How many lock nuts are required at each end of IMC or RMC conduit that enter an enclosure. One outside and one inside of the enclosure or just one inside?

I always have installed 2 (outside and inside), but i vaguely remember that i had read somewhere that only one is required on the inside.

I could be wrong.

I'm lookin @ 342 & 344 and I don' t seewhere locknuts are required on both sides

You may be thinking about this



Some inspectors want this whenever the raceway is the sole bonding means on any line voltage circuit.

Am I missing something? Installing IMC/RGC into an enclosure, IF you don't have a locknut on the outside what prevents the raceway from falling into the enclosure?

the straps and the other end of the conduit that is held in place by one locknut in the interior of the other box.

don't get me wrong i always install 2, but was just wondering if it is a code issue or UL issue to have only one.

The words in 300.10 Make it very hard to install RMC or IMC using only one locknut (assume it's not a bonding locknut) and maintain continuity of the grounding conductor if the raceway is the grounding conductor. I would not approve an installation of RMC or IMC using a single standard locknut on the inside of an enclosure and nothing on the outside.



300.10 NEC 2008 Edition

George:

This part will work for me.

From 300.10 (2011)
"....shall be mechanically secured to boxes, fittings,
cabinets, and other enclosures."

Niko:
Using your scenario above, it sounds like a heck of a lot of work to save 2 locknuts. I have to say, I have not had anyone try that kind of install.

Could you apply the same mechanical logic with EMT fittings, without locknuts inside the boxes, and the shoulder of the connectors making contact??

GRC needs two (2) locknuts at each connection, unless the terminus is hubbed.

And don't forget your ( plastic ) bushings, too. I'm amazed at how often they're forgotten.

( GRC nipples require bushings across the entire range of wire sizes. 344.46 )

This also applies to running threads.

-------

That you are even spending one second on this line of inquiry -- is a miss-application of time.

One does NOT make it in the trades by trying to value engineer out locknuts.

It is in the nature of the beast that electrical contractors always have a surfeit of said locknuts since so many connectors end up terminating in PVC female adapters. ( F/As )

Hence, locknuts hold nothing but Scrap Value -- particularly now that China is exporting steel fittings on the cheap.

Guys dont get me wrong i always install outside and inside. furthermore i agree it would be a waste of time as tesla said.

I also agree what 300.10, but if a single lock nut is used and the conduit is secured then i can't fail the inspections.

Sure you can. "secured" is one of those terms that end up being defined by the AHJ. They could easily call this "loosely jointed" and want to see bonding jumpers with bushings. Lock nuts are cheap, Use 2 and make everyone happy.

Niko:

Do you have a comment regarding my EMT question?

As to failing inspection for one locknut,I would definetly write a red tag, citing 300.10.

well..... not really

I am guessing the one lock nut guys assume the tension between the 2 boxes will maintain the intimate contact necessary. I guess they do not understand the (thermal) linear expansion thing.

Actually I was surprised that the "two locknut" language was only in the over 250v article. I have heard it so often I thought it was any time you used RMC/IMC directly into a box through a KO. Usually when I saw threaded steel pipe it is going into a hub tho.

It's a interesting discussion. If I have a little down time, I'm gonna do some research on that. Any ideas on the age of the rules calling for only a single locknut

Sparkyinak:
There is no 'rule' stating 'one locknut'!

This is one of the 'can I get away with this' debates, IMHO.

Let this tale put things in perspective ...

In 1967, a very competent electrician - the firm was the same one that got the "Ufer" into the NEC - installed several panels side-by-side. The panels were connected to each other with nipples and locknuts.

Thanksgiving, 2004 .... I get the call that this customer has lost power to half the building and UPS alarms were going off. Many 'surge suppressor' power strips had failed.

Ever look at the obvious for hours and not see it? Fail to see the elephant in the room? That's how I spent my Thanksgiving.

What had happened was .... the guy had not separated the ground from the neutral in the adjoining panels- instead, treating the whole assembly as a 'main panel.' Over time, simple corrosion and vibration had set the stage for one of those nipples to lose good contact with the sheet metal. High impedence ground = open neutral.

Power down, separate the neutrals from the grounds, run a few feet of wire between the panels ... and all was good. The IT department noted that all their 'power quality' issues had also disappeared. Imagine that.

This happened with opposing locknuts on each enclosure. Now, do you really want to claim that one locknut, alone, is enough to secure things?

If this was all a "main" panel, those were service conductors and he needed bonding bushings to start with. Also the grounded conductor needs to be a wire, it is only grounding conductors that can be pipe.

Reminds me of a 30 amp, 120 volt circuit run with only one (1) conductor, using the EMT for a 'neutral'!!

Really nice arcing when that big ol' AC was running!

Thanks gfretwell For 250.97 I have always used 2 locknuts on RMC but just did cuz that is how I was taught. I never new the NEC Art. B'Fore

An emt connector is installed with one locknut, but it's considered a bond - only one locknut. With rigid pipe it was the norm to install a steel bushing on the inside and tighten the single locknut on the outside. One locknut is a bond, but something needs to be on the other side.