404.2

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George Stolz

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When the light is extinguished by flipping both threeways to the "common-to-hot" position, then the receptacle in the detached building (I'm thinking of the full schematic, with receptacles included) is supplied by two hot conductors.

They are smaller than 1/0, probably not the same length, and might not even be terminated in the same manner. :D
 

winnie

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I agree that this creates a parallel circuit conductor, and thus is a 310.4 violation...but there isn't a safety issue.

Generally when parallel conductors are use, it is to 'create' a single conductor with greater ampacity. This conductor is then protected by OCPD for the larger conductor.

If there is imbalance between the conductors being used in the parallel set, then one conductor might be overloaded without any indication at the OCPD.

But in this case, the conductors are properly protected by the connected OCPD, even if there is extreme imbalance between the two paths.

This is similar to the exception to 310.4 permitted for control wiring, where you can have parallel conductors smaller than 1/0 as long as the _individual_ conductors would be properly protected by the OCPD. However this is a home, and if you feed a receptacle it isn't control wiring, so the exception doesn't apply.

I'd worry about the EMFs (which are not a violation), but not about the 'parallel' conductors, even though that is a violation, and as I said before, if the cables involved are run side by side, there is no EMF concern. Running 2 x/2 cables side by side is probably cheaper than carrying x/3 and x/4 cables if you use such very infrequently.

-Jon

[edited to correct code reference number]
 
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George Stolz

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Jon, you bring up an interesting point I honestly hadn't thought of. Should the NEC permit parallel conductors that are protected at their individual ampacities? There is no way for the conductors of a travelling bus to be overloaded, if they are all supplied from the same OCPD.

Then again, is there another application (other than the bus) that would make use of this, or is this an isolated case?
 

winnie

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See 310.4 exceptions 2 and 3. The code already permits parallel conductors when the supplied OCPD is sufficient for individual conductors, in two cases: instrumentation wiring and high frequency wiring. I don't see why it shouldn't be permitted in this case as well.

I recall a discussion on some forum where someone wanted to run a common exhaust fan from several bathrooms, with a single pole switch run as a switch loop to each bathroom. We came up with several wiring methods that didn't even smell of violating 310.4, but the fact was that the simplest approach did appear to be a violation.

I don't see many situations where this is a problem, and you can always weasel around the words: a 'switch' is not a 'conductor', so these are not conductors 'joined at both ends to form a single conductor', and all of the _separate_ conductors present are protected at their ampacity.

-Jon
 

Jljohnson

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Colorado
I can remember, as an apprentice about 20 years ago, wiring a barn that had 5 sets of 3-ways. They were in 5-gang boxes at each entry to the barn. The master that I was working under had me run hot/N to the 1st box, take all 5 switchlegs from the 2nd, then run one 12/3 and four 12/2's from box to box. Made perfect sense to me since there really was no point in sending the N to the 2nd 5-gang switchbox more than once. Neutral and one set of travellers were sent on the 3-wire, the remaining 4 sets of travellers on the 2-wires.
 

Dennis Alwon

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Jim

Is that really any different from dropping a 2 wire down to a switch with the feed in the light. The conductor is just extended but my understanding is that would not cause unwanted EMF's. It appears that the neutral is being carried back with the condutors eliminating EMF's. Otherwise, we would have emf's everytime we hook up a dead end 3 way. ?????
 

al hildenbrand

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georgestolz said:
When the light is extinguished by flipping both threeways to the "common-to-hot" position, then the receptacle in the detached building (I'm thinking of the full schematic, with receptacles included) is supplied by two hot conductors.

They are smaller than 1/0, probably not the same length, and might not even be terminated in the same manner. :D
:confused:

This still doesn't make sense to me. Well. . .I get the sense of what you are saying, but. . .When I read 310.4 I see a set (a group) of all conductors "1/0 and larger" being talked about. The exceptions add conditions that add some more conductors to this group.

Then.

Then the conditions that this group of conductors must meet are described.

The general group of conductors smaller than 1/0, those not falling into the exception categories, are not mentioned.

Therefore, referencing my circuit sketch above, the parallel conductors aren't covered by 310.4.
 

al hildenbrand

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TwoWireTravellingBus3Way.jpg


Even more to the point: When both switches are down (as drawn), the Hot is paralleled in the same cable.

And when both switches are up, the two wires that are paralleled aren't used as conductors, so no current is paralleled.
 

iwire

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al hildenbrand said:
Therefore, referencing my circuit sketch above, the parallel conductors aren't covered by 310.4.

I agree that is how it reads but I don't see it as the intent or the way it is enforced.

IMO opinion only the conductors specifically referenced by 310.4 can be in parallel and then they must follow 310.4

It seems to me that to interpret as your suggesting would lead to a say a 100 amp feeder supplied by two 8 AWGs in parallel that did not have to follow any rules such as routing and characteristics.

We could start running British ring circuits. :smile:
 

al hildenbrand

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iwire said:
We could start running British ring circuits. :smile:
:) The ring circuit is just another form of the unfamiliarity that one finds in the "travelling bus" 3-way setup in my diagram.
 

al hildenbrand

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stickboy1375 said:
I was thinking of a violation of 300.20 but only if metal boxes are used and the cables enter the box through separate knockouts.
To that I would add: "only if metal boxes are used and the cables enter the box through separate knockouts and a notch is not cut between the entries (300.20(B)) and the conductors are not grouped to avoid inductive heating (300.20(A))."

That's a lot to show.

And, remembering the opening post, 300.20(A) is worded loosely enough that one can argue that any 15 or 20 amp 120 volt branch circuit doesn't have current densities great enough to cause inductive heating of any consequence. As a reality check, consider all the countless millions of existing switches and receptacles installed in black enamel painted steel wall cases wired with Knob & Tube. The K&T method has single conductors per box entry hole with no notches between holes.

The black enamel boxes are the oldest, having been installed in the first part of the 1900s. The current through them, in spite of decades of time, has not resulted in NEC changes to correct inductive heating on these 15 and 20 amp branch circuits.
300.20 Induced Currents in Metal Enclosures or Metal Raceways

(A) Conductors Grouped Together Where conductors carrying alternating current are installed in metal enclosures or metal raceways, they shall be arranged so as to avoid heating the surrounding metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be grouped together.
The continued service of all of the oldest K&T metal boxes on 15 and 20 Amp circuits, to me, is prima facie evidence that, even though the K&T single conductors enter separate holes in ferrous metal, the conductors are "grouped together" "to avoid heating the surrounding metal by induction."

300.20(A) only says "grouped together", not, specifically, "a common hole".
 

don_resqcapt19

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Al,
The continued service of all of the oldest K&T metal boxes on 15 and 20 Amp circuits, to me, is prima facie evidence that, even though the K&T single conductors enter separate holes in ferrous metal, the conductors are "grouped together" "to avoid heating the surrounding metal by induction."
Look at 300.20(B). The wires are required to enter through a common hole or have slots cut between the holes. As a practical matter, the heating is not a problem at the current level that would exist in K&T wiring systems. I have been told that the Canadian code does not require slots unless the current is in excess of 200 amps.
Don
 

al hildenbrand

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don_resqcapt19 said:
I have been told that the Canadian code does not require slots unless the current is in excess of 200 amps.
:smile: Interesting.
 
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