Excessive Heating?

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The Iceman

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I'm confused on 300.3 (B). I understand that all conductors of the same circuit including grounded, and equipment grounding conductors, need to be in the same raceway. I also understand doing this reduces heating especially when run in a metallic raceway. What about, for example, if a 3/4" EMT raceway was run to a device box, from the panel, with six separate switches all on separate circuits, then all the switch legs were run back through the same 3/4" EMT back to the panel, then to six separate lights. If no grounded conductors were run in this 3/4" EMT raceway, to cancel the magnetic fields, wouldn't that cause excessive heating?
I was asked to do this on a service job a few weeks ago and I asked this same question. Nobody could give my an answer. I think I was told to get my head out of that darn code book and just doit. :-?
 
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The Iceman said:
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If no grounded conductors were run in this 3/4" EMT raceway, to cancel the magnetic fields, wouldn't that cause excessive heating?
:-?
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Branch wire multi circuit should always be derated...( let me quailify that with individual wires that are not in a bundle wrap IE Romex but in a conduit)

The magnetic field is canceled in which the aspect of a complete circuit is used hot and neutral,
the Ground should not be caring current, and is run with any modern day service , if not install one.

:-? < > not at a Code Book
 
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480sparky said:
'Cause there isn't one.
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i went to a seminar last april on the proposed code changes for the 2008 cycle. one change they were talking about is they wanted us to run a grounded conductor to all switches because a lot of low voltage lighting controllers need a neutral. it was never put in the 2008 nec.
 
The Iceman said:
If no grounded conductors were run in this 3/4" EMT raceway, to cancel the magnetic fields, wouldn't that cause excessive heating?
:-?
Click to expand...

Although the switch legs are run in separate conduits just think of the feed and switch legs as an extra long conductor, ie the ungrounded conductor is just longer than the grounded conductor. The grounded conductor is run with the ungrounded at the beginning and end of the circuit. I don't believe emf's would be a problem here.
 
When a normal circuit is run with the 'hot' and 'neutral' conductors, then the current flow on the hot and neutral will be of equal magnitude but opposite directions. The _net_ current is zero, the magnetic field cancels out, and you avoid heating the conduit.

This generalizes to circuits of more than two conductors, eg a 120/240V range circuit, where the bulk of the current is flowing on the two hot conductors, suppling 240V for heating, but a bit of current is flowing between one of the hot conductors and neutral. If you look at the total of all current flowing on all of the conductors at the same time, the net current will be zero.

In the case of a properly installed 'switch loop', you don't have the 'neutral' conductor. You simply have the unswitched hot feed to the switch, and the _switched_ hot feed from the switch back to the circuit. But think about the direction of current flow in these two wires. Any current flowing toward the switch on the hot feed is balanced by current flowing from the switch to the load. The _net_ current is zero, the magnetic field cancels out, and you have no heating of the conduit.

-Jon
 
winnie said:
When a normal circuit is run with the 'hot' and 'neutral' conductors, then the current flow on the hot and neutral will be of equal magnitude but opposite directions. The _net_ current is zero, the magnetic field cancels out, and you avoid heating the conduit.

This generalizes to circuits of more than two conductors, eg a 120/240V range circuit, where the bulk of the current is flowing on the two hot conductors, suppling 240V for heating, but a bit of current is flowing between one of the hot conductors and neutral. If you look at the total of all current flowing on all of the conductors at the same time, the net current will be zero.

In the case of a properly installed 'switch loop', you don't have the 'neutral' conductor. You simply have the unswitched hot feed to the switch, and the _switched_ hot feed from the switch back to the circuit. But think about the direction of current flow in these two wires. Any current flowing toward the switch on the hot feed is balanced by current flowing from the switch to the load. The _net_ current is zero, the magnetic field cancels out, and you have no heating of the conduit.

-Jon
Click to expand...

Thank you for explaining that. It is exactly what I thought
 
electricalperson said:
i went to a seminar last april on the proposed code changes for the 2008 cycle. one change they were talking about is they wanted us to run a grounded conductor to all switches because a lot of low voltage lighting controllers need a neutral. it was never put in the 2008 nec.
Click to expand...

Not sure how things are were you are but i only wire for whats being installed today.
 
electricalperson said:
i went to a seminar last april on the proposed code changes for the 2008 cycle. one change they were talking about is they wanted us to run a grounded conductor to all switches because a lot of low voltage lighting controllers need a neutral. it was never put in the 2008 nec.
Click to expand...

The low-voltage industry has been proposing that every code cycle for years.
 
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