250.96A

[RRJ]

Do I need bonding bushings on a 30 amp 480 volts motor disconnect with 1/2 rigid metallic pipe in concentric k.o. Or does it only apply when you’re trying to use the raceway as the GEC?


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[Dennis Alwon]

Here is the section that applies

250.97 Bonding for Over 250 Volts. For circuits of over
250 volts to ground, the electrical continuity 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 connectors, flexible metal
conduit connectors, and cable connectors, with one locknut on
the inside of boxes and cabinets
(4) Listed fittings

 

[texie]

Do I need bonding bushings on a 30 amp 480 volts motor disconnect with 1/2 rigid metallic pipe in concentric k.o. Or does it only apply when you’re trying to use the raceway as the GEC?


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Yes unless the concentric KO is listed for for bonding over 250 volts. The fact that you have a separate wire type EGC does not eliminate the requirement. Very few enclosures are so listed. Most 4 square boxes are however.

 

[infinity]

I agree with Texie, 4" and 4 11/16" boxes are listed for bonding over 250 volts disconnect switches are not.

 

[RRJ]

Thank you guys. I’m putting reducing washers because I don’t have bonding bushings. They are rated for 600v.


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[texie]

Thank you guys. I’m putting reducing washers because I don’t have bonding bushings. They are rated for 600v.


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You can do that and comply but be aware that the listing requires all the concentrics be removed. In other words if you have a 2" concentric that goes to smaller sizes you must remove them all and the use a washer that is 2" X whatever size your raceway is.

 

[Carultch]

You can do that and comply but be aware that the listing requires all the concentrics be removed. In other words if you have a 2" concentric that goes to smaller sizes you must remove them all and the use a washer that is 2" X whatever size your raceway is.

So in other words, you don't only have to cover all the ring knockouts with the reducing washers, but you ALSO have to remove them? Such that it is not enough to just leave the unused KO's in place, and cover over them with reducing washers. Is this what you mean?

 

[texie]

So in other words, you don't only have to cover all the ring knockouts with the reducing washers, but you ALSO have to remove them? Such that it is not enough to just leave the unused KO's in place, and cover over them with reducing washers. Is this what you mean?

Correct.

 

[RRJ]

So in other words, you don't only have to cover all the ring knockouts with the reducing washers, but you ALSO have to remove them? Such that it is not enough to just leave the unused KO's in place, and cover over them with reducing washers. Is this what you mean?

Here is the wording from the UL website:

OTHER TYPES OF METALLIC ENCLOSURES

Cabinets and cutout boxes, in accordance with NEC Article 312, as well as junction and pull boxes, in accordance with NEC Article 314, may also employ concentric or eccentric knockouts, and are evaluated for UL Listing in accordance with UL 50, the Standard for Safety for Enclosures for Electrical Equipment (CYIV and BGUZ). In contrast with metallic outlet boxes, these other types of metallic enclosures with concentric or eccentric knockouts are not required to be subjected to a short time current test. As such, NEC Section 250.97 would require that bonding jumpers be used unless all concentric or eccentric knockouts were removed. For the UL Guide Information, please refer to UL Product iQTM enter “CYIV” or “BGUZ” into the search box.


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[infinity]

Here is the wording from the UL website:

OTHER TYPES OF METALLIC ENCLOSURES

Cabinets and cutout boxes, in accordance with NEC Article 312, as well as junction and pull boxes, in accordance with NEC Article 314, may also employ concentric or eccentric knockouts, and are evaluated for UL Listing in accordance with UL 50, the Standard for Safety for Enclosures for Electrical Equipment (CYIV and BGUZ). In contrast with metallic outlet boxes, these other types of metallic enclosures with concentric or eccentric knockouts are not required to be subjected to a short time current test. As such, NEC Section 250.97 would require that bonding jumpers be used unless all concentric or eccentric knockouts were removed. For the UL Guide Information, please refer to UL Product iQTM enter “CYIV” or “BGUZ” into the search box.

I don't see anything in there about reducing washers.

 

[RRJ]

I don't see anything in there about reducing washers.

Removing the knockouts to comply.


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[infinity]

Removing the knockouts to comply.

It says remove all the KO's so you do not need a bonding jumper, where is the wording about reducing washers?

 

[RRJ]

It says remove all the KO's so you do not need a bonding jumper, where is the wording about reducing washers?

I saw it in mike holts 2020 understanding the NEC book picture and also in an article online.

After All These Years

Adequacy of reducing washers questioned When removing concentric or eccentric knockouts from a sheet-metal enclosure, it sometimes happens that a larger size than desired comes out or the only available knockout is larger than needed. This is where reducing washers, one inside the enclosure...

www.ecmag.com

The UL White Book paragraph about reducing washers reads, “Metal reducing washers are considered suitable for grounding for use in circuits over and under 250V and where installed in accordance with ANSI/NFPA 70, National Electrical Code.”


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[Dell3c]

It's also in a Mike Holt Forum, thread under Bonding & Grounding.

Date of thread, June 7th, 2008
Thread title of : UL listed Reducing Washers. Thread Author: Tom Baker.
*See post #6 in thread.

 

[electrofelon]

I assume the paint would need to removed from the mating surfaces as well?

By the way RRJ, if you were not aware, split bonding bushings are available so you could always put those on later.

 

[RRJ]

I assume the paint would need to removed from the mating surfaces as well?

By the way RRJ, if you were not aware, split bonding bushings are available so you could always put those on later.

True,i will probably end up installing then later.


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