Parallel Equipment Grounding Conductors, Short Circuit

[Leyden]

TL;DR Summary

Seeking information on parallel equipment grounding conductors fault characteristics.

I am seeking direction, information, resources and thoughts on the following subject. The common interpretation of the National Electrical Code does not allow you to use multiple smaller equipment grounding conductors in place of one larger one (except in a few cases such as a cable assembly, this thread is specifically dealing with single conductors in conduit). So, could anyone point me to any information, studies, etc. that would lead me to believe either this standard is appropriate or an overburden?

The issue I see at hand is fault current and the level at which a particular size of wire fuses/melts/fails. I think the major factor in the question is inductive reactance, some things that I think will play a large role in this are the proximity of the the equipment grounding conductor to the ungrounded conductors under fault and the proximity to grounded equipment foreign to the designed circuit (local metal objects), also carrying varying levels of fault current.

Some background information; You are allowed to use multiple smaller wires for ungrounded current carrying conductors. I believe the thought behind the holdout on equipment grounding conductors is because it is the safety conductor, so it is more safe for an ungrounded conductor to fail than an equipment grounding conductor.

Under the common interpretation of the standard, an equipment grounding conductor larger than the ungrounded circuit conductors could be required under certain circumstances, another less commonly accepted interpretation is that the ground does not have to be larger than the circuit conductors(in more common installations the ground is smaller than the circuit conductors).

It is already a related requirement for conductors to be able to withstand the available fault current but the basis of the question is the fault current dividing among the parallel equipment grounding conductors. The equipment grounding conductor size is based on the size of the overcurrent protection device, if you have parallel feeders say twelve sets of 600kcmill copper circuit conductors you may need a 800kcmill copper equipment grounding conductor in each of the twelve conduits.

Also, this is an old code requirement and has been contested through the years, the only legitimate argument I have seen to keep it is not an argument based on science that it should be required, but that to get rid of it requires clear scientific proof that it should not be required, that fault current will divide adequately and one parallel conductor will not bear too much of the brunt and fuse.Thank you

 

[anorlunda]

There's a pretty good discussion here https://iaeimagazine.org/magazine/2...ductors-for-parallel-conductor-installations/

including the criteria for exceptions to the rule.

 

[dlgoff]

There's a pretty good discussion here https://iaeimagazine.org/magazine/2...ductors-for-parallel-conductor-installations/

including the criteria for exceptions to the rule.

Yes. A good discussion. I thought I'd add this image as a reference.

image compliments of https://www.ecmweb.com/qa/stumped-code-installing-circuit-conductors-parallel-separate-raceways

 

[Dullard]

If I were forced to defend the status quo:
A connection failure in 1 (or more) power conductors in a multi-conductor feed may result in conductor overheating or system performance issues. That's bad, but probably won't kill anyone. A similar connection failure in safety grounds may prevent a breaker trip and/or allow a substantial voltage (to ground) to exist on things that are theoretically 'grounded.' Most important, the ground issue is nearly impossible to detect, unless you go looking for it - that usually happens as part of the accident investigation / lawsuit.

 

[Leyden]

I agree, i believe i mentioned that in the original post. but there is already another requirement which both the equipment grounding conductor and the ungrounded conductors must both be able to survive the available fault current, so really if each of the grounds need to be that big then the ungrounded circuit conductors should be as well.

Technically I see a good argument that the ungrounded should be the bigger ones. For example, say an ungrounded circuit conductor of a parallel feeder installation somehow severs and the upstream side connects to grounded metal equipment , the entire available fault current is borne on the individual wire, while the current on the equipment grounds will divide(probably somewhat evenly, but I'm not sure enough about this part, this is the part the entire thread is about). just to try to illustrate the extreme, NEC allows you to parallel 1/0, so say the scenario has 10 parallel conduits of 1/0 ungrounded circuit conductors and 500Kcmil equipment grounding conductors in each conduit. a single 1/0 of a long run may posses enough impedance for a system like that to keep the protective device from opening, all while possibly introducing deadly voltage gradients in the area.