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I need to install a whole house fan and would like to know if I am required to have that fan on its own circuit. I am in Los Angeles County if you happen to know of any L.A./California deviations.

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    What do the installation instructions for the fan say? I don't think NEC says a whole house fan must be on a dedicated circuit, but the manufacturer's documentation might.
    – Tester101
    Jul 9, 2015 at 5:12
  • Those I have installed do not require a dedicated circuit, but one should keep in mind the amperage draw and assume normal electric motor issues on the circuit. Jul 9, 2015 at 19:13
  • I installed it recently on an existing circuit--I don't believe the instructions said to do otherwise. Since then, I have become more familiar with the code, and started wondering if there were some language about it within the context of HVAC or small appliance--in general I've been surprised by the number of things that require their own circuit now.
    – Phil Esra
    Jul 10, 2015 at 7:35
  • Related: physical construction requirements for a generic attic appliance. mybuildingpermit.com/sites/default/files/documentation/…
    – bishop
    Dec 2, 2019 at 19:15
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    How many HP is the fan motor? That's probably what's going to govern this... Dec 3, 2019 at 0:49

2 Answers 2

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Sounds simple, but it's not. Assuming this is a "normal" attic fan, one manufactured recently and packaged for that purpose (not a super-charged home built model or a last-century model reclaimed from a 1950's estate sale), then let's start with the NEC:

430.42 (A) Motors not over 1 Horsepower. One or more motors without individual overload protection shall be permitted to be connected to a general purpose branch circuit only where the installation complies with the limiting conditions specified in 430.32(B) and (D) and 430.53(A)(1) and (A)(2).

Per that, you can add the attic fan to an existing circuit, unless it's limited by:

  • 430.32(B), (D): These apply to auto-starting motors and requires they have a separate overload device. Modern, commercial fans with a thermostat on have a thermal fuse and comply this parts B and D.
  • 430.53(A)(1), (A)(2): These apply to multiple motor arrangements and do not apply in this configuration.

However, when adding a device to a general circuit, you must perform either a VA/sf calculation (which isn't ideal for attic appliances, so I'll skip that) or a whole-circuit calculation. For that, see section 210.11, but in short divide the total calculated load in amperes by the ampere rating of the circuit itself. Ensure your calculation is for the fan on its most consumptive setting (ie "high"). It'll be something like 115V @ 7A.

If you have your attic lights, an outlet, and your attic fan on a 15A circuit, you're probably fine.

You only need an individual branch circuit (what's called "dedicated" in the trade) in specific scenarios. Kitchens and similar areas need a small appliance outlet dedication. Laundry rooms similarly. Neither of these dedications can loop to the lights: last thing you want when your washer faults is to be in a dark closet with an electrically hazardous situation around you. More restrictive rules apply to bathrooms, in that neither can the circuit supply lights nor other outlets elsewhere. GFCI considerations also apply in the bathroom and, sometimes, the laundry room.

That said, I would put the fan on a dedicated circuit with a manual disconnect no matter what, for a several reasons. First, unless it maxes out your panel, an inspector can't argue with this wiring. Second, fans generate a lot of line noise, which interferes with signal quality on downstream devices. Third, if the fan starts while I'm working in the attic, I don't want to have to worry about any tools I'm using tripping the circuit and leaving me in the dark.

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I don't believe the fan mfr will call for a dedicated circuit but you may want to contact them to be sure. More than likely, they'll defer to local codes so also check with your local inspector. As with any other device, consider the other loads on that circuit, and which ones could be operating at the same time, to ensure you don't overload the circuit.

Anecdotally... My own house fan is on the same circuit with my dining room & hallway. In normal use, I'll have up to 11 led lights (154 Watts, about 1.5 Amps) and the 1/3 hp fan (6.7 Amps published, unknown actual) running and can add a vacuum cleaner with no problem on a 20 A circuit.

Fan specifics... The install instructions for a Dayton 1LXN7, 1LXN8, 2EAX4, and 2EAX5 don't say anything about a dedicated circuit. Their manual just says... "Install the fan control(s) in the desired location and wire according to the appropriate wiring diagram (following applicable NEC and local codes). Connect power to motor and 2-speed switch with timer using approved wiring methods."

The instructions for an Attic Cool/Ventamatic CX242DDWT, CX302DDWT, CX24BD2SPD, CX30BD2SPD, CX36BD2SPD, CX242DDWTHUB, CX302DDWTHUB, CX24BDM-2SPDHUB, CX30BD-2SPDHUB, and CX36BD2SPDHUB also don't mention a dedicated circuit... "Note All wiring supplies and installations must meet or exceed the requirements of local electrical and fire codes. Wire to 120 Volt, 60 Hz circuit only, using 14-3 two-conductor wiring with ground." ... "Install included switches in a double gang UL listed wiring box, in a wall close to the fan."

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