I now have both fan and light connected to a three way switch. It works fine. I would like to keep the three way with light, and use the fan on another on/off single switch. A three way duplex would do this. The question is, will I have to run another 12/2 line to the fan
And can I use the other half of the three way duplex just as an on off switch?
Christopher

Yes, you could use the second half of a duplex 3-way switch as a single pole switch to control the light. Assuming a "duplex 3-way switch" means two 3-way switches in one device.

Yes, you will have to run a second cable from the switch to the ceiling box if power is being fed to the ceiling box.

If power is being fed to one of the switch locations, the duplex 3-way can be put at that location with out running another cable. If you wish to put the duplex 3-way at the other switch location, you will have to run another cable from the ceiling box to that switch location.

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Originally Posted by hkstroud

Yes, you could use the second half of a duplex 3-way switch as a single pole switch to control the light. Assuming a "duplex 3-way switch" means two 3-way switches in one device.

Yes, you will have to run a second cable from the switch to the ceiling box if power is being fed to the ceiling box.

If power is being fed to one of the switch locations, the duplex 3-way can be put at that location with out running another cable. If you wish to put the duplex 3-way at the other switch location, you will have to run another cable from the ceiling box to that switch location.

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O.k. I have power in the box in which I intend to change to a three way duplex.(Yes, I did mean two, three way switches, in a duplex) What changes are needed in the ceiling box for light and fan, to separate the power flow to light and to fan. This combo is in the first box of two(another light is further down the line). This is a Hunter fan, if that makes a difference. Once change are made here, how is the three way duplex wired?

Present wiring should be a 2-wire cable bring power to the switch box. A 3-wire cable to the second 3-way switch. Another 3-wire cable to the ceiling fan/light box.

The black of the 2-wire power in cable should be connected to the common of the 3-way switch and to either the red or black of the 3-wire cable going to the ceiling box. The white of the power in cable should be connected to the white of the 3-wire cable going to the ceiling box. The red and white of the 3-wire cable going to the other 3-way switch should be connected to the traveler screws of the first 3-way switch. The black of that 3-wire cable should be connected to the red or the black of the 3-wire cable to the ceiling.

To replace the present 3-way switch,
Remove and take note of the present wiring.
Connect the black of the power in cable to the common terminals of both halves of the duplex 3-way switch. Move the wires connected to the traveler screws of the old 3-way to the traveler screws of one half of the new duplex 3-way switch. Connect the black or red (which ever one that is presently connect to the black of the 2-wire power in cable) to one of the traveler screws of the other half of the duplex 3-way switch. The remaining traveler screw of that half will be unused. If you don't like the on position of that switch, move the wire to the other traveler screw.

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Originally Posted by 1max1

This combo is in the first box of two(another light is further down the line).

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Does that mean that the first 3-way switch in in a 2-gang box along with another switch that controls another light. If so that only means that there will be an additional cable going to that light. That wiring should remain undisturbed.

No changes in the ceiling box should be necessary.