Common or Multiwired Circuits


Old 09-26-06, 06:49 PM
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Common or Multiwired Circuits

I have a house where much of the wiring is BX type cable. I don't understand the presumably "common neutral" or "multi wire" circuits that I may have with it.

For example, the ceiling light that I accessed had 4 BX cables (each 2 wire) going to it:

4 blacks were tied together.
3 whites were together, with one tap off wire going to the light.
1 solo white remaining also fed the light. (He and his respective black partner went to the controlling switch)

Also 2 of the incoming BX were coming from 2 other individually controlled ceiling lights.

Please, can anyone explain, or link me to a diagram that explains this type of wiring. I find it baffling. THANKS!
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Old 09-26-06, 07:33 PM
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I see no sign of a multi-wire/shared neutral circuit in this situation. This is just a case of a box that is used as a junction box: One power in feed, two power out feeds (to two other lights), plus a switch loop.

A switch loop takes power from the incoming power in the light box to the switch, then back from the switch to the light itself. In your case, that "solo" white is the wire bringing power to the light from the switch (but only when the switch is "on"), and the corresponding black is the wire taking power from the source to the switch.

Whoever did the switch loop should have wired it differently than they did: They should have tied the switch loops white wire to the other the BX run's black wires, and then its black would go to the light. They should have also "reidentified" the white wire of the switch loop as a hot wire (by marking it black, red, or blue with a marker or electrical tape) since in a switch loop, the white wire is hot (NOT a neutral).

Basically, your wire nuts with black wires includes one black wire feeding two blacks that go to other fixtures and the wire going to the switch (black in your case). The "hot" feed to the light is the wire returning from the switch (white in your case). As mentioned before, these two wires are being used improperly, though it works anyway.

The white wires that are nutted together includes one white wire feeding two whites that go to other fixtures and a white pigtail going to the fixture to provides it's neutral connection.

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