# Passing current through threaded nut and bolt



## EJPHI (May 7, 2008)

We ran into a problem where we may need to pass AC and DC through a threaded assembly.

Usually we use the threaded nut/bolt to press two conductive element together, but we may not have that luxury here.

Does anyone have experience with this problem?
Not NEC.
Female threaded copper to male stainless steel thread.
Temperature cycling for sure - heater power interface:vs_mad:

Many thanks in advance.

EJPHI


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## Wirenuting (Sep 12, 2010)

Not NEC you said?

Hmmm, a plumbing code maybe?

Please explain further.


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## EJPHI (May 7, 2008)

Internal to our product. So we follow other guidelines.
However, an NEC perspective would be interesting.


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## VELOCI3 (Aug 15, 2019)

Product? You would need to refer to one of the listing/safety bodies for a ruling. Underwriters Laboratories for example. 


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## EJPHI (May 7, 2008)

OK, but is there some experience here?


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## cuba_pete (Dec 8, 2011)

Your conductor in your assembly is your business, unless someone tells you different. 

I have equipment that does this, threaded metallic tubing that carries potentials from different conductors internal to the equipment.

Now, it’s military grade, but still has UL labeling from the 80’s.


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## em158 (Jul 7, 2016)

Is a steel bolt structurally necessary? Aluminum, brass, or silicon bronze fastener possibly?


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## splatz (May 23, 2015)

It's an interesting question, nothing I've done but there are two places I can think of in regular electrical installations where a threaded mating is the current carrying interface. Just on face, I'd be very surprised if a tight threaded connection wasn't an excellent connection. 

One is RMC, the threaded mating in the couplings is the connection. There are threads in the archives that discuss tapered threads versus straight threads for electrical connection. Occasionally @brian john mentions tests that show pipe is a better conductor than a wire, but I don't know if that test was done with RMC or whether there were couplings in the pipe. 

The other is the code that requires a ground screw make two thread impressions in the metal, with standard sheet metal boxes etc. 32 tpi makes it 24 tpi does not. I imagine there may have been some testing to base that rule, but it may have been just for mechanical security. However I wouldn't rule out that it is baseless and someone just pulled it out of their ass when they made the rule. 

I don't know about copper to steel threads. I just looked it up and apparently copper's coefficient of expansion is quite a bit higher than steel's. But it doesn't appear to make a difference with billions of screw terminal connections where a copper wire loop is under a steel screw. Maybe the elasticity of the metals is enough to make up for the expansion and contraction. 

I have to believe if you wanted to, you could add a spring contact that bears on the threaded screw shaft to make contact, as well as the threaded mating.


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## VELOCI3 (Aug 15, 2019)

splatz said:


> It's an interesting question, nothing I've done but there are two places I can think of in regular electrical installations where a threaded mating is the current carrying interface. Just on face, I'd be very surprised if a tight threaded connection wasn't an excellent connection.
> 
> One is RMC, the threaded mating in the couplings is the connection. There are threads in the archives that discuss tapered threads versus straight threads for electrical connection. Occasionally @brian john mentions tests that show pipe is a better conductor than a wire, but I don't know if that test was done with RMC or whether there were couplings in the pipe.
> 
> ...



RMC, IMC, EMT can use a set screw or compression connector for this also


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## paulengr (Oct 8, 2017)

If the bolt is part of a joint typically that’s when you use silicon bronze. Normally bus bars and other bolted connections rely on the bolt for tension, not conductivity. Some engineers spec silicon bronze because they don’t know that and some contractors don’t either and put steel washers under lugs! Use Belleville washers if you are worried about thermal expansion. The copper institute has details about this kind of busbar joint.


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## joe-nwt (Mar 28, 2019)

I've heard a 1/2" grade 8 bolt can carry about 800A before it starts to glow orange.....:vs_blush:


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## CoolWill (Jan 5, 2019)

It's hard to imagine a better connection than wrench-tight threads. Welded or soldered would be better, but not very versatile.

Use just a touch of this if you're paranoid:

https://www.specialized.net/cp8-tb-thomas-betts-kopr-shield-compound-8-oz.html

Edit: Also, I know that the coefficient of expansion for copper is larger than that of steel, not sure about stainless. Too lazy to look. But with a male copper to female steel connection, it will actually get tighter as the temp rises.


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## Jlarson (Jun 28, 2009)

Like for a pass thru connector for the heater power? I know on equipment we've built in the past that had similar connections we just used nord-lock washers and torqued for the given bolt size, adjusted for anti-seize since I'd always hit all that stuff with a coating of nickle anti-seize.


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