Bath Exhaust Fan Power

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Old 04-24-11, 01:46 PM
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Bath Exhaust Fan Power

I want to add a bath exhaust fan in a 90 year old house with primarily armored cable wiring. It is practically impossible to fish wires through these plaster walls, so I am looking for a way to connect power with a minimum of wall destruction, and leave the fan on the same switch as the bath light. My thought was to connect power to another line in the same circuit in the attic (in a box of course), run the hot from that box to another junction box where I would tie in to the light switch leg, and then the hot to the fan box. Is there anything unacceptable about this method?
 
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Old 04-24-11, 02:17 PM
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run the hot from that box to another junction box where I would tie in to the light switch leg
Explain light switch leg. Do you mean a switch loop or a switched power source to the light? Either way you would not tie a hot into either of those. You are going to need a separate switch and cable unless you tie the fan into the existing light and use that switch.
 
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Old 04-24-11, 02:46 PM
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A switch loop/leg, one wire in/out. Which is the hot wire to the existing light, isn't it? So my theory was that tapping into that switch loop puts the fan on the same switch. So I'm getting power from that circuit, just not directly from the light, but putting it on the same switch loop.
 
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Old 04-24-11, 04:40 PM
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A switch loop/leg, one wire in/out. Which is the hot wire to the existing light, isn't it?
No it is simply a way of switching the power cable to the light on and off. The power cable is at the light. The neutral connects to the light. The hot side of that cable connects to one side of the switch loop. The light connects to the other side of the switch loop.

So my theory was that tapping into that switch loop puts the fan on the same switch.
No. A switch loop has no neutral. You need both a hot and a neutral for the fan.

Nothing you have written tells me if the hot comes into the switch or the light. Is there only a 2-conductor cable at the switch? If so it is a switch loop.

Switch Loop
 
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Old 04-24-11, 04:59 PM
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Here is a crude mash-up diagram of what I envision as a switch loop with two fixtures on it, with the power coming to the fixtures from two different places on the same circuit.

 
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Old 04-24-11, 06:35 PM
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Your diagram is incorrect. You would not need to bring power into the switch again.

Is your wiring method grounded? If it is ungrounded it should not be extended. I doubt that the metallic cable is original to a 90 year old house.
 
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Old 04-24-11, 06:43 PM
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You have a neutral connected to the switch switching terminals. A neutral is never connected to the switching terminals.

As stated you need to come off the light. Your way will not work.

There is another problem. In a 90 year old house the cable may be Bx not AC. It is debatable if Bx qualifies as grounded. Ungrounded circuits can't be extended. Adding the fan would be extending the circuit.

A new circuit is your best option. If you can't run cable in the wall just do as much as possible in the attic then use surface race such as Wiremold to run down the wall to a surface mounted switch.

Another option is to use a remote control for the fan instead of a hard wired switch.
 
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Old 04-24-11, 07:15 PM
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It's not a neutral, it's a black wire remarked white for purposes of diagramming the switch leg. That's the original diagrammer's protocol, not mine. You're right, it's BX not AC, don't know if it's original but I don't see any remains of anything earlier.
 
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Old 04-24-11, 07:55 PM
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You need to check to see if your cable has the thin bond conductor. Typically this was backwrapped at the connectors. Check near your panel. It will be very thin.
 
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Old 04-25-11, 07:17 AM
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I'm sure there is none, but my intent wasn't to rehash the grounding issue, I know there is a separate issue there. My only question was, aside from the grounding issue, is it feasible to tap power from a different point in the circuit (other than the light itself), and still run it through the light switch loop. My fault for leaving out the detail that the light fixture is a wall fixture, and I can't tell the direction the feed comes from. If it were a ceiling fixture, I would simply tap in to the light in the attic. Hence the question.
 
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Old 04-25-11, 07:22 AM
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My only question was, aside from the grounding issue, is it feasible to tap power from a different point in the circuit (other than the light itself),
Only if power comes in at the switch rather then the light. We've asked but you have not told us where power comes in.
 
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Old 04-25-11, 08:40 AM
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That was my last post, I can't tell exactly where power comes from, but it looks like the light is the last stop on the circuit, and that power is most likely fed from an outlet in an adjacent bedroom, with a switch leg that runs up into the attic and back down the adjacent wall. But I don't understand why it would work only if power comes in at the switch, if I already have power fed to the fan from a different point on the same circuit. In other words, if I simply directly connected the fan to that different point in the circuit, the fan would run constantly. But if I run the hot wire from that power connection through the light switch leg first before it gets to the fan, it's switched with the light. It's essentially the same as the following, the exception being that the power feed for the second fixture comes from a different point in the circuit, but is connected into the same switch loop. What am I missing?

 
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Old 04-25-11, 11:34 AM
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In refference to my question power can come in at only one of two places, the switch or the light. Where it comes from before that is not relevant. If it comes in at the light as you seem to be saying then the cable you are referring to is a switch loop and a switch loop can control only one power source. In this case it is the original light.
 
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Old 04-30-11, 08:07 PM
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I finally got this wired up and it works just like I envisioned, with the qualification from pcboss that I didn't need to run power TO the switch loop, just run it FROM the loop to the fan. So the answer I was looking for, and I may have asked the question poorly, is no, the neutral/ground do not need to come from the light, they can come from any point in the circuit to the fan. But the hot comes from the switch loop in order to make it switched with the light.
 
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Old 05-01-11, 07:41 AM
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Both the hot and the neutral need to be in the same cable when using metallic cables to prevent heating.
 
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