- #1
Leyden
- 39
- 4
- 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
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
Last edited by a moderator: