wiring in EMT conduit - induction or other issue?

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Old 03-17-07, 09:20 AM
ddr
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wiring in EMT conduit - induction or other issue?

I know that NEC article 300.20 states you must group the hot and neutral of the same circuit together when running multiple circuits in conduit (especially metallic tubing) because of induction. This in mind, are there any code violations regarding hot and neutral lengths of a single circuit running in different directions in the same EMT conduit as in the following examples (I know these arenít the easiest or ďbestĒ ways to run a circuit, Iím just wondering if they are legal):

A: The 12/2 feed for a workbench light and receptacles in an unfinished basement runs in conduit along a floor joist above, enters the receptacle box for a switched receptacle (for light) and pass through (no breaks) to a conduit running down to the workbench GFCI receptacle. The load side of the GFCI powers other receptacles and finally the switch for the light receptacle, with the neutral and switched hot going up to the switched receptacle using the same conduit the feed came down in.

B: Same as above except while the hot passes through the box for the switched receptacle, the neutral goes through the receptacle, then continues down the conduit with the hot to the GFCI. The neutral would terminate at the receptacle before the switch but a hot feed would go from that receptacle to the switch, then the switched hot would run back up through the same conduit to the switched receptacle which got its neutral at the beginning. (Would this layout cause induction the switched hot run and, if so, would it matter since it is only powering a light fixture?)

C. Several receptacles in a room are fed from wires run in conduit in a tight crawl space behind the wall. Instead of splicing the feeds for the receptacles in junction boxes in the crawl space, the wires feed from the conduit to a junction box, through the box to a conduit going to the receptacle where they feed through the receptacle and then back down the same conduit to the junction box where another conduit takes them to the next junction box and so on (so as to keep the connections in the receptacle boxes rather than in the crawl space).

Again, I know the above ideas arenít the most logical, Iím just curious if they violate any codes.

Thanks in advance for any input.
 
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Old 03-17-07, 10:39 AM
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The Eddy Effect that can cause inductive heating of metal parts will never be a problem as long as the conduits are run in branch circuit fashion.

Meaning one main trunk goes to boxes that T to other boxes. Regardless of if the conductor is a hot or a neutral does not matter. What matters is that the same current on the circuit flows in both directions inside the conduit with the same magnitude.

A problem does occur if there are parallel conduits or cables and the flow of current is going to the field device in one and returning to the source in another. A problem can also occur if a conduit is added directly from one branch box to another, cutting out the main trunk boxes.

Hope this helps. It is very difficult to explain.

On a side note: the same circuit amps going back and forth, even in the same conduit does add to the number of current carrying conductors in the conduit. Resistive heating is still a problem, even though inductive heating is not.
 
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Old 03-17-07, 05:39 PM
ddr
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jwhite,

Thanks for the reply. I didnít grasp all of it but I think I get the gist:

Hot and neutral wires run as in example C (going up to and coming down from a receptacle using the same conduit ) would balance (or cancel) each other regardless of the reversed direction and not cause inductive heating or magnetism but, as there are four current carrying conductors, the heat caused by resistance would be more than if there were just two feed wires going up to the receptacle. Correct?

As for example A, itís pretty much the same: if the light is off the feed wires balance each other and if the light is on the same current is added to the hot and neutral so they are canceled, again regardless of the fact that the flow of the two hots and neutrals are going in opposite directions. Resistive heat will be added. Correct?

But in example B, the hot wire from the switch is traveling the conduit to the switched receptacle which has the neutral feed attaching at the receptacle itself. Wouldnít this cause an induction issue since the hot is running by itself essentially as the neutral picks up at the switched receptacle itself?

Like I said, just curious how it would work. Would be interested if you (or anyone else) has the time, but nothing critical.

Thanks again.
 
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Old 03-18-07, 06:10 AM
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Maybe I misunderstood example B

Where is the origin of the neutral. Have you another conduit run from the panel?
 
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Old 03-18-07, 11:36 AM
ddr
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jwhite,

Thanks for the reply. Actually, I probably didnít make the example clear enough if the first place. I hope the description below clears things up.

The feed hot and neutral arrive in a conduit running along a floor joist above the workbench and enter a box for a switched receptacle (for the light) which is also attached to the floor joist.

The hot wire passes through the box for the switched receptacle and enters a conduit which runs down to the box for the GFCI. The neutral wire, however, connects to one terminal of the switched receptacle and then another neutral connects to the other terminal and proceeds down the conduit to the GFCI.

The hot and neutral daisy chain from the GFCI through the other receptacles. The neutral run terminates at the last receptacle before the switch. A hot runs from the last receptacle to the switch and from there the loop for the switch runs back through the boxes for the receptacles and up the same conduit to the switched receptacle.

The reason Iím wondering if this would cause any type of induction is that the current of the loop wire is not balanced by either the hot or neutral feeds: the neutral for the switched receptacle is the first connection made in the run, right at the switched receptacle.

Okay, wait, I think I just answered my own question:

When the switch is off, everything is balanced. When the switch is on, the looped part of the switch loop is carrying whatever current is pulled by the light but the hot run feeding the switch must supply that extra current so the feed hot and loop hot cancel each other as they travel in opposite directions in the same conduit. Right?
 
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Old 03-18-07, 02:38 PM
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I think you got it.

The only way this can get messed up is if the conduits or cables are not run in a branch fasson.

If you draw your cable run on paper and can make a triangle or square then it is possible to have a problem. If when you draw your run everything resembles the a main trunk with branches, then you cannot have a problem.
 
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Old 03-18-07, 04:04 PM
ddr
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Okay, I think Iím clear on it now. Thanks for all your time jwhite.
 
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