I am replacing an existing heater, light, night light, exhaust fan unit with a new unit that only contains a light and exhaust fan. The existing wiring has a 12-3 coming straight from the panel with red and black on separate circuit breakers. The existing 12-3 is too short for me to pull it down to the switch box. My plan is to install a metal junction box in the attic and the run a 12-3 wire to the switch box. In the switch box I plan to install 2 switches. The first switch will be fed by the black power source to control the light and the red power source will control the fan. I will run another 12-3 wire to the fan/light unit and tie together the neutral wires. I know this is over kill but I don’t want to leave a unused hot wire in the switch box if I don’t have to. I will install a handle tie to the existing 2 breakers. I would appreciate any thoughts or suggestions.
Electrical – Separating 12-3 wire from panel to feed a bathroom fan and light
electrical-panelexhaust-fanjunction-boxwire
Related Solutions
It is possible that the two black wires (other than that going to the switch) are going separately to the fan and light. If so, the change is easy.
To test this do the following:
- turn off the breaker
- separate the three black wires nutted together
- wirenut the black to the switch with one of the others and insulate the other in a separate wirenut
- turn the breaker back on test the switch. Hopefully, the switch will operate only one of the fan or light.
- turn the breaker back off
- switch the black wires so the other one is connected to the wire to the switch
- turn the breaker back on and test the switch. Hopefully, it will only control the other of the light or fan
- Turn the breaker back off and optionally restore the original wiring
If the test results followed what I marked "hopefully", you are in business. Remove the other wire to the switch and wirenut it to two wires to run to one side of the dual switch (if one side of the switch has a removable bridge, just connect the single wire to that side). Connect the two blacks originally wirenutted together to the other side of the switches (if that side has a removable bridge, remove it).
For the pigtail wires, you should use the same guage wire as the rest. Examine the wires for printing on the insulation (look for an AWG number).
I'd also recommend against using the back push-in connectors. It's better to use the screw terminals.
This is a bit more annoying than it looks (spoiler: the obvious way to wire it may not pass muster with all inspectors)
The power feed to this all comes in at box 1 (vanity light) and feeds the /2 switch loop to box 3 as well as the /2 feed to box 2. Box 2 and Box 3 are connected by a /3 cable, apparently intended to be a switch loop.
With all this, you'd think you'd be able to:
- Turn the power off, of course
- Connect the white wire from the /2 in box 2 to the white wires from the light and fan there.
- Connect all grounds together in box 2 and to a box ground pigtail (if it's a metal box)
- Connect the black wire from the /2 cable in box 2 to the white wire in the /3 cable and mark that white wire as a hot (permanent marker as you have done should be fine -- it just needs to be a permanent/indelible marking, visible from all angles).
- Connect the red wire from the /3 cable in box 2 to the blue wire from the overhead light.
- Connect the black wire from the /3 cable in box 2 to the black wire from the bath fan.
- Connect all the grounds in box 3 together.
- Connect the white wire from the /3 cable in box 3 to the HOT terminal on the fan controller after marking it as a hot.
- Connect the black wire from the /3 cable in box 3 to the 1-POLE terminal on the fan controller.
- Connect the white wire from the /2 cable in box 3 to the switch HOT terminal, again after marking it hot.
- Connect the black wire from the /2 cable in box 3 to the switch 1-POLE terminal.
- Connect the red wire from the /3 cable in box 3 to the other switch 1-pole terminal -- the Adorne parts can terminate two wires on each terminal.
- Button everything back up and turn the power on.
and have it work. In fact, it will work if you do this; however, there's a catch! This arrangement takes one switch loop feed (from the vanity light) and returns it back down two different cables (to the vanity light and to the overhead light), which can be seen as a Code violation (of the 300.3(B)/310.10(H)/300.20 complex) as it creates a large loop area that puts out stray magnetic fields in your bathroom (good for ruining the picture on that old TV you have mounted in the wall in there ;).
A more conservative approach would be to utilize a dual pole switch to break the two hot feeds simultaneously while sending the return paths back the way they came, as follows:
- Connect all the grounds in box 3 together, and to grounding (green/bare) pigtails to any switches that need them.
- Connect the white wire from the /3 cable in box 3 to two (preferably black) pigtails after marking it as a hot. One pigtail goes to the HOT terminal on the fan controller and the other goes to one side of the dual pole switch.
- Connect the black wire from the /3 cable in box 3 to the 1-POLE terminal on the fan controller.
- Connect the red wire from the /3 cable in box 3 to the other terminal on the same side of the dual pole switch as the pigtail.
- Connect the black wire from the /2 cable in box 3 to the other side of the dual pole switch.
- Connect the white wire from the /2 cable in box 3 to the other terminal on the other side of the dual pole switch.
- Button everything back up and turn the power on.
This arrangement also works, and keeps the two light switch loops neatly separated. However, Legrand does not make a two pole Adorne switch, and I do not believe there are faceplates available for the combination of an Adorne device side by side with a standard switch in a two gang box. You'd have to get a standard-form-factor (i.e. not Adorne) fan controller to do this.
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Best Answer
You're using the word "wire" to describe what is actually "cable". 12/3 is a cable that contains 3 conductor wires plus a ground wire. This matters here.
First, find all the other multi-wire branch circuits
OK, this is priority one. You need to get inside the panel, and search the panel for more circuits like this one. You stated they are on independent breakers; they need not to be.
They need to be on 2-pole breakers, which is a double breaker which fits in 2 spaces (or straddles 2 spaces, as in GE Qline and Crouse Hinds). The most important feature of this breaker is that the two handles will be factory-tied together.
This is to provide common maintenance shutoff, but it also assures they are on opposite poles of the circuit. That is to say, there must be 240V measured between the two hots. If there is 0 volts, that will overload the neutral wire. A classic blunder is to free up space by moving these from a 2-pole to a tandem breaker (note handles not tied).
Further, anywhere both hots are present, the neutral wire must be pigtailed. That way, any device can be removed without breaking the other half of the circuit.
Other than that, Mrs. Lincoln, Multi-Wire Branch Circuits are rather wonderful, in my opinion, and a great way to get 2 circuits almost for the price of 1.
Option 1: Cap it off and don't use it
In this case, you simply stop using the red wire. Cap it off on both ends, stick a wire nut to cap it off, and tape the wire nut on the wire (they tend to fall off when put on a single wire).
Don't cut it - for one thing, it is hard to insulate a wire that's been cut back to the sheath, and for another, the next resident may want a heater and wouldn't that spare wire be a nice Christmas present for them! Or, your wife could get sick of the hair dryer and curler tripping the breaker constantly, and look at that! Spare circuit right up in the ceiling!
Use the spare half-circuit for something else
You can't have bathroom receptacle loads power anything in any other room. That restriction does not apply to bathroom hardwired loads. Since half the circuit only powers bathroom light and fan, you have loads of capacity on the black half-circuit and 100% capacity on the red half-circuit. You can get 20A out of both of them!
So add 2 circuits to anything you want. It could be more kitchen countertop receps. It could be 2 bedroom circuits.
Heck, you could bring them both down to the bathroom receptacles and now you have 3 circuits for bathroom receps. That takes care of the curling iron + hair dryer problem! Honestly, 3 circuits in a bathroom is a little excessive, even for me, so I would then liberate the original recep circuit for something else.
Do the Harper Shuffle
If routing makes this feasible, I would do a swaperoo. I would dedicate the MWBC to bathroom receptacles, so you have 2 full circuits for receps.
Then, I would take the original bathroom receptacle circuit, and use it for lights, fan and room for heater. Because in the modern age with LED lighting, a whole MWBC is overkill for that. A plain 120V/20A circuit has 1920W of provisionable power; a heater takes 1500, a fan takes ~150, and LED lights won't be near 270W.
And -- this solves the problem of hair dryer overload killing the bathroom lights. A receptacle overload will most likely trip both half-circuits; but since the lights are on a totally separate circuit, non-issue.
If you abandon the MWBC, you could still do the recep.
Suppose you decide to cap off the red wire and just not use it. You still have well in excess of 1500W of usable power on the lighting circuit, that's just being wasted. You might as well bring that down to a receptacle and let someone use it!
However, you don't want them overloading that circuit and killing the lights, so I'd arrange it so there's only a single receptacle socket available to them. (plug-in devices are UL-restricted to 1500W). They make 1-socket plain receptacles, but you'll need GFCI protection for a bathroom recep. That's a bit harder, but they do make combination GFCI + receptacle + switch devices, with 1 switch and 1 GFCI-protected socket. Perfect, and you can even put the switch to good use!
Also, once any part of the MWBC has a bathroom recep on it, the other half can't be used for anything outside that bathroom. So this depends on you abandoning the other half of the MWBC (or using it in this bathroom).
Remember, this is the 2010's oh wait. You need switch neutral.
You mentioned running a single 12/3 down to the switch. Nuh-uh. Consider the wires that need to be at the switch:
Further, you're up against a different rule, that says Currents must be equal in each cable or conduit. So you can't just use two 12/2 cables. There are several ways to skin this cat.
Run two switch loops. However, due to the 2011 rules, both switch loops will need to use 12/3 cable, so you'd run two 12/3 cables. Each loop needs its own neutral; you can't shortcut and use a 12/2 for the fan, because if someone installs one of those pushbutton timers that needs neutral, it is not allowed to poach neutral from the switch 12/3 (that would make currents unequal in each cable).
Extend always-hot power from the fan box, down to the switch box (say, in a 12/2 cable, or a 12/3 if you wanted the option to bring down the other hot, say, for receps). Then, use a second cable (12/3) to power fan and light, in the usual way. However, this means you have two different things going on in the ceiling box: extending the supply cable, and supplying the fan and light. These should be separated 100%, i.e. extend the supply, then lay a piece of saran wrap in there to encapsulate that wiring since you won't touch it again, then wire the fan/light normally. You will need a deep box for this since you'll add a bunch of cubic inches for all that splicing.