12

Question:

What are the rules for how to attach a wire to an already grounded EMT conduit? Can I use stainless steel host clamp and tighten down?

Background:

I was installing a metal box for a light switch in our attic and had to terminate existing old NM cables that had no ground. Since there was no ground in the cable to ground the enclosure, a hot shorted to the box would be dangerous and would not pop the breaker.

As a temporary measure I used a metal hose clamp to run a bare #12 copper wire between the box ground screw and a nearby EMT conduit that was already grounded. The wire is never getting out, it is cinched under the clamp quite hard.

However, are there rules on what you are allowed to use fastened a wire to an EMT conduit for grounding? If so, then I can order a real ground clamp and go back up there, but if this is acceptable, then it would be convenient to leave it the way it is.

What I would have used if I had one:

2" grounding clamp

[ image source ]

What I used (at least temporarily):

hose clamp

[ image source ]

7
  • I am quite sure it the same as using plumbing pipe instead of electrical pipe for conduit. They look the same, they do the same job, but they do not quack like a duck the same. Temporary should be the time between holding the ground in place and going to the store to get the proper part.
    – crip659
    Commented May 13 at 20:36
  • 6
    In general you have to do things with parts and techniques that have been tested and approved. It doesn't mean your approach won't work, you just can't use it til it has gone through the painful approval process.
    – jay613
    Commented May 13 at 20:38
  • @jay613 You forgot 'and expensive' after painful, or how to add 50$ to a 50 cent hose clamp.
    – crip659
    Commented May 13 at 21:04
  • 3
    You should be using a retrofit ground method listed in NEC 250.130(C). Commented May 14 at 0:33
  • 1
    I am dubious about the explanation calling for the pressure of the contact to be greater than air pressure. It must be sufficient to maintain a firm mechanical contact. But I don't believe the physical effect can be what the writer thinks. Air pressure does not separate the wire and the bonding equipment, since it pushes from all directions. Commented May 16 at 5:24

3 Answers 3

10

To give a bit on insight, electrical connections are degraded by oxidation of the conductors. If two bare metal conductors have a good, low-resistance connection at first, the resistance between the two will get higher as air causes the conductors to corrode.

The most common way to prevent this corrosion is to have high-pressure contact between the two conductors. If the pressure between the two conductors is greater than air pressure, air can't get in between and cause oxidation.

The ground clamp has teeth that will cause an air-free connection to the pipe when the screws are tightened sufficiently. Likewise, the screw connection to the conductor will create an air-free connection when tightened sufficiently. This creates a long-lasting corrosion-free bond between the pipe and the wire.

A hose clamp cannot create enough pressure between the wire and the pipe to prevent corrosion forming in the interface between the two.

Go with the safe route. Install a proper ground clamp.

2
  • 1
    You are underestimating how tight hose clamps get, they are often used to tighten hoses on that need to withstand hundreds of PSI. Commented May 16 at 6:53
  • NEC allows hose clamp style connections. Not for what OP is using, but for communication grounding. If what you said was true, then why does 250.70 b allow them? This answer is not correct.
    – cde
    Commented May 18 at 0:37
17

There's the Code answer, and the practical one:

Start from NEC 110.2:

The conductors and equipment required or permitted by this Code shall be acceptable only if approved.

Approved means listed and often means labeled. If the hose clamp has a listing number, it meets that bar.

Then add in NEC 110.8:

Only wiring methods recognized as suitable are included in this Code. The recognized methods of wiring shall be permitted to be installed in any type of building or occupancy, except as otherwise provided in this Code.

If you find a Code section that allows you to clamp the wire directly to EMT with a hose clamp, that would be an approved method.

Most likely your hose clamp is not listed, and I'm not familiar with any Code provision that allows use of a hose clamp for grounding.

Now, the practical answer: if it's genuinely solid this will probably be fine. Hopefully the clamp doesn't corrode, the wire against the conduit doesn't end up with high resistance due to the interaction between the metals, and if it does you never need the ground. That's a lot of items which need to line up.

Personally, it would lead me to pick up one of the right clamps the next time I'm getting other electrical supplies, then replace my hack with the right item. In fact, I did exactly this with a ground wire last year. I used a hose clamp on it to get things up and going, then bought a clamp like the one in your picture and swapped it out a few days later.

6
  • Buy one of the correct clamps? I tend to buy three, or 10, or 1200 of a thing, depending on the likelyhood I'll spontaneous need another at a later job.
    – Criggie
    Commented May 14 at 22:41
  • 4
    I'm a home gamer - in ten years I've used three or four total of these ground clamps to fix things. Buying one at a time is fine.
    – KMJ
    Commented May 15 at 3:39
  • @Criggie - 1200? Just trying to think what I have used 1200 of??? Nails for the nail gun and drywall screws... can't think of one other thing. I am sure I have used 1200 4x8s of drywall. Man I would get a huge discount if buying 1200 of them and it would be nice to always have a sheet... Just need to convince wife to convert 3 car garage to drywall storage
    – DMoore
    Commented May 15 at 19:55
  • @DMoore A number of years ago I needed some ferrules for power cabling. It turned out a pack of 1200 cable ferrules was less than double the cost of a 40 pack. I only needed 3 at the time, so now have a lifetime supply. Since then I've probably used 40 or so.
    – Criggie
    Commented May 16 at 0:56
  • Approved does not mean listed. And can you find a code section that defines what types of specific grounding connectors are required?
    – cde
    Commented May 17 at 23:40
2

Sheet metal style strap clamps are allowed in NEC. Just not for what you are doing. Communications grounds for TV or satellite or telephone can use a listed version per 250.70(B). A non-listed hose clamp isn't enough though.

For outlets and other devices NEC 250.8(A) requires a few types of connections. Retrofitting is per 2014 or newer under NEC 250.130(C) allowing to connect to grounded emt.

Code limits you on how you can connect the equipment grounding conductor. You can weld it, you can use a terminal bar, screw/nut with atleast 3 threads or you have to use a listed device (meaning certified by a agency that your local inspector approves of). A hose clamp wasn't tested for electrical conductivity. Even though you know it works, I know it works, everybody knows it works, the manufacturer isn't interested in risking themselves to say it works.

Frankly, reading this you can drill a hole in your emt and use a self tapping machine screw and that would be enough. But just do it at a junction box instead to not get side eyed.

Or do what you already know, get a listed, designed for electrical grounding pipe clamp.

You don't need to buy the expensive bronze direct burial version though. A cheaper zinc one is fine for a dry location.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.