we have a lot of 480v stuff at our plant and never use bonding bushings because we alway pull a Grd wire thru every conduit. Do we also need to be bonding the conduit?

Travis

 

[Cow]

I believe they are worried about the conduit becoming isolated without an effective ground fault current path. Using a bond bushing creates that path you may lose if the concentrics bust out. I personally like using reducing washers large enough to cover the biggest ko. It's easier to install than a bond bushing, cheaper too, and this way you know the concentrics won't bust out. Not necessarily so, with a bond bushing.

 

[travish]

we generally are knocking holes in our cabinets (machine control panels), so this doesn't apply, but if we feed from a 480 panelboard in the plant somewhere that had factory concentric knockouts we would need to bond the conduit. Makes sense to me

Appologies to the OP for hijacking the thread

Travis