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Joined: Apr 2002
Posts: 7,382 Likes: 7
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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.
John
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Joined: Aug 2006
Posts: 356
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Do you have a comment regarding my EMT question?
well..... not really
Be kind to your neighbor, he knows where you live
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Joined: Jul 2004
Posts: 9,931 Likes: 34
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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.
Greg Fretwell
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Joined: Jul 2007
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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
"Live Awesome!" - Kevin Carosa
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Joined: Apr 2002
Posts: 7,382 Likes: 7
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Sparkyinak:
There is no 'rule' stating 'one locknut'!
This is one of the 'can I get away with this' debates, IMHO.
John
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Joined: Jan 2005
Posts: 5,445 Likes: 3
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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?
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Joined: Jul 2004
Posts: 9,931 Likes: 34
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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.
Greg Fretwell
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Joined: Apr 2002
Posts: 7,382 Likes: 7
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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!
John
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Joined: Mar 2007
Posts: 101
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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
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Joined: Mar 2004
Posts: 947
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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.
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Posts: 57
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