I am looking to run a new 15amp circuit to my bathroom and want to split it in attic so one sub-circuit goes to exhaust fan and another to lamp. First, does this circuit look correct?

One logistical problem is that in both circuits, there is a LOAD wire traveling from the switch to the equipment (fan or lamp). How would you run this single cable? It seems wasteful to run the 12/2 romex and then cap off ground and neutral cables in it at both ends.

Essentially, is there a smarter way of doing this?

Note: The fan switch I am using requires 4 leads (ground, neutral, hot and load).

Bathroom 15amp Circuit

Updated Circuit


Your diagram is incorrect but close. Get rid of the "junction box". Run your wire from panel to device box you are using for switches. At device box put two pig tails on each wire. You now have two "sub circuits". Connect one set to fan running hot through switch. Same for light. Since you are using Romex make sure all grounds are connected.

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  • So far I have been going through 4 different diy books on electrical and a bunch of info on the internet and somehow I came to conclusion that splitting circuits in the junction box was the way to keep things clean. But what you and others have suggested certainly makes a lot of sense. – cryptic0 Jun 17 '19 at 13:15
  • Romex is irrelevant - all grounds must always be connected correctly, whatever type of wire you are using. – J... Jun 17 '19 at 14:44
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    Because you're going in circles and that's not allowed. Nothing wrong with a junction box, just remember you can never cover it over, the junction box cover must always be accessible without tools, so it's an aesthetic issue. Also remember you must bring neutrals to switches, so that means /3 cable to the switch. – Harper - Reinstate Monica Jun 17 '19 at 15:07
  • @Harper Because it's worded somewhat confusingly, it's worth stressing that junction boxes definitely need their metal covers installed, they just can't be buried in finishings (ie: the properly installed metal cover must always be accessible). I know that's what you were trying to say, but someone might read this and think they shouldn't install the cover plates on their junction boxes. – J... Jun 17 '19 at 18:33
  • Added new circuit. Does that look correct? – cryptic0 Jun 17 '19 at 22:21

I am very confused by your diagram. Here is how I would do it. FYI, grounds ignored but implied and always connected to each other and (as appropriate) to switches, devices, metal boxes, etc.

Combined Fan/Light - i.e., two devices in one box

  • Panel to switch box in the bathroom: 12/2 - Black (hot)/White (neutral)
  • Switch box:
    • Attach two black pigtails to the black wire with a wire nut. One goes to each switch.
    • Switch box to fan/light: 12/3 - Black (switched hot 1), Red (switched hot 2), White (neutral)
    • Attach all white/neutral wires together with a wire nut - incoming from panel, outgoing to fan/light, fan switch.
    • Attach black switched hot to one switch. Attach red switched hot to the other switch.
  • Fan/light box:
    • Attach fan hot to one switched hot (black or red). Attach light hot the other switched hot (black or red).
    • Depending on the fan/light design, it may have one neutral wire or it may have two neutrals. However many there are, attach them to the white wire.

Separate Fan/Light - i.e., two devices in separate boxes

  • Panel to switch box in the bathroom: 12/2 - Black (hot)/White (neutral)
  • Switch box:
    • Attach two black pigtails to the black wire with a wire nut. One goes to each switch.
    • Switch box to fan: 12/2 - Black (switched hot), White (neutral)
    • Switch box to light: 12/2 - Black (switched hot), White (neutral)
    • Attach all white/neutral wires together with a wire nut - incoming from panel, outgoing to fan, outgoing to light, fan switch.
    • Attach each switched hot to one switch.
  • Fan box:
    • Attach fan hot to one switched hot and attach neutral to neutral of same cable.
  • Light box:
    • Attach light hot to one switched hot and attach neutral to neutral of same cable.
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  • I am getting a bit lost in terminology. By 'switched hot', do you mean the load? At least on my fan timer switch, there are 4 terminals. – cryptic0 Jun 17 '19 at 0:35
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    Line = hot. Load =switched hot. With a timer you may need neutral - you can pigtail to the other neutrals. – manassehkatz-Moving 2 Codidact Jun 17 '19 at 0:51
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    @RobertMoody It is just one circuit. Two separate switches - which I prefer and the OP's original diagram includes two switches as well. – manassehkatz-Moving 2 Codidact Jun 17 '19 at 1:27
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    Ya got messed up on question. sub circuit and both circuits. May need a little edit to be easier to read. – user101687 Jun 17 '19 at 1:37
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    Switched hot is the better term @cryptic0... the term "Load" becomes rather important when you start dealing with GFCI... and it does not have the same meaning. – Harper - Reinstate Monica Jun 17 '19 at 4:11

You better put a DC power supply at the junction box site. You can't do this with AC!

The same thing that makes transformers work with AC makes this unsafe and illegal. There's a fundamental rule:

Currents must be equal in each cable or conduit

It's okay for that current to be split across multiple wires, say you have a double switch powering a light at low/high mode, in both-on you have 1A on hot1, 2A on hot2, and 3A returning on neutral. That is balanced.

It gets a little weird with multi-wire branch circuits (15A on pole 1, 2A on pole 2, 13A on neutral) and gets psychedelic with 3-phase power. But the rule is the same. If you stick a clamp meter around the whole cable, it will read ~0 because currents are canceling each other out.

A "tree" topology enforces this

If you look at almost all physical wiring topology, they are a "tree" topology - one or more cables branch from any point, but no branch loops back onto another branch. This effectively enforces "currents must be equal" because any current going up a branch must come back that same way.

In other words, if you take your drawing and drop it into Paint or Photoshop and do a "paint can fill", no areas should be left un-filled. Note that in your diagram that does not work - the j-box, switch, fan triangle will not fill, ditto light.

If you re-scope your plan with the "Currents must be equal" rule in mind, your choices should be obvious.

Remember you need to provision a neutral at EACH switch location*, even ones which do not yet have smart switches. I presume these switches are not adjacent, if they were adjacent there'd be better ways to do this.

* there are some weird exceptions.

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  • While loops will wreak havoc with anyone trying to play a single-coil electric guitar, I think a bigger problem--which would still exist with DC--is that they obscure the required 1:1 relationship between power feeds and returns. If the only things that connect to breaker X's' neutral are fed from breaker X, the total current in breaker X's neutral can't exceed the current through breaker X. If that neutral wire is connected to other things, however, it could be overloaded. – supercat Jun 17 '19 at 15:35
  • @supercat that is a very good point. This overloaded neutral issue is sometimes seen in knob and tube wiring, where power was DC in the early years, and often, neutrals were fused. – Harper - Reinstate Monica Jun 17 '19 at 15:45

You don't specify where you are, but be aware that some bathroom ventilation fans require they be on GFCI-protected circuits. This can be accomplished either by an in-panel GFCI breaker ($$$), or by supplying the fan's switch from a GFCI outlet in the bathroom if one is available. If a GFCI outlet isn't already in the bathroom, that's another issue entirely. You could replace the two-gang switch box with a three-gang box, then wire from panel to a GFCI outlet in the switch box, then pigtail from the GFCI to the switches for the fan and light, and then from those switches to the fan and lamp, respectively.

If you choose to add the GFCI outlet (as above), you will need to run 12-2 Romex from a 20-amp breaker in the panel to the GFCI, then 12-2 or 14-2 from the GFCI to the switches and further downstream.

If your fan doesn't require GFCI, then the minimum you'll need to run 14-2 Romex from a 15-amp breaker in the panel all the way out. But if you do it this way, you CANNOT put an outlet on that circuit (violates code). If you might put an outlet on that circuit later on, it has to be a GFCI outlet, the breaker has to be 20-amp, and wire has to be 12-2.

Edit: as pointed out, it's a code violation to put 14-2 wiring downstream of a 20-amp breaker. Don't do that. I've edited my answer.

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  • I am in the U.S. The panasonic fan I am using says UL listed for tub/shower enclosure when GFCI protected. Not sure what qualifies as tub/shower enclosure. My bathroom is full size with toilet and the tub. There is currently a gfci outlet in the bathroom, but it's not grounded (60s home). So my plan was to take it out and run a separate 20amp circuit just for the gfci outlet. Though I do like your idea of doing a three gang switchbox and running 12/2 to the first gang (gfci outlet). However, is it okay to run 14/2 after that or does it violate code? – cryptic0 Jun 17 '19 at 17:50
  • 1 - You can't run 14/2 anywhere in a circuit if the circuit has a 20A breaker. 2 - My understanding (from my electrician, but years ago) is that if the fan is above a tub or shower then it needs GFCI, but if it is above some other part of the bathroom then it does not need GFCI. 3 - Sharing a circuit between receptacles and fan is OK if it is a plain fan. If it is a heat fan then I wouldn't recommend that as you would trip the circuit breaker if you used a hair dryer while the fan was on. – manassehkatz-Moving 2 Codidact Jun 17 '19 at 18:01
  • You can't run 14-2 from a 20-amp breaker if the load could potentially draw more than 15 amps. I doubt that the fan or the lamp could draw more than 15 amps, but manassehkatz's point isn't unreasonable -- most inspectors would flag 14-2 on a 20-amp breaker as a violation. So to avoid inspection violations, if you use a 20-amp breaker (which you must on outlet circuits), then use all 12-2 Romex. – Russell Jun 17 '19 at 18:07
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    re: "tub/shower enclosure". That wording in the instructions means "if you install this fan directly over the tub or shower, or anyplace nearby that has a good chance of getting wet, you must use a GFCI circuit." So in a normal bathroom where you put the fan in ceiling in roughly the middle, and that area is curtained off from the tub/shower, then you are not required to use a GFCI circuit. – Russell Jun 17 '19 at 18:10
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    After thinking about it over lunch, I've concluded my original advice about 14-2 pigtailing the fan and lamp, is wrong. If the fan fails in such a way that it draws more than 15 amps, but less than 20 amps, the pigtail can overheat and fail. The 20 amp breaker won't protect in this niche case. The NEC exists because when dealing with very large numbers of installs, even the niche cases will happen. – Russell Jun 17 '19 at 19:05

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