There is a ceiling fan up and a set of four recessed lights. The lights are controlled from a dimmer/switch in the wall and they work fine. Next to the switch for the light there is room to install the control for the fan (has knob for light and one for motor). That's the state of things as of now.

I'm trying to install the control for the fan and got it working but have some questions.

In the wall outlet I see two live wires and it seems there is only one common. The two live wires are on two different 15 amp circuits. Currently one live wire is used to power the switch for the recessed lights. The common is then used for return path.

For the ceiling fan switch should I use the other live wire and share the common or should I share the live wire and just cap off the second live wire on the other circuit.

The wiring should be about 15 years old. Why would there be a live wire without common?

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    You know they are on two 15A circuits. Are the circuit breakers right next to each other in the panel? (Or opposite each other on a Pushmatic)? – Harper - Reinstate Monica Jul 9 '17 at 13:00
  • They are right next to each other. – JustAnotherDad Jul 9 '17 at 16:13
  • Did you ever get this resolved? If so, please give a check-mark to the answer or write up your own answer explaining what you did to get it fixed and give yourself a check mark. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer. – FreeMan Aug 4 '20 at 12:50

It is common for a neutral to be shared by two circuits if they are different phases (see @Harper's comments). Your house was probably built under the 2002 NEC code so it may not require that these circuits share a common trip device. NEC Article 210.4 (B) now considers this a multiwire branch circuit and requires that both ungrounded conductors be equipped with a simultaneous disconnecting means. This is more to protect someone from a shock hazard caused by breaking the neutral and getting caught up in a series hazard between the load side of the neutral and the line side. Basically it makes you or part of you the load.

So yes it is not unusual, but if the circuits do not share a common trip, you should replace them with one that does. Hope this helps stay safe.

  • To energize my fan/light combo switch can I just run the live wire from the light switch? So I'd be using one 15 amp circuit for the 4 recessed lights in ceiling and the fan/light combo. There may be a wall outlet on that circuit also but the most load it would take is a iPhone/kindle charger. I would just cap/tape the other live circuit and switch off the breaker. – JustAnotherDad Jul 9 '17 at 16:19
  • 240.15(B)(1) allows you to use an identified handle tie in lieu of a common trip breaker. – ArchonOSX Jul 9 '17 at 18:19
  • Sorry but I'm new to this. @ArchonOSX are you saying better tie my fan/light combo switch to hot let of other light switch than share the common and use hot leg from other circuit? – JustAnotherDad Jul 9 '17 at 22:21
  • Either way you would be ok. Most recessed cans are now rated for no larger than 90w incandescent load. which would be about 2A. Even at 150W it would only be about 5A plus 1.5A for the receptacle you would be at 6.5A calculated load which is plenty of room on a 15A breaker for the additional fan/light. – Retired Master Electrician Jul 10 '17 at 2:07
  • My comment was to @RetiredMasterElectrician who said you must replace the breaker with a two pole common trip breaker. That is not required even in new installations you can use an identified handle tie between the two single pole breakers. You can choose either hot wire, with the load your putting on it there won't be much difference, but I would use the other hot leg and try to balance the circuit. – ArchonOSX Jul 10 '17 at 8:43

Your recessed lights are hooked up in a "switch-loop", identified on this link as the third example (at the bottom): Link to important stuff

The previous poster was trying to say that per code, switch loops are no longer allowed; the 2nd example is now the standard (using 14-3 wiring; has an extra red wire). What happened is not a crime. Switch loops were used by electricians as a smart way to wire; whenever power goes directly to a light (and not a switch before the light), using the white neutral as a power allows the switch to work without having to string the extra length of wire. Today's code prefers "a safer" approach with 14-3 wiring, although it doesn't really look safer.

With the fan switch, you should pull out the wiring further to find where the white wire went. With two separate breakers, it shouldn't be hooked to the recessed lights, or it will trip the breaker.

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    You're misunderstanding the Code -- the post-2011 configuration is still a "switch loop" in the sense that power comes in at the light first and then is brought down to the switch, it's just that neutral has to be brought to the switch location too – ThreePhaseEel Dec 2 '20 at 13:23

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