EMT as ground - For how much amperage?

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Doc13067

Doc13067

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Bucks County/Philadelphia, PA USA
I know that EMT can be used as a ground if properly bonded and secured at the fittings (250.118 (4)). Does anyone know if there is a limit to the amount of amperage running through the EMT without a grounding conductor in the form of a wire? For example, if I run a single light fixture with EMT and just have a hot and a neutral wire with the pipe acting as ground that is fine - could I also run 20 circuits with just hots and neutrals with the only ground being the EMT? Or is there a certain point where you need to run ground wires in the pipe?

What I’m trying to do is run circuits from a new panel to an old panel, all in one length of EMT, because the old panel contains all the branch circuits and the old panel is being converted to a junction box. Trying to avoid running grounds for every circuit since the pipe can be used as a ground.

keep in mind this is assuming all other variables are done properly so conduit fill, secure fittings, ground wires ran from boxes to devices etc. I am strictly asking about the ability of the pipe (EMT specifically) to serve as the ground and any limitations that exist. Can’t find this info anywhere. Thanks guys!
 
There is no NEC ampacity limit on the EMT being used as an EGC although there may be a practical limit on it's length.
 
Code considers EMT an acceptable EGC for any circuits that can fit inside the EMT.

There clearly are limits, but they are implied. You can't run an 100A feeder in 1/2" EMT, so 1/2 EMT is not a suitable EGC for 100A.

In general, because of the relatively large cross section, the EMT will be a better conductor than the wires it contains, even though steel is a poorer conductor than copper.

A side thought: for _paralleled_ installations, code generally requires that _each_ parallel EGC is sufficient for the _entire_ circuit rating. I bet that in a large installation with several large parallel EMTs that the individual EMTs don't meet this requirement, although I don't believe code addresses this.

-Jon
 
@Doc13067 as @electrofelon mentioned above you only need one ground wire if you do pull a ground for example. If you had 5 15am circuits, 1 30 amp circuit and 1 50 am circuit and 6 20 amp circuits you would only need 1 equipment ground sized for the largest circuit (the 50 amp circuit) your ground would be a #10 rom the neutral bar in the new service panel to a ground bar (bonded to the panel/junction box)

Or you can just use the emt.

#10 copper on the ground
 
Doc13067 said:
@Eddie702 I’d like to just use the EMT but I wanted to make sure There wasnt a limit on amperage. Interesting that it’s a better conductor than the wire. Makes life much easier not needing those wires in there.
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It is a better conductor than the wire because it has more cross sectional area than the wire you would typically run, and this even after considering that copper is a better conductor than steel.
 
kwired said:
It is a better conductor than the wire because it has more cross sectional area than the wire you would typically run, and this even after considering that copper is a better conductor than steel.
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Be careful here and this is the dark side. There is a limit that conductor length to be an effective ground. NEC just waves the magic grounding wand. Steel Tube Institute is probably the best data source on the subject. It definitely makes you question ever using PVC for anything electrical.
 
Please consider the data from the Steel Tube Institute and the Copper Development Association and whatever the equivalent aluminum association is, from what their lobbying intent is, and use your intellect above the minimum code.
 
My issue about pulling a ground (which I prefer) is the amount of EMT I see that is pulled out of connectors or poorly supported. But bad workmanship is just that
 
Eddie702 said:
My issue about pulling a ground (which I prefer) is the amount of EMT I see that is pulled out of connectors or poorly supported. But bad workmanship is just that
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Not just bad workmanship. The set screw has been questioned but if you push in one little spot on one side obviously lots of coverage on the other. Even with RMC though the fact is that conduit gets abused. A lot of plants have a minimum 3/4” RMC or IMC and should probably increase it to 1” EMT for good reason. Not that anyone should be doing this but IMC will support someone’s weight if properly supported every 6 feet if they step on it. 1/2” can be ripped down with very little effort. Either way the impedance is far lower than a separate ground wire just due to massively larger area. The problems are that fault currents are a problem with increased length in raceways irrespective of the design. The only major difference is that magnetic (steel)raceways do a lot better, which obviously the steel tubing guys heavily promote. Strength wise IMC is stronger and lighter than RMC but EMT still holds its own. If you’ve done much beam loading you know that OD has a much larger influence than wall thickness or material.’so EMT is light but not terrible. There are just terrible installs.
 
Eddie702 said:
My issue about pulling a ground (which I prefer) is the amount of EMT I see that is pulled out of connectors or poorly supported. But bad workmanship is just that
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Not really an issue in metal structure and metal framed buildings, raceways that are completely separated are still electrically connected by default, this is the reason 517.13(A) is more important than (B).

Note, bad workmanship can be found in wire connections

Roger
 
I don't know if stronger is the right word. I think RMC is more than strong enough to do its job. Compared to IMC, RMC is pretty soft, but so what. You are protecting wires not building a suspension bridge.

Bending and threading IMC is the worst. You have to over bend it just right right because it springs back so much and if you over bend it too much it almost impossible to tweak back. For threading you want the real deal Rigid pipe threader. If you try to use a chain vise and a power pony you'll end up spinning the pipe inside the vise instead of cutting threads, and if you manage to get it tight enough to not spin the pipe you'll throw the vise off its feet and on to the ground like a pro wrestler doing a body slam.
 
Doc13067 said:
Does anyone know if there is a limit to the amount of amperage running through the EMT without a grounding conductor in the form of a wire
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Well .. from what I've understood in the past, for one thing the fault current would run on the surface of the conduit not through it, that's just the nature of electricity, with that said I've heard that some conductors probably major utility power lines have at times been basically hollow tubing, because just like a conductor the circular mils of the diameter is what determines the current flow ease.

So off the hip I'd guess a 1/2" Emt is about the equivalent to a 2/0 although the material is not aluminum so that would be a factor with conductivity, my guess is probably roughly 150A of current carrying capability although that conduit would start to glow orange I'm sure.
 
Another C10 said:
Well .. from what I've understood in the past, for one thing the fault current would run on the surface of the conduit not through it, that's just the nature of electricity, with that said I've heard that some conductors probably major utility power lines have at times been basically hollow tubing, because just like a conductor the circular mils of the diameter is what determines the current flow ease.

So off the hip I'd guess a 1/2" Emt is about the equivalent to a 2/0 although the material is not aluminum so that would be a factor with conductivity, my guess is probably roughly 150A of current carrying capability although that conduit would start to glow orange I'm sure.
Click to expand...
See the links in posts #5 and #7

Roger
 
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