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Sizing bonding jumper for 250.92(B)(4)

BN4537

BN4537

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Is it necessary to size a bonding jumper to 250.122 if the only purpose of the bonding jumper is to connect to the bonding bushing on a metal raceway for a service? No concentric knockouts or impaired connection, the only purpose is to comply with the section that states "Standard locknuts or bushings shall not be the only means for bonding required by this section".

The only reason I am wondering is that the metal raceway "is" bonded by standard locknuts etc. and the bonding jumper is added on services to make a lower impedance connection so that you can pull enough amps to blow the fuses on the primary side of the utility transformer, which can take 2-4 times the amount of current that your main breaker would require.

This photo is an example from my old photos to demonstrate basically what I am talking about, but not a current install and may not fit my question entirely.


Bonding bushing.jpg
 
Could you instead use a product like this on large parallel conductor installations, threaded onto an EMT male adapter? Is there an amperage limit on these I can't find? Just doesn't seem equivalent to a large bonding jumper on, say, a 1200 amp service.

Seems to be listed for RMC and IMC conduit, is an EMT male adapter considered the same thing for listing purposes of this type of locknut?


Screenshot 2025-09-23 082808.png
 
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In my opinion, I believe you can because it’s mentioned in 250.92(B)(4) - “bonding type locknuts.”

I’m leaning toward no on use with anything else unless it’s listed as an assembly for bonding.

What’s an EMT male adapter?
 
bump... let's hear about it. Are there reasons to like them? Reasons to reject them? Does that screw get past paint and what actual contact is made?
 
BN4537 said:
Something like this: Male Adapter. Wondering if you can even use those bonding type locknuts
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Oh a connector, gotcha, ty! I personally would not use them with EMT connectors unless the manufacturer instructions for the bonding locknuts say that you can 110.3(B). Now if they make an EMT connector with bonding locknut as a listed assembly, I would be down with that. But not if they’re listed separately and the instructions don’t specify use with EMT.

If you really wanna go down a rabbit hole you could read through the UL standards 467 and 514B. You can view the digital versions of the standards for free, you just need an account. If I were to find something in there (probably 467?) that says the device being marked for use with IMC/ RMC automatically includes EMT unless specifically marked “not for use with EMT”… Then I might reconsider.
 
ICE said:
bump... let's hear about it. Are there reasons to like them? Reasons to reject them? Does that screw get past paint and what actual contact is made?
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The screws are pointy to bite into the metal. Other bonding locknuts don’t have a screw, the actual lock nut is shaped in a way to bite into the metal as it’s tightened on. Kinda neat and pretty sweet in a tight spot.
 
250.12 Clean Surfaces. Nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded or bonded shall be removed from threads and other contact surfaces to ensure good electrical continuity or shall be connected by means of fittings designed so as to make such removal unnecessary.
 
Per UL 514B "Conduit, Tubing, and Cable Fittings," a threadless fitting is "A fitting intended for use with nonthreaded rigid metal conduit, intermediate metal conduit, or electrical metallic tubing. So all EMT fittings are threadless fittings, and an EMT connector is a threadless connector.

For service bonding, NEC 250.92 covers the requirements. 2023 NEC 250.92(B)(3) says it is OK to use "Threadless couplings and connectors if made up tight for metal raceways and metal-clad cables." So an EMT connector and its provided locknut suffice for service bonding, if "made up tight."

Per UL 514B, fittings for metal raceways undergo resistance and current tests.

Cheers, Wayne
 
Wayne, I respectfully disagree. 250.92(B) states that “standard locknuts or bushings shall not be the only means for the bonding required…” Per this section, additional means of bonding are required when service conductors are involved.
 
warmer said:
250.92(B) states that “standard locknuts or bushings shall not be the only means for the bonding required…”
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Right, that refers to using 2 locknuts on rigid metal conduit. IIRC UL 514B only has mechanical tests for locknuts, no resistance or current tests.

But an EMT connector plus its locknut is not "only" standard locknuts. It's a listed combination tested under UL 514B for resistance and current. 250.92(B)(3) refers to threadless connectors, and such connectors always come with a locknut.

Cheers, Wayne
 
wwhitney said:
Right, that refers to using 2 locknuts on rigid metal conduit. IIRC UL 514B only has mechanical tests for locknuts, no resistance or current tests.

But an EMT connector plus its locknut is not "only" standard locknuts. It's a listed combination tested under UL 514B for resistance and current. 250.92(B)(3) refers to threadless connectors, and such connectors always come with a locknut.

Cheers, Wayne
Click to expand...

Interesting.

So that is why the body of the text says that bonding jumpers are required for impaired connections, period, then goes on to say that (even if there isn't an impaired connection) you can't consider it bonded by just a standard locknut and have to use one of the 4 methods - because not all of the 4 methods would work if there was an impaired connection. If there is an impaired connection, you will need a bonding jumper regardless of the method you have used.

If it isn't an impaired connection, standard EMT fittings would work per item (3). If it's RMC or IMC, you would need a threaded hub per item (2), or a bonding bushing, or a bonded locknut per item (4).

Interestingly, item (1) refers to 250.8(A), which allows listed pressure connectors, terminal bars, exothermic welding, machine screws engaging at least 2 threads or secured with a nut, or listed assemblies or means.

It appears that the weakest bonding method listed is EMT threadless connectors with a standard locknut as an assembly per item (3). By comparison, an offset steel nipple that is attached with a bonding type locknut like that pictured above seems like it is probably a slightly better connection.

Further, I would assume that if you can find a bonding type locknut large enough for the pipe you are connecting, and the pipe is properly sized for the conductors inside it, you would be compliant with such a locknut even on very large services. The quality of the connection does not depend as much on the screw from the locknut biting into the cabinet as it does the full circumference of the pipe being tightly mashed into cabinet, thereby creating a connection with a lot of surface area.
 
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wwhitney said:
Right, that refers to using 2 locknuts on rigid metal
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It’s not limited to that though and the wording I quoted was originally part of what is now 250.92(B)(3) in 1996. In 1998, the change was proposed so that it wouldn’t only apply to threadless connectors so they moved it to main text in 250.92(B). The only vote against was due to clarity - they said putting that sentence in the main text of (B) made it look like it only applied where there were impaired connections, which was not the intent.

I think this needs to be revisited for the 2029 NEC.

To clarify, ALL standards locknuts are not enough to meet the bonding requirements at the service, including the locknuts that come with the EMT connectors, regardless if it’s a perfectly sized punched hole or concentric knockout. If it’s at the service, an enhanced UL 467 listed method of bonding IS required.

I will attach a photo from the 1996 NEC showing how this information was originally organized and I will also attach a photo of the accepted proposal from the 98 ROC. Anyone following this conversation can access the same information for free via the NFPA website. If you get lost trying to find it, message me.
 

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warmer said:
To clarify, ALL standards locknuts are not enough to meet the bonding requirements at the service, including the locknuts that come with the EMT connectors, regardless if it’s a perfectly sized punched hole or concentric knockout.
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Obviously it needs to be an unimpaired KO, otherwise other bonding around the impaired KO will be required.

But I disagree that any of the text you imaged (other than one unsubstantiated sentence in the PI's substantation) says that one locknut opposite a threadless connector is not sufficient. Everything just says that one standard locknut opposite another standard locknut is not sufficient.

UL 514(B) tells us that a threadless connector opposite its locknut has been tested for current and resistance for the purposes of bonding. IIRC, for sizes of 4" and under, those tests required by 514(B) are identical to those required by UL 467. So for those sizes, a UL 467 listing is not "enhanced."

Let me ask you this, under your interpretation, what is an example of a threadless connector install to which 250.92(B)(3) would apply? If there are none, so that connectors could be removed from 250.92(B)(3) without changing the meaning, that says to me that your interpretation is not what the writers of 250.92(B)(3) intend.

Cheers, Wayne
 
To reiterate my basic point succinctly, the phrase in 250.92(B) " Standard locknuts or bushings shall not be the only means for the bonding required by this section" refers to a standard locknut or bushing opposite a standard locknut or bushing on a threaded raceway.

A connector opposite a locknut or bushing is not "only" standard locknuts or bushings. The connector is part of the "means for the bonding" and has been tested as such under UL 514(B).

Cheers, Wayne
 
wwhitney said:
Let me ask you this, under your interpretation, what is an example of a threadless connector install to which 250.92(B)(3) would apply? If there are none, so that connectors could be removed from 250.92(B)(3) without changing the meaning, that says to me that your interpretation is not what the writers of 250.92(B)(3) intend.
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You can use a threadless connector and the locknut it comes with for compliance with 300.10, but to comply with the bonding requirements of this section you would need to also install a bonding bushing.

Or use a threadless connector with a listed bonding type locknut and comply with both.
 
warmer said:
You can use a threadless connector and the locknut it comes with for compliance with 300.10, but to comply with the bonding requirements of this section you would need to also install a bonding bushing.
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But 250.92(B)(3) says a threadess connector is an allowable method. So I don't see how that conclusion fits the text of the NEC.

You seem to be reading 250.92(B)(3) as if the text "and connectors" were omitted.

Cheers, Wayne
 
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