bonding j boxes and concentric knockouts

[rplumer]

I was recently told by an electrical inspector that I need a bond bushing on a 480v motor circuit (f.l.a. 3.5 amps) where 1/2" sealtight leaves a 3/4-1/2 eccentric knockout in an 11-b box. He pointed out this deficiency in numerous spots including a 125 amp circuit in EMT. My question, is the inspector asking for more than the NEC requires? We looked up the issue in 250 and found in 250.97 has requirements for circuits over 250v which he could be citing.

 

[al hildenbrand]

You found the right reference.

The inspector is right.

Given a fault resistance of X Ohms, doubling the voltage from 240 to 480 quadruples the energy in the fault. The concentric KO tabs need help to carry the fault otherwise they'll tend to become open arcing points at they melt like fuse links.

 

[pierre]

According to UL, most boxes today are listed as suitable for bonding with voltages "under and over 250V".
(QCIT)
All boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for bonding without any additional bonding means around concentric (or eccentric) knockouts where used in circuits above or below 250 V, and may be marked as such.
Bonding is always a good thing in my book, but making someone change an installation that may meet code is not a good thing. It may be bare minimum code, but...

 

[al hildenbrand]

Pierre,

I find it interesting that, even though the box and the concentric KOs may have been investigated for reliable ground above 250 V, that indicating such is optional. . .

Is it reasonable to trust that the box one is inspecting is, in fact, "reliable" when the box is new, when the box is old?

 

[pierre]

I am not too sure when UL started testing boxes for concentric/eccentric knockouts, qualifying them as suitable for grounding. I wonder if the boxes are manufactured any different than they were before the testing. It is a good question - as grounding is the most important part of our installations.

 

[don_resqcapt19]

Pierre,

I wonder if the boxes are manufactured any different than they were before the testing.

The box is different. The KO is much tighter than the old ones. There is a lot more contact between the rings and the box itself to provide an effective fault clearing path.
It is my understanding that there are no panel or disconnect enclosures that have their multi-sized KOs listed for bonding. Only the device, conduit and outlet boxes are listed for this purpose.
The following wording appears in both "Boxes, Junction and Pull" (BGUZ) and in "Cabinets and Cutout Boxes" (CYIV)

Boxes with concentric or eccentric knockouts suitable for bonding conduits for non-service conductors are marked as such and do not require bonding jumpers to be installed around remaining concentric or eccentric knockouts.

I read this as saying you can't use these knockouts for bonding on systems over 250 volts because they are not suitable for use with service conductors and 250.97 requires bonding that is suitable for use with service conductors.
Don