Bathroom vent fan wired in series - ok?


Old 11-15-15, 10:28 AM
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Bathroom vent fan wired in series - ok?


Were replacing our old Bathroom vent fan during a remodel and when I pulled the old one out it was wired differently than expected. The fan has only one switch but there are two pieces of Romex that are wired together inside the fan electrical box. The two pieces or romex are wired together (Black to white) and the remaining black and white wires from the two different pieces of romex are wired to the fan. The old fan always worked I just figured it was time for a new fan with the update. My question is - is it ok for a fan to part of a series, as it appears that power is going to the fan and then back out to somewhere else. Where it goes is unknown. it's buried under tons of insulation.

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Old 11-15-15, 10:32 AM
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Welcome to the forums.

That is called a switch loop. That is where power comes into the device and a two wire cable is run to the switch. My diagram shows a lamp. You have an exhaust fan.

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Old 11-15-15, 10:47 AM
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Thank you so much. I greatly appreciate your help. -Kris
Old 11-17-15, 07:33 PM
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Obscure technical note: A switch and the thing (light, fan, etc.) the switch controls are wired in series, namely the current has to go through each in turn. Another common use of series circuits nowadays is in Christmas lights; many individual lamps are wired in series.

Obscure technical note: The hot wire connected to the light fixture (or fan) must not be white. So, in a switch loop, the white wire carries the raw hot current (unswitched current) to the switch while the black wire carries the switched current back to the light, etc. As shown in the diagram above.

Obscure technical note: Any wire attached to a switch terminal or to other hot wires it not a neutral. So the use of a white wire for these purposes is not ambiguous to electricians although should be avoided whenever possible. However the latest code requires that both ends of a hot white wire be marked with colored (not green) tape or stain.

Obscure technical note: The very latest code requires a neutral in switch boxes. So new switch loops need at least 3 wires plus ground. Needed cables will have red, white, and black conductors and then use of a white wire as a hot wire would not be applicable. It doesn't matter whether the red is the switched hot or the unswitched hot.

Obscure technical note: Light fixtures (and fans) and receptacles are never connected in series with respect to each other. A sequence of receptacles all on one circuit and "in a line" may be referred to as a daisy chain.

Last edited by AllanJ; 11-17-15 at 07:54 PM.
Old 11-17-15, 08:37 PM
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It doesn't matter whether the red is the switched hot or the unswitched hot.
Obscure technical note: That is code but usual wiring convention is red switched and black constant.

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