How not to create a magnetic field.

[Electric Sam]

I have 8 - 20 amp circuits in a j box 25' above finish floor. Both hots & neutrals are there. I'm installing 2 - 4 pole contacts in an enclosure to control the circuits. I would like to install the contacts lower about 15' run away from all the neutrals. If i extend the feeds & the loads in the same conduit with only 1 neutral for the 110 volt coil, will the magnetic field be zero because the feed & load are at 180 degrees?

 

[JoeStillman]

I have 8 - 20 amp circuits in a j box 25' above finish floor. Both hots & neutrals are there. I'm installing 2 - 4 pole contacts in an enclosure to control the circuits. I would like to install the contacts lower about 15' run away from all the neutrals. If i extend the feeds & the loads in the same conduit with only 1 neutral for the 110 volt coil, will the magnetic field be zero because the feed & load are at 180 degrees?

Yes.

 

[gar]

130911-1106 EDT

Electric Sam:

Take a length of wire and form this into a tightly packed hairpin shape. Run a DC or AC current thru the wire. The current flowing toward the closed end is exactly opposite in direction to the current flowing from the closed end, and of equal magnitude. A short distance sideways from this loop these two fields essentially cancel one another.

Next expand the hairpin shape into a circular path. At the center of the circle the fields add, and at any point within the circular path. Go outside the current path and the fields start to cancel. Go sufficiently far away and there is almost complete cancellation.

An informative problem would be to write an equation for the magnetic field intensity as a function of the radial distance from the center of the circle. This might be in radial units normalized to the circle radius. Easier, if you make it an infinitely long rectangle. You can also do this experimentally with an N turn loop R1 in radius, possibly a 1000 turn sensing coil, and using AC excitation.

.

 

[steve66]

I have 8 - 20 amp circuits in a j box 25' above finish floor. Both hots & neutrals are there. I'm installing 2 - 4 pole contacts in an enclosure to control the circuits. I would like to install the contacts lower about 15' run away from all the neutrals. If i extend the feeds & the loads in the same conduit with only 1 neutral for the 110 volt coil, will the magnetic field be zero because the feed & load are at 180 degrees?

I'm not quite following exactly what you want to extend down.

If the neutral is terminated on one side of coils, then yes, it would need to be extended down.

But if you aren't terminating the neutral somewhere in your contact enclosure, you don't need to extend it. As long as your wires come down in pairs, so the same current that comes down goes back up, you won't have a problem.

For example, two wires going down to a contact are not problem, neither are 2 wires going to a coil. Same with 4 wires to 2 contacts, or 2 coils.

Actually, as long as a single conduit runs down to this contact enclosure, and there aren't any other conduits running out of the enclosure, you won't have a problem. Kirchoffs current law says the current into a box must equal the current out of that box. With a single conduit, the current that comes down on the wires in that conduit must flow back up on the wires in the same conduit. The opposite directions will cause the fields to cancel.

 

[GoldDigger]

For example, two wires going down to a contact are not problem, neither are 2 wires going to a coil. Same with 4 wires to 2 contacts, or 2 coils.

Or three wires going to two contacts, with one wire being a common return.
The general principle is that any current going in one direction must be matched by the return current coming back through a nearby wire.