3

I'm trying to understand the exception in the NEC, subsection 250.32(B)(1), which gives permission to use the grounded conductor as the ground fault current path. Apparently, in earlier code cycles this was allowed, and I run across it in the field often where an outbuilding, like a detached garage or shed, has triplex (2 hots, 1 neutral) running to it as a feeder from the main panel in the house. It is also common on farms to have a central pole with a main panel and three-wire feeders branching off to feed each building. Normally, a single phase feeder like this should use four wires (2 hots, 1 neutral, and 1 equipment grounding conductor(EGC)) with the EGC not bonded back to the neutral ever again after the one time at the main disconnect. But, there does not seem to be an issue with using the three-wire setup if the grounded (neutral) conductor is allowed to double as the one designated fault current path back to the source. At each sub-panel, the grounded conductor, then, is bonded to the equipment grounding bar with a jumper and also to the sub-fed building's grounding electrode. All of that makes sense to me.

My problem is with the language used in condition 2, allowing the exception. It is confusing and does not seem to make sense when taken literally. It reads:

"(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved."

If it said "between the two buildings" it would make sense to me. But it doesn't. It says "in each building. I can't believe that the earlier editions of the Code required no continuous metallic paths in each building, and it doesn't make sense logically.

Up to this point, everything I've read from different community forums interprets the language as if it said "between," and their reasoning makes sense for the most part. Of course, there are different opinions out there that differ slightly. Some say the reason is that parallel ground fault current paths would be dangerous. Others, say that parallel neutral paths is the greater danger. It just bothers me that in the year 2023 we have something so dumb as this in the NEC, unless I'm missing something like a technical definition for continuous metallic path meaning between each building or something else that is obvious to everyone but me.

Am I wrong in saying that the language is wrong or too vague to be useful, or can someone please set me straight on this? I'm in a situation where I need to be able to explain this code to other people and all I can say right now is that it appears the language in the code should be worded differently.

0

3 Answers 3

7

I'm trying to understand the exception in the NEC, subsection 250.32(B)(1), which gives permission to use the NEUTRAL (in NEC lawyer-speak: "grounded conductor") as the ground fault current path.

There is no permission to do that. That is forbidden. It has been forbidden since NEC 2008 for inter-building feeders with no metallic paths between buildings. And before that for everything else.

That language is only there to clarify that the pre-2008 language did actually exist, and to state the terms under which it was then permitted. So that inspectors could distinguish between installations which complied with pre-2008 code and as a result "grandfathered", versus installations which never complied and are therefore not grandfathered. And also to cover new conditions which void the grandfathering.

But, there does not seem to be an issue with using the three-wire setup if the NEUTRAL ("grounded conductor") is allowed to double as the one designated fault current path back to the source. At each sub-panel, the NEUTRAL, then, is bonded to the equipment grounding bar with a jumper and also to the sub-fed building's grounding electrode. All of that makes sense to me.

That's fine as long as everything is working properly.

I'll refer to the combined conductor as PEN (Protective Earth & Neutral). That is a British expression, and the British are acutely aware of why combining them is foolhardy. They can't even properly charge EVs without bonkers safety systems.

Browse this forum for long enough and you'll notice we get a Lost Neutral question every other week. Why does that happen? Because the utility uses triplex to your home, and the bare wire is both the physical carrier wire (so it carries the brunt of metal fatigue) and also the neutral. And that isn't so bad because the utility doesn't supply ground.

"(2) There are no continuous metallic paths bonded to the grounding system in each building or structure involved."

If it said "between the two buildings" it would make sense to me. But it doesn't. It says "in each building. I can't believe that the earlier editions of the Code required no continuous metallic paths in each building, and it doesn't make sense logically.

OK. Here's what that was about, from the frame-of-mind of the pre-2008 rule.

Imagine it's prior to 2008. You have cable TV running between two buildings, with the jacket correctly bonded to safety ground in both buildings. Which remember, is bonded to the PEN wire. The carrier wire in the triplex, which takes the brunt of metal fatigue. So PEN breaks, and neutral current goes where? Through the cable TV jacket, of course!

So the code is identifying cases of you having a metallic "other thing" (gas pipe, water pipe, ethernet) that could potentially become an alternate neutral path. It is saying "if you have an alternate neutral path, PEN is forbidden and you must run a separate neutral and ground with them un-bonded in the subpanel".

As you can see, this does not help someone run triplex in 2023.

Much the opposite, it's bad news for someone who ran triplex prior to 2008 and now wants to run cable TV to the outbuilding. The crux of the above is to require them to retrofit proper ground.

And that's why it's still described in full in the codebook, so you know when new, unrelated work (copper water pipe) might break the grandfathering.

2
  • 3
    There you go, putting words in my mouth. I'm not looking for loopholes in physics, only trying to wrap my head around the wording. But anyway, this is pretty helpful! Thanks for the history lesson. Sep 22, 2023 at 23:51
  • 1
    @Nick OK then, removed. Sorry, about that. Unfortunately the British thing is not history, it's their current practice. Maybe they'll rethink now with EVs. Sep 23, 2023 at 18:22
2

You might be over-thinking this. I read it as, "there's no metallic path connected to grounding system at both ends."

Whether or not it's "between" the buildings would be satisfied by the context of requirement above the exception to be "run with supply conductors." In other words, this is about existing supply runs, not the individual building, and not a new supply run to a building.

2

The whole context leads most people to expect the net effect is that existing 3 wire subfeeds are only legal is there aren't parallel grounding paths.

The author/owner of the NEC, the NFPA, has no legal authority. Legal authority resides with the State and it's law making privilege. This is recognized by the 2023 NEC in 90.4.

90.4 Enforcement.

(A) Application. This Code is intended to be suitable for mandatory application by governmental bodies that exercise legal jurisdiction over electrical installations, including signaling and communications systems, and for use by insurance inspectors.

(B) Interpretations. The authority having jurisdiction for enforcement of the Code has the responsibility for making interpretations of the rules, for deciding on the approval of equipment and materials, and for granting the special permission contemplated in a number of the rules.

The only interpretation that counts is your local inspector. Call him and ask what and why, then you can use his answer to frame your conversation with the people you need to convince.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.