Parallel Equipment Grounding Conductors

[stevee]

This may seem like a stupid question, but I'm looking for NEC clarification. Bear with me, this info was brought to me second hand so the details may seem a little sketchy.

One of our electricians is working on a school project. They are installing EMT to feed a couple of different loads, a lighting circuit (15A) and a receptacle circuit (20A). Both are supplied from the same panel.

The blueprints call for two separate equipment grounding conductors. One for each circuit. A #14 for the lighting and a #12 for the receptacle circuit. The electrician did submit an RFI and was told this was how it was to be installed but was not told why. The receptacles being fed also are not Isolated Ground Type.

In the NEC I was unable to find any section that specifically prohibits the use of two separate EGC's in the same conduit. But I was taught that using two equipment grounding conductors in the same circuit would cause additional impedance in the grounding circuit, hence, causing the OCPD to operate slower under ground fault conditions due to the increased impedance of the circuit. This would seem like an unsafe practice to me, but cannot find any specific code section that prohibits this practice.

I know 250.122(C) describes using a single EGC for multiple circuits. This is the method that I have always used.

Can anyone tell me if the practice of using multiple EGC's in the same conduit is prohibited by the NEC. If so, what section? And also what your personal belief is of using this practice.

 

[M. D.]

Not prohibited, might be a bit of a waste copper , but that's it

 

[rcarroll]

The code does not prohibit multiple egc's in pipe. Thus, it's allowed. Now since the conductors are run in EMT, an equipment grounding conductor, you have three egc's. No problem.

 

[charlie b]

This may seem like a stupid question, but . . . .

Do not be concerned, as there is no such thing.

I was taught that using two equipment grounding conductors in the same circuit would cause additional impedance in the grounding circuit, hence, causing the OCPD to operate slower under ground fault conditions due to the increased impedance of the circuit.

You were taught wrong, I fear. If the two wires are connected in parallel (not the case in the installation you described, since they are not connected at both ends), then the total impedance always goes down when conductors are in parallel. If the two are only connected at the panel, then the existence of one has no impact on the other, even though both are in the same conduit.

 

[winnie]

I see no way, on the scale of a school building at power line frequencies, that having a second EGC could actually _increase_ fault impedance and slow OCPD operation.

For the circuits you describe, the vast bulk of circuit impedance is the simple resistance of the conductors. Put another conductor in parallel, and you lower impedance.

It is true that if the EGC does not follow the same path as the circuit conductors, you can get some serious inductance and a change in fault current. Additionally if the frequency is high enough (or the circuits long enough) you could see transmission line effects where stubs of one circuit might change the impedance of the other circuit. But I don't see either of these being a significant factor.

I totally agree with M.D. It's a waste of copper. It increases conduit fill. It will make splicing the EGCs a pain. You might want to find out if the CDA has bribed the spec writer

-Jon

 

[stevee]

You were taught wrong, I fear. If the two wires are connected in parallel (not the case in the installation you described, since they are not connected at both ends), then the total impedance always goes down when conductors are in parallel. If the two are only connected at the panel, then the existence of one has no impact on the other, even though both are in the same conduit.

I agree that the total resistance should go down when using conductors in parallel but what about added impedance due to capacitive reactance? Aren't two conductors separated by an insulator (air) the definition of a capacitor. If so wouldn't the capacitive reactance created cause additional impedance?

I've never run a test but I suppose it is possible that the added reactance and the reduced resistance cancel each other out and the overall impedance could remain close to the same.

By the way, I agree with all of you that it is a waste of not only copper but the increased labor costs associated also. The electrician who asked me the question wasn't sure why they wanted parallel conductors he only knew that they wanted it done that way.

I assumed it was not prohibited but was unsure if it was, so that's why I asked. But I do appreciate the feedback. It seems as though there are several different thoughts when it comes to the topic of electrical "theory." I suppose that's why they call it "theory."