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#196917 11/01/10 06:02 PM
Joined: Feb 2002
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Are you allowed to use a bonding locknuts with concentric/eccentric knockouts?

P.S. Greg, you know where I am going with this, let's see what others have to say.

harold endean #196919 11/01/10 06:09 PM
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I gave you my cite, lets see what everyone else has to say.


Greg Fretwell
harold endean #196931 11/01/10 09:26 PM
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UL White Book seems to indicate no problem for metal outlet boxes, regardless of voltage, but then 250.97 doesn’t specifically list them as one the methods in the exceptions for bonding over 250V.

KJay #197010 11/04/10 11:06 PM
Joined: Jun 2004
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Raco junction boxes are rated/listed for use with 277V circuits even with concentrics.

Ditto the competition, IIRC.

The only time I face issues is with disconnects -- especially NEMA3R for RTU.

I've never been given the opportunity to install bonding locknuts. Out my way, the only ones I've seen are in Code Handbooks.

-----------

I can say from witnessing aged rooftop installations that bonding takes a beating over time. Hence, I always run a separate bonding conductor for all circuits that leave the building.

If you've never seen it, what happens to roof top EMT runs is trips and falls. Over the years various HVAC boys manage to bump what ever EMT can possibly be bumped.

As for current installations, root top EMT runs are either contract prohibited or banned by the AHJ. Only short, to the point, EMT is permitted.

BTW, old disconnects/fuse holders are scary. Generally they are too small and too often they do not keep rain or leaves out. And then the labels are all ruined. This is not the type of stuff to handle on a wet roof.



Tesla
Tesla #197014 11/05/10 08:48 AM
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The situation I am writing about deals with a new panel installed before that main service breaker. It feeds a PV system. So the feeder to this new PV panel has no overcurrent protection. Now Would a bonding locknut be suitable for use on a panel with concentric knockouts?

harold endean #197016 11/05/10 12:31 PM
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Harold I have modified my position. I am thinking it will be acceptable. The function of the bonding locknut is to bond the raceway and if it is made up tight against the connector lip and the concentric ring with a 250.66 sized jumper, the raceway is bonded, even if the ring breaks out completely.

For some reason I had in it my mind that the concentric locknut would loosen if the eccentric broke. I guess I should not type after midnight wink


Greg Fretwell
gfretwell #197020 11/05/10 12:57 PM
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Greg,

If the concentric KO pops out, the grounding would be lost. The bonded locknut will still be tight against the shoulder of the connector, but the metal to metal grounding will be missing because the KO busted loose. Right?

harold endean #197021 11/05/10 01:15 PM
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Not if there is a jumper from the locknut to the can.


Greg Fretwell
gfretwell #197028 11/05/10 07:33 PM
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BTW, it is important to acknowledge that PV systems -- as generators of power are NOT infinite busses.

Further, solid-state current saturation effects mean that the kind of peak arc power available from an infinite bus does not exist.

This is true whether you're on the DC or AC side of the inverter.



Tesla
Tesla #197031 11/05/10 08:44 PM
Joined: Nov 2007
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I thought bonding locknuts just had a screw that you tighten against the box after tightening them down. If it has lug with a bonding jumper attached, wouldn’t that be a bonding bushing?

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