Bonding locknuts and concentric knockouts

[suemarkp]

Are bonding type locknuts equired around concentric knockouts when doing 120V wiring, or only in Service conductors or conductors over 250V?

Also, why are two locknuts specified for bonding over 250V? How could you use a threaded conduit and not use a locknut on both sides? How is EMT bonding different for over 250V circuits -vs- circuits under 250V. It seems like a normal EMT threaded end is sufficient if no concentric knockout. Is 250.97 (Bonding over 250V) basically saying you bond things as you normally would unless there are concentric knockouts?

[mdshunk]

How could you use a threaded conduit and not use a locknut on both sides?

I've seen guys "cheat" when putting short RMC nipples between a panel and a wireway. They use one locknut and bushing inside the wireway, and one locknut and bushing inside the panel. They don't use the required locknuts on the outside of the panel and wireway, but the nipple still feels tight... it's just not bonded well at all. I'm not sure what the major mental malfinction is that would cause a person to skip such a simple thing as adding the extra 2 locknuts.

[Wgoodrich]

The two locknuts are to ensure a strong bite into the metal of the panel. However any time a concentric or eccentric is present and that conduit is depended on to be a part of the grounding path a bonding bushing is required to carry the current across those knock outs left over. This runs the faulted current on the bonding bushing and bonding jumper wire jumping over those knockouts. Knockouts are not a reliable path.

This is true on any conduit line or load that is being used as a grounding connective path.

The emt connector has a smooth backing. The bonding locknut has angled tabs that bite into the panel. Check your manufacturer's installation instructions concerning EMT connectors to find if they are approved as a bonding path with only one grounding lock nut. I believe you will find connectors for EMT connectors will be allowed with only one locknut.

Be aware 250.97 is over 250 volt to ground. This means any one line being over 250 volts to ground not two conductor making 250 volts. What is being talked about is 480 volt corner grounded delta or 480 volt wye connection measuring 277 volts to ground or higher. 240 volt corner grounded delta and the like that measures voltage from one hot conductor to ground at 250 volts or single phase 120/240 does not apply to this rule. Those read 250 volts or less to ground from any hot single hot conductor.

Be aware there are now panels that have concentrics and eccentrics that are rated as a grounding path. Few exist but they are out there. Again check the manufacturer if they specifically approve their knock outs as grounding paths. If you can't back it up then don't rely on a knock out as a grounding path.

Good Luck

Wg

[suemarkp]

You've pretty much confirmed what I thought and how I do things, however I see no place in the code that mandates grounding bushings on concentric knockouts. Is this one of those implicit rules where "effectively bonded" comes into play?

And it seems that 250.97 is redundant if you wire things as expected (two nuts on each threaded conduit, using modern conduit fittings, threaded bosses, etc). What does 250.97 require that is not already being done when bonding any non-service conduit?

[Wgoodrich]

Read 250.92 for services.

COPIED SECTION NEC 2000;

Bonding jumpers meeting the other requirements of this article shall be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair the electrical connection to ground. Standard locknuts or bushings shall not be the sole means for the bonding required by this section.


Notice 250.97 states other than services and also states that in conditions of OVER 250 volts to ground [meaning one hot conductor measured voltage to ground not two hot conductors read between].

250.86 states all metal conduits must be grounded.

Hope this helps

Wg