Using Conduit as EGC 1

[JTS]

We are working on a project where a new 3000A swtichboard is being installed at an industrial facility, but the customer wants to use the existing conductors and rigid steel conduit for the feeders to all of the loads. None of them have equipment grounding conductors (EGC's) installed. While using the conduit as an EGC is legal per code, in almost all of our installations we do not do it; however, in this instance it might be necessary.

We can't find any sizing requirements for the conduit to be used in this way. NEC 250.122 does not give a specific size for the conduit based off of the breaker ampacity. Does such a table exist that we are missing, or is there an industry standard that's out there?

 

[waross]

I have always assumed that if it is large enough to contain the conductors, it is large enough to serve as the equipment ground.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

SI this EMT or GRC? Then it can be used as an EGC no questions asked.

 

[7anoter4]

See:

https://steeltubeinstitute.org/stee...ites/4/2015/09/STIConduitInstallation2015.pdf

Annex B

 

[buzzp]

What Waross said. It is all handled when you size the conduit for the wire you are using. It doesn't matter if it is RMC, IMC, etc.

 

[GroovyGuy]

Howdy JTS,
Are these conduits installed above ground for their entire length (ie no underground portion). I am not sure of what the NEC says wrt U/G conduits, but in Canada (eh) the conduit cannot be used as an EGC if the conduit is installed U/G.
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[buzzp]

I don't think the NEC limits it to above ground only. Although, it does make some sense since corrosion over a period of time would render your EGC useless. I try to avoid any metal UG and is why we always use the wire type of EGC. It does add some labor and material cost but at least I know it will be there and I don't have to depend on the electricians so much to make sure it is made up correctly.

 

[7anoter4]

I think the following article could be interesting:

https://steeltubeinstitute.org/steel-conduit/wp-content/uploads/sites/4/2014/12/GROUNDIN.pdf

See Table 6.1 for instance.

 

[waross]

Table 6.1 seems a little strange.
For 3/4 inch Electrical Metallic Tubing with 4 x #10 AWG conductors, the maximum allowable run is more than Galvanized Rigid Conduit.
For 3/4 inch Electrical Metallic Tubing with 4 x #8 AWG conductors, the maximum allowable run is less than Galvanized Rigid Conduit.
Is this effect related to possible magnetic saturation of the conduits?
Comparing the results with rule 10-002 and table D3 of the Canadian Electrical Code, it appears that for loaded circuits it is safe to use the conduit as a bonding/grounding conductor.
In the case where a circuit is lightly loaded and the length based on the allowable voltage drop is increased, there may be issues, despite being allowed by the Code.
I did find the information interesting.
Thanks 7anoter4. lps

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

If any of those machines have power electronics on them, such as VFDs or Servo drives, this is a horrible idea. They will eventually end up with common mode noise issues that will affect not only the machinery with the electronics, but other equipment as well. The NEC is only about minimum standards to prevent fires, it's not a best practice guide.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

I work to different codes, but the basic principles and materials involved are similar. We're having a tormented time at site where galvanised steel conduit was installed many years ago in a code-compliant installation, but years later the galvanised coating is failing and we are seeing loss of any effective earth return path. It's primarily the outdoor runs which are suffering most, but we've had some crappy results in internal installations.

Given the choice I'd install the earth continuity conductor, at least on the bigger circuits where there is less room for the earth loop impedance to rise due to corrosion and oxidation before the maximum disconnection time under fault conditions is exceeded.