ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

I do not believe there is any blatant danger in using EMT to protect the GEC.

The reason this EMT is required to be bonded to the conductor has to do with impedance.

It is possible that the metal raceway could increase the impedance of the GEC under fault conditions.

The raceway could get hot if enough continuous current is flowing through the EGC. Bonding the ends of the raceway will not have any effect on this type of heating.

Bob

Because of magnetic fields, the impedance of a GEC with steel conduit that's not bonded at both ends is approx. double that of a properly bonded GEC installed per 250.64(E)

Scott where did you get your figure of double?

I have seen some knowledgeable people state the effect is negligible, certainly not double.

Not busting your chops, I really want to know the truth and am curious to what you have seen.

I do agree it should be bonded, no matter what else the code requires it to be bonded.

I still do not see a terribly dangerous situation.

I was actually coming back here with some code references but you already provided one of them.

Here is another, 250.64(B) tells us that when the GEC needs to be protected from physical damage it "shall be in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing, or cable armor."

Bob

I agree with you Iwire.
Scott,
I find that idea hard to believe really.
The reason I say that is, because in any healthy Electrical Installation the GEC shouldn't be carrying any current anyway.
It's only under Fault conditions that the GEC should carry current (Fault current).
That's what causes a CB to trip.

This is for an Unhealthy System with current flowing on the GEC.

From "Soares Book on Grounding":

Metal enclosers for grounding conductors shall be continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. If mechanical protection is required for the grounding conductor it shall be a non-magnetic enclosure such as pvc conduit. The reason is that the increased impedance in the grounding conductor, by itself in a metal raceway, could so restrict the flow of fault current that a great deal of damage is done before a fault could be cleared, if it was cleared at all. This information is from the canadian electrical code, but the theory is the same anywhere.

The theory is the same but the code is not.

We can use metal conduit and it does not have to be continuous if we bond it at the ends.

Bob

What size is that wire?

I see no bushings

Bushings are not required for physical protection of grounded conductors. 300.4(F)

Just to make life easy.... I use schedule 40 for things like this Not to mention the corrosion factor for my area... If necessary I just use the armored ground wire with the proper clamps, nice & clean

Randy

I was watching a video Mike Holt produced, and he claimed to have read from IEEE that the inductive choke can be as high as 97% on this type of installation.

I've never seen that figure personally, but that is what Mr. Holt says.

Ryan hit it on the head. When I was still teaching high school electricity we had a video that had a bit about a hi amperage short through a metal pipe. The choke was so great it blew a hole in the pipe and the blew the wire in half. With the bushings/bonding in place the pipe becomes a part of the wire and the choke factor goes away.

Cool guys, I had heard from Bennie and an engineer that the choke effect was not as much of a problem as the NEC would have us believe.

Regardless of that the code requires the bonding so bond I will.

Bob