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I have a dedicated 15A circuit for a top-floor bathroom fan with heat lamps which draws much less than capacity. There is nothing else on this circuit.

I know that code for a 15A 14ga circuit at 80% load should be 12A max.

This is the quandry. I'm looking at a Panasonic Heater Fan (FV-11VH2) that draws, only when both the heater and exhaust fan is running, 12.12A (1454.5W). 95% of the time, only the vent fan will be running (<1A). When it's running the heat it'll be on a wall timer.

In a perfect world, I'd run a new 12ga 20A circuit, but I'm not too keen on ripping out 3 floors of drywall to run a new wire.

So run it anyways on the 14ga/15A breaker or abandon it? Hard to believe an extra 120mA run occasionally is a big deal ...

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I would guess that the initial draw on the fan + heater could reach or exceed 15A easily if it is typically running at 12.12A. You could end up tripping the breaker the moment you turn them both on at the same time. –  maple_shaft Jan 14 '13 at 16:14
    
What do the manufacturers installation instructions say? –  Tester101 Jan 15 '13 at 15:05
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2 Answers

Should be no problem to run the load that calculates out at 12.12A as long as the power-on current surge of the device is within the 15A rating of the circuit.

The device will actually draw more current than the 12.12A when the thing is first turned on. The amount will be dependent upon the internal design. A cheaper unit is likely to be a problem and may very well trip your 15A circuit breaker. A unit with a more elegant design with a soft start capability may not exceed the 15A limit.

If it was me in your situation I would think hard about the long term safety of running a load bigger than you probably should be placing on your existing 15A circuit. There is also the nuisance factor to consider if the start-up surge is marginal and you end up tripping the breaker one out of every 20 times you power the fan and heater on at the same time.

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When it is first turned on it will be a good deal more than 12.12A meaning it very well could be a problem on a 15A circuit. Not sure why you think otherwise. –  maple_shaft Jan 14 '13 at 16:11
    
@maple_shaft -- If indeed the turn on surge exceeds the 15A rating of the circuit the breaker is likely to trip. However if the unit has a soft start that is designed to keep the power on surge within the normal 20% margin for a 15A circuit then things should be OK. The .12A worst case running overage is within the tolerance margins. –  Michael Karas Jan 14 '13 at 16:51
    
Well it would be too close for comfort for myself personally, but I guess the only way to know for sure would be to try it out and see what happens. –  maple_shaft Jan 14 '13 at 16:56
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I don't see what you guys are on about. I'm well aware of motor startup loads. Are you all saying a puny little fan motor that draws less than 1A running is going to have more than a 2.88A additional startup load sufficient to cause nuisance tripping? I would normally allow 2.5x full load current on thermo-magnetic breakers. Or is there some start up load for heat lamps I'm unaware of? I'd also be really surprised if any of these sorts of fans have soft start controllers. –  bcworkz Jan 14 '13 at 18:37
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@bcworkz - The heat lamps will also have some startup surge. Incandescant type lamps start out at power on cold filements that are lower resistance than they are once they become heated up. –  Michael Karas Jan 14 '13 at 23:31
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In this case, it looks like you'll have to run a new 20A circuit. As per Article 110.3(B) of the National Electrical Code (NEC), and the manufacturers instructions.

NEC 2008

110.3 Examination, Identification, Installation, and Use of Equipment.
(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

On page 5 of the manufacturers installation manual (PDF), it clearly states that you must use a 20A circuit.

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