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I upgraded my bathroom fan to a bigger, more powerful fan. The problem is it is on the same 15a breaker as the 2 bedrooms and the bathroom. It is connected to a light above my tub and the three wires that come up from the dual switch on the wall via a junction box.

The house was built in 1942 and the wiring from the switch to the junction box is original, so the three wires have the cloth coating but look to be copper on the inside. The wire from the fan and the wire to the light above the tub are both newer.

The problem is every time I turn on the fan or the light it blows the breaker.

I even disconnected the light above the tub and replaced the wall switch with a single switch and when I turn the fan on, all the lights dim and it then throws the breaker.

I am wondering if I can replace the 15a breaker with a 20a breaker and how would I go about doing that? I already purchased the new 20a breaker.

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    How many HP/watts is this new bath fan? How long does it take the breaker to trip from when the fan is turned on with nothing else on this circuit? What make and model is your breaker panel? – ThreePhaseEel Aug 16 '19 at 2:39
  • Is this "just a fan" or is a heat fan? – manassehkatz-Moving 2 Codidact Aug 16 '19 at 4:30
  • Can you post more details/specifications for the fan? What's running in the two bedrooms? – Greg Nickoloff Aug 16 '19 at 19:14
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No, you can't just "upsize a breaker". The breaker has a job, and you would break its ability to do that job. I gather you don't understand what that is, but fooling around with creaky old wiring... Well, I'm glad you asked first.

No, take the breaker back to the store. The only breaker you should be installing is a 15 amp AFCI. Where you have old/suspect wiring, AFCI is a fantastic safety system.

Anyway, that won't fix your problem

Breaker size is not the issue. Bathroom fans are considerably less than 1 amp, they certainly shouldn't be tripping a 15A breaker, for Pete's sake. Your fan has a big problem. It's not supposed to do that at all.

So either the fan has been miswired, or, the fan is defective.

Maybe it's time to call a competent handyman or electrician.

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Breakers protect the equipment on the circuit but they also protect the wires. That is always important, but perhaps extra important with old cloth-covered wires. The end result though, with old wires or new, is that if you have 14 AWG wire in any part of the circuit then the largest breaker you can use is 15A. If you have 12 AWG (or larger) wire in the entire circuit then you can use a 20A breaker

What size wire do you have? You could measure it, but I'd bet you have 14 AWG, because:

  • that is the most common
  • if you had 12 AWG then you very likely already have a 20A breaker and
  • you said when I turn the fan on all the lights dim. That statement indicates the breaker is NOT the only problem. A breaker will NOT cause dimming - it is either "on" or "off". But too much current on the circuit - beyond the normal capacity of the wire - will cause dimming.

Don't Replace the Breaker Without Changing the Circuit

Your best solution is likely to be a new circuit - effectively splitting the load from the existing circuit. You could replace the wire of the existing circuit, but that will be basically the same work as adding a new circuit, so you might as well add a new 12 AWG, 20A circuit and leave the existing circuit powering the bedrooms.

A new circuit will also let you get up to code, or at least close to code, regarding power supplied to bathrooms - ideally separate from other rooms. Don't forget GFCI if any bathroom receptacles are involved.

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