resistance in circuit

[innov]

Hi, I installed light fixtures in a large facility using a pre-fabricated cabling system consisting of mc cableand plugs at each fixture.
The cables come in standard lengths, which was about 5 feet too long for the span I'm crossing, so I coiled and wire-tied them above each fixture (2 loops). I now have 14 of these windings; what I'm worrying about is this:

Am I creating any neglible resistance?
And will there be any neglible inductance?

I'm running 480/277 volt multi-wire circuits,.83amps/fixture @277v in 12 ga. wires.

 

[waross]

The short answer. NO
The induction is negligible and what there is is self cancelling.
The 14 x 5' = 70 feet of cable is not a problem by itself, but you may want to check the voltage drop tables to determine the maximum footage allowable for your circuit configuration.
yours

 

[innov]

Thanks for the quick response

I did calculate the voltage drop, and am getting under 1 percent VD.

Can you explain how the induction cancels itself?

 

[waross]

The current in the neutral is opposite and equal to the current in the hot wire. The magnetic fields created by these two equal and opposite currents cancel each other. This holds true for single phase, two phase wires and the neutral of a three phase circuit and a three or four wire three phase circuit.
No net induction. No problem.
yours

 

[innov]

You got me thinking now.....you said they cancel each other out; that means they do create an inductive current, which would impy I will get a net loss through my circuit. Is that correct?
just curious

respectfully

 

[Skogsgurra]

No. As Waross says. No inductive current (should be inductive voltage drop).

A cable is in practice a bifilar winding. And as such not having any inductivity. At least not one that increases when the cable is coiled up.

Gunnar Englund

http://www.gke.org

 

[innov]

thanks, skogsgurra and waross


Maybe I didn't make myself clear enough; my circuit consists of 2 (hot) wires and 1 grounded (neutral) wire.

In a perfect world the 2 hot wires will carry all of the current, and the neutral wire none at all. This would be 2 circuits flowing in the same direction, and thus amplifying the inductive current.
Being this is not a perfect world, I imagine the neutral wire will have to carry any unbalanced portion that may exist. This is what would (would!!) cancel the inductive current.
Am I right here?



I copied the following out of an encyclopedia


Some bifilars have adjacent coils in which the convolutions are arranged so that the potential difference is magnified (eg., the current flows in same parallel direction). The magnetic field created by one winding is multiplied with that created by the other, resulting in a greater net magnetic field.

Others are wound so that the current flows in opposite directions. The magnetic field created by one winding is
therefore equal and opposite to that created by the other, resulting in a net magnetic field of zero (eg., neutralizing any negative effects in the coil). In electrical terms, this means that the self-inductance of the coil is zero.


respectfully,

 

[jraef]

Without getting into the math, you're trying to make a mountain out of a mole hill. IF there did end up being any inductance, the amount produced by 2 loops would be so minuscule as to probably not be measurable without the most sensitive of instruments. Its net effect on your light fixtures would be close to nil as said earlier. The extra wire resistance would be more significant, and you already know that to be insignificant.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

Think of it this way: for there to be any net current to cause inductive volt-drop, the vector sum of the three currents would have to be non-zero. Assuming that the circuit is not faulted to earth, whatever goes down the live wires must return up either the other live or the neutral. Otherwise Mr. Kirchoff is wrong, and that doesn't seem likely!

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I don't suffer from insanity. I enjoy it...

 

[waross]

The current in the hot wires is not in the same direction. It is in the opposite dirrection and so it cancels.
If the current in the hot wires is not equal, the difference will flow in the neutral in such a direction as to cancel.

Otherwise Mr. Kirchoff is wrong, and that doesn't seem likely!

yours