300.20 Gone Wild

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Smart $ said:
That's one interpretation.
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Smart $ said:
Could be construed as a conductor and/or its insulation as being the common partition or barrier. And that is the problem as presented.
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No, I don't think so. The NFPA Glossary definition excludes that (the "bolded" part of your statement) by saying that "grouped" conductors are NOT in continuous contact.


What is "common" to two conductors? Why, the container that both are within.
 
don_resqcapt19 said:
There is just not enough inductive heating at the lower current levels for this to be a real issue.
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Look at all the words in 300.20, not just specific ones taken out of context.

This section clearly requires that conductors be arranged to minimize inductive heating. As you have just stated, inductive heating is a non-issue below 200A so the spacing or twisting of conductors in N-M and MC cables inherently irrelevant.
Then the section continues on requiring that conductors be grouped. The definition of grouping, used in the NEC, does not require the conductors to be in actual contact. UL standards, among others, clearly require an amount of conductor separation (e.g. the A-B-C-N spacing of bussing within an enclosure means the A and N conductors are not grouped with each other).

For most part, I cannot remember seeing much in the way of mathematical formulas that are to be used to determine the magnetic field strength between conductors internal to a common 'ferrous' enclosure.
 
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iwire said:
Don you are tossing aside basic commonsense in this thread.

Do you really think the CMP that put that rule in place was so removed from the trade they asked for the impossible?


I find the whole notion just silly.
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But then comes today's legal application of any rule, no matter how simple it may seem someone will find the most unexpected version of interpretation and have some ground to stand on with it. It is how attorneys make their living in most cases:happyyes:
 
al hildenbrand said:
No, I don't think so. The NFPA Glossary definition excludes that (the "bolded" part of your statement) by saying that "grouped" conductors are NOT in continuous contact.


What is "common" to two conductors? Why, the container that both are within.
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I believe you are highly opinionated on the subject, to the degree you will not consider other viewpoints without bias.

The insulation of one conductor is common to all other conductors. That is its essential purpose!!!
 
Smart $ said:
The insulation of one conductor is common to all other conductors. That is its essential purpose!!!
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The insulation is invisible to magnetic field. . . . The border of the ferrous metal enclosure, however, is visible and common to the magnetic fields of each conductor. This is 300.20.
 
iwire said:
Don you are tossing aside basic commonsense in this thread.

Do you really think the CMP that put that rule in place was so removed from the trade they asked for the impossible?

I find the whole notion just silly.
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I believe that this is no different from many other rules, where the CMP really has no idea of what the words they accepted actually mean.
 
al hildenbrand said:
The insulation is invisible to magnetic field. . . . The border of the ferrous metal enclosure, however, is visible and common to the magnetic fields of each conductor. This is 300.20.
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True... but you don't get induction heating of the ferromagnetic enclosure unless circuit conductors are separated by distance (i.e. not in close proximity to each other along their length)... so the grouping will be with respect to the circuit conductors (and EGC per Code section).

FWIW, you can have induction heating of the enclosure when circuit conductors are separated by distance within the enclosure...

Say for instance you have a high-current, two-wire circuit that passess completely through a panelboard. You run one circuit wire clockwise from entry to exit on opposite side, and the other CCW. You will have induction heating of the enclosure... but that is if that's the only circuit. Energize other circuits with circuit conductos in the panelboard and you could get a lot of magnetic field cancellation, and usually do without even thinking about it... but you could also increase the heating effect under the right conditions... so this is where the thinking about it part comes in.
 
jim dungar said:
Look at all the words in 300.20, not just specific ones taken out of context.

This section clearly requires that conductors be arranged to minimize inductive heating. As you have just stated, inductive heating is a non-issue below 200A so the spacing or twisting of conductors in N-M and MC cables inherently irrelevant.
Then the section continues on requiring that conductors be grouped. The definition of grouping, used in the NEC, does not require the conductors to be in actual contact. UL standards, among others, clearly require an amount of conductor separation (e.g. the A-B-C-N spacing of bussing within an enclosure means the A and N conductors are not grouped with each other).

For most part, I cannot remember seeing much in the way of mathematical formulas that are to be used to determine the magnetic field strength between conductors internal to a common 'ferrous' enclosure.
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My point is that there are rules in 300.20 that require specific actions to be taken where single conductors are run through ferrous metals no matter what the current level is. There is no real issue with lower currents.
The following is from an IAEI magazine article.
Rule 12-3024(7) and (8) of the Canadian Electrical Code provides a solution to prevent harmful eddy current heating effects:

  • For loads up to 200 amperes, no special precautions are required;
  • For loads over 200 amperes, cables must enter all metal enclosures through a non-ferrous plate (normally aluminum), and any connectors, bushings, etc., that completely surround the conductors must be of non-ferrous materials.
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Smart, Don, the OP is about cable with twisted conductors being "required by 300.20" to remain twisted.

infinity said:
I recently had someone tell me that they've been told that it's an NEC requirement to keep the conductors of a multi-conductor cable (like MC) twisted together within a panel because of 300.20. Please stop the insanity.
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infinity said:
I guess that we now need a clear definition of grouped. But even so if the conductors are in a cable we're being told to leave them twisted together, if they're in a raceway then it's not required. BTW this came from a few different sources including an apprentice school teacher.

Will anyone take a shot at defining the word grouped as used in the section ActionDave posted?
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By this point, I am finding you both off the subject.

Smart $ said:
you can have induction heating of the enclosure when circuit conductors are separated by distance within the enclosure...

Say for instance you have a high-current, two-wire circuit that passess completely through a panelboard. You run one circuit wire clockwise from entry to exit on opposite side, and the other CCW. You will have induction heating of the enclosure... but that is if that's the only circuit.
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don_resqcapt19 said:
My point is that there are rules in 300.20 that require specific actions to be taken where single conductors are run through ferrous metals no matter what the current level is.
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don_resqcapt19 said:
I think it moved on to the requirement that the conductors be grouped within the enclosure and what "grouped" means.
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I get that. But with the OP focus on some entities enforcing the continued twist of multi-conductor cable once the sheath is removed as a requirement of 300.20 turns the focus away from single conductor or specialty two-conductor configurations to something much different, IMO.
 
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don_resqcapt19 said:
My point is that there are rules in 300.20 that require specific actions to be taken where single conductors are run through ferrous metals no matter what the current level is.
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No, the provisions of 300.20 only come into play when the arrangement of the conductors can lead to inductive heating. If the conductors, as installed, in a common ferrous enclosure or raceway do not result in inductive heating then, be default, their grouping (what ever it actually is) is acceptable to the NEC. However, if inductive heating is present, then the conductor grouping must be modified (i.e. reduced spacing) so the heating is no longer a problem.

Don, can you show us a sample calculation you use to 'cover yourself'?
 
Conductors shall be arranged to avoid inductive heating, and, "to accomplish this", the conductors shall be grouped together.

With your reference to the NFPA Glossary we have moved from "grouped" to adjacent. . .
 
jim dungar said:
No, the provisions of 300.20 only come into play when the arrangement of the conductors can lead to inductive heating. If the conductors, as installed, in a common ferrous enclosure or raceway do not result in inductive heating then, be default, their grouping (what ever it actually is) is acceptable to the NEC. However, if inductive heating is present, then the conductor grouping must be modified (i.e. reduced spacing) so the heating is no longer a problem.

Don, can you show us a sample calculation you use to 'cover yourself'?
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Jim,
I don't see the section being enforced that way. It is enforced any time there is a single conductor passing through ferrous metal. I don't see any inspector ever permitting a single AC conductor through ferrous metal, no matter what the current level is, even though the CEC clearly says there is no issue with currents less than 200 amps. I also don't see very many inspectors that would require grouping within a common enclosure.

I don't have any calculations as my installations comply with the way the rule is commonly enforced...that is no single AC conductor through ferrous metal.

As far as the original question, I really don't see any real world issue with inductive heating where all of the conductors are within the same ferrous metal conductors, but I see the first and second sentences of 300.20(A) as standing alone, and the second gives a prescriptive requirement, that the conductors be grouped without exception.

If that second part was not there, then I would agree that you would only need to take action where there would be inductive heating, and that is why I see no need for the second sentence.
 
don_resqcapt19 said:
J... my installations comply with the way the rule is commonly enforced...that is no single AC conductor through ferrous metal.
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What part of your installation techniques will you now be changing to comply with your arguments/positions in this thread?
 
jim dungar said:
What part of your installation techniques will you now be changing to comply with your arguments/positions in this thread?
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Very few of my comments have anything to do with how I would make the installation...they only have to do with what I believe the code actually says.
 
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