Grounding conductors and metallic enclosures

[Grouch1980]

Hey gang,
So my question this time is the following:

Suppose you have a feeder made up of 10 sets (EMT conduit), and it was upsized to 10 sets to account for voltage drop. According to NEC section 250.122(B), you have to upsize the equipment grounding conductors also, in proportion to the upsizing of the phase conductors. So now you have upsized equipment grounding conductors inside each set of conduit (EGC's).

1) Let's say you decide NOT to use the grounding conductors, and decide that the EMT metallic enclosure will serve as the grounding.... so you get rid of the EGC's inside the conduits. How does the metallic enclosure / EMT conduits achieve the same effect as the upsized EGC's? is it because we have more surface area with the 10 sets of conduits?
2) Why do you even have to upsize the EGC's (obviously i will anyway since it's required by code), if the metallic conduits can already serve as the path for ground faults? If I have an EGC inside the conduit, wouldn't the ground fault just travel across the EGC and the conduit together?

Just trying to understand the concepts.

 

[LarryFine]

Simple answer: As long as you bond the EMT correctly, don't run redundant EGCs.

Yes, conduit has proven to out-perform an appropriately-sized EGC for the conductors the conduit is sized for.

With a group of parallel feeders, each EGC is sized as if it was the only one for the entire group.

 

[tom baker]

EMT makes an excellent EGC, as it is very low impedance. There is a very good explanation of this in Soares Book on Grounding, and search IAEI Magazine for article on the test project.

 

[Grouch1980]

Simple answer: As long as you bond the EMT correctly, don't run redundant EGCs.

Yes, conduit has proven to out-perform an appropriately-sized EGC for the conductors the conduit is sized for.

With a group of parallel feeders, each EGC is sized as if it was the only one for the entire group.

Ah. That answers my 2nd question... yes, it's redundant. so if the conduit is not bonded correctly, you'll have the redundant EGC's upsized appropriately to carry the ground fault.

 

[Grouch1980]

EMT makes an excellent EGC, as it is very low impedance. There is a very good explanation of this in Soares Book on Grounding, and search IAEI Magazine for article on the test project.

and I think that answers my 1st question... EMT has very low impedance. so if you get rid of the upsized EGC conductors inside the conduits, the EMT STILL acts as an efficient pathway to bring the ground fault back to the utility, since it's low impedance. But the EGC conductors are NOT as efficient, so if they act as a redundant pathway, if the metallic enclosure pathway fails, then they have to be upsized since the conductors are not as efficient as the EMT enclosure. do i have all that right? i've heard of that book, i'll look into it.

 

[tom baker]

Note table 250.122 refers to a wire type EGC, if thats what you have in EMT, the note requires it to be sized larger for 250.4 A.

 

[Grouch1980]

yep, got it. thanks!