What is the difference between NM and SE.

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220/221

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I know the physical difference between #8 SE and 8-2 NM but what is it about SE that makes it acceptable to use as a (existing) range or dryer circuit while an existing 8-2 NM is not?

I understand 250.140 (3) but am asking ....why is SE OK and NM is a hazard.

My contention is that both have 2 insulated conductors, a bare ground and thermoplastic sheathing which USED to be acceptable but is not now (for obvious reasons).

I am open to all theories...theorys...whatever.

If your thoughts are "That's just the way it is, accept it without question", I have already heard that and I understand your position. If you have ANY ideas other than that, I'd like to learn about them.
 
I'll venture a couple of opinions:

First, the physical difference means that something conductive penetrating the cable will contact the grounded conductor first, similarly to concentric, shielded, coaxial MV cable. (What's in a name?)

Second, when the rule came about, NM had undersized EGC's, but an insulated grounded conductor would be full-sized. The bare conductor in SE was always larger than an undersized bare EGC.

Third, keeping in mind that the NEC is always evolving, it may be that the original 240v electric stoves and dryers had no line-to-neutral loads, and allowing that use was a change reflecting existing installations.
 
Keep in mind that the NEC is written by people that represent various interests. Larry gave you some good insight. Why is NM limited to the 60 degree ampacity rating? Why was it prohibited for years to install NM in any building more than three stories above grade? Why is NM restricted from being installed above a suspended ceiling in a commercial building, but not in a dwelling? Some of it makes little sense...
 
After reading that section I'm with 220 on this. My take is if it is existing and say a remodel, don't touch it until it's inspected. If it's not getting inspected I wouldn't worry about it. Mileage may vary in different jurisdictions.
 
220/221 said:
....I am open to all theories...theorys...whatever......
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I have an interesting question/statement coming from your questions.

I'm thinking of an 8-2 w/grd NM used in the scenario of 250.140(3). The white conductor (being used as a hot) would have to be re-identified to a phase color, and the bare equipment grounding conductor would actually be serving as a grounded conductor, so it would have to be re-identified with white tape, I guess. This, it seems to me, would create some confusing issues for untrained installers which could create potentially dangerous conditions down the road for future troubleshooters, etc.

Panel members may have believed that the make up of the SE cable was more easily understood since the concentric conductor was more readily recognized as a grounded conductor, and when used as a branch circuit having its' origin in the main panel, would only carry neutral current and serve as an equipment grounding conductor for the length of the BC directly to the Main service equipment.:smile:

So there may have been some of this type reasoning injected into the deliberations way back when these provisions were adopted.
This may have prompted this practice. Just a thought.
 
lpelectric said:
I'm thinking of an 8-2 w/grd NM used in the scenario of 250.140(3). The white conductor (being used as a hot) would have to be re-identified to a phase color, and the bare equipment grounding conductor would actually be serving as a grounded conductor, so it would have to be re-identified with white tape,
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Why would you have to do that? Of course it is hypothetical since we cannot use nm in this situation. When you use SE cable we don't identify the bare conductor with white tape.
 
As for existing installations, if I came upon, for example, a 10/2 feeding an appliance, I'd leave it if possible, unless it was non-compliant at the time of the original installation.

I've even seen paralleled 10/3's w/o ground used to supply ranges. I wouldn't change them unless required due to, say, an upgrade that rendered the existing panel a sub-panel.
 
Dennis Alwon said:
....... When you use SE cable we don't identify the bare conductor with white tape.
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I appreciate that. And when you use SE cable, I won't identify the bare conductor with white tape, either. :grin: ;)
 
Do you suppose that the insulation on the conductors gives it the higher rating or the construction of the cable?
 
220/221 said:
My contention is that both have 2 insulated conductors, a bare ground and thermoplastic sheathing which USED to be acceptable but is not now (for obvious reasons).
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NO. It NEVER used to be allowed. This is the major flaw in your argument.
PLEASE go back as far as you can in the NEC and you'll see the grounded conductor was ALWAYS required to be insulated unless SE cable was used.

Exception #3 in 250.140 says an existing range circuit can use a 3-prong receptacle IF the grounded conductor is INSULATED. The exception to that is if SE cable was used. Why? I don't know.

I also know the bare wire in NM cable was NEVER intended to carry current. To even suggest that it is OK to use the bare equipment grounding conductor in NM cable as a neutral is dumb at best and criminal at worst.
Using the logic that if it is OK for SE then it is OK for NM holds NO water IMO. It is expressly forbidden.


Why is SEU cable even still being used? Again, I don' know, but it is. And the bare braided grounded conductor is a current carrying conductor.
I do know the sheath on SE cable is tougher than NM, and the grounded conductor is many individual strands as opposed to one for the EGC in NM.
 
To even suggest that it is OK to use the bare equipment grounding conductor in NM cable as a neutral is dumb at best and criminal at worst.
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But....do you know why? That was my only question.





This is the major flaw in your argument.
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I am not arguing, I am merely stating my opinion and asking for other people's ideas. It is the basis for discussion and can lead to understanding and knowledge.
 
220/221 said:
But....do you know why? That was my only question.
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Specifically, no. But I have enough common sense to realize that current through a single BARE wire is not a good thing. Never was.

Why SEU was allowed is beyond me. Why do they still allow it for services? Obviously the grounded conductor has been considered safe to carry current for probably 60-70 years. Considered safe by folks way smarter than me.





220/221 said:
I am not arguing, I am merely stating my opinion and asking for other people's ideas. It is the basis for discussion and can lead to understanding and knowledge.
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Wow. No colorful words or insults. I'm impressed.
 
iwire said:
Does that mean we are not supposed to?

I am not sure, just asking....:smile:
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Well, if it is seu which is what I thought we were talking about what color would you tape the bare conductor and why. There would be no purpose in taping it white because what is the conductor-- grounded or grounding. In this case it is both, I presume.
 
Maybe my #2 reason in post #2 is why.
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I did read your post and appreciate your insight. I'd say it was the best I've heard yet.



Wow. No colorful words or insults. I'm impressed.
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You can continue to drag me into an exchange of insults on the other boards if you like.


I have enough common sense to realize that current through a single BARE wire is not a good thing.
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The issue can NOT be the BARE part as both have BARE conductors.

Common sense tells me that a single wire carrys the same current as a solid one. Is size the issue like Larry suggested? Could be. With all the SE I have stripped and made up I don't honestly know what size the bare wire equals. It seems like a 6 or 8 but maybe that's because it gets twisted up and looks bigger. I will pay close attention next time.
 
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