We recently installed a whole house fan that has a max current of 6.5A at 120V. Due to the dangers of whole-house fans accelerating fires, we wanted to be safe and wire in a smoke alarm relay so that if the smoke alarms go off the fan is automatically shut off.

Our smoke alarms are hardwired Kidde alarms and we found the SM120X device to perform the relay. We were able to put the smoke alarms and the whole house fan on their own 15A circuit.† The relay seemed to work the first time we turned on the fan (on the normally closed / NC wire) but the second time the fan lasted less than one second before shutting off. It appears as though the relay has been killed since the NC wire now no longer carries any current as tested with a multimeter and after having reset the entire circuit. When the relay is bypassed, the fan works fine.

Then doing more research after the fact, the problem is likely that the relay is only rated for 10A of non-inductive load and a whole house fan is a fairly large inductive load... this also fits with the fact that the second time it lasted a moment before dying since the inductive spike had some time to travel back and fry the relay.

One suggestion I read was that if the inductive load is only a "few" amps that an RC snubber (a resistor and capacitor in series placed in parallel to the load) could protect the relay. However, they did not mention what a "few" was or what size snubber.

So, what should be used to protect the relay from the inductive load?

† We made sure to install the relay in the living space (a closet) and not the unfinished attic since we noted that the documentation states "Do not exceed the temperature or humidity limits of +40°F (4.4°C) to 100°F (37.8°C) (such as in garages and unfinished attics)". So, we didn't completely ignore the manual...

  • 7
    If the goal is to reduce fires, mashing together homebrew components is not the way to go LOL... Jun 26 at 0:32
  • @Harper-ReinstateMonica Not planning to use homebrew components for this, they make pre-packaged RC snubbers meant for this (at least I found a few).
    – thaimin
    Jun 26 at 1:33
  • @knowitall The SM120X is not being used for that. The fire alarms are all there and interconnected without the relay. We added the relay afterward. Also, there is no fire alarm panel. It is being used as a relay (its marketed purpose).
    – thaimin
    Jun 26 at 1:35
  • Confused, so who or what is giving power to that relay, if it is not the fire sensors
    – Ruskes
    Jun 26 at 1:56
  • 2
    @knowitall it is, but what you quoted from the document was that SM120X should not be used to bridge otherwise independent groups of alarms together. This is a single group of alarms and the relay is using their interconnect signal to flip another device that isn't a fire alarm panel. In fact, the circuit we are using is nearly Figure 1 on the second page (except we are using NC instead of NO and we have an additional alarm in the main circuit).
    – thaimin
    Jun 26 at 2:37

2 Answers 2


Sounds like a job for a RiB or Aube relay. These are both UL-Listed general purpose relays made specifically for applications like this. You must use UL listed equipment (not UR-Recognized components) NEC 110.2, and you must install in all respects according to labeling and instructions NEC 110.3.

Any snubber you might use would need to be UL Listed for use in residential AC electrical, and that will narrow your product selection to nil most likely.

The relay will take care of any snubbing issues via its rating, which you will assure is sufficient when you conform with 110.3.

The Aube contains its own transformer and you simply shunt 2 wires together to throw the relay.

The RiB requires power be supplied from elsewhere, such as 120V from the fire alarm circuit. The RiB must be chosen to have the appropriate coil voltage.

Both of them are designed to be mounted in a standard steel junction box with knockouts, with Romex cables entering through cable clamps. As required by the electrical code.

The Aube intentionally keeps the low voltage terminals on the outside of the junction box and the high voltage pigtails on the inside. This doesn't matter here since the fire alarm module does not provide separation, therefore if 24V relay coil voltage was used, it would still need to be in AC rated cable (Romex).

  • Yea. Don't start messing around with snubbers, use a relay that's properly rated for the job instead.
    – Mast
    Jun 26 at 10:28
  • Yeah, all of the snubbers I saw were only UL-Recognized which is kind of what led me to ask if that was at all right. I am looking into the RIB relays right now, but getting a bit lost in all of their parameters (Grainger has 16 different ones with 120V coil voltage).
    – thaimin
    Jun 26 at 16:11
  • @Thaimin well you don't have to closely match anything but the coil voltage; all other parameters only need to be >=. In fact, an abundance of switching amps only helps durability. Jun 26 at 22:25

Motors need beefier switching to maintain controllability. In industrial situations, devices called 'contactors' are used, essentially relays that can handle the inductive kickback from a motor turning off.

The contactor has a small relay coil that can probably be safely driven by your SM120X, and the contactor can disconnect power to the whole-house fan.

One thing of concern might be this:

We were able to put the smoke alarms and the whole house fan on their own 15A circuit.

If I accurately understand this to mean that you run both the fan and the smoke detectors from a single circuit breaker, that may be a code violation and dangerous. I'd suspect that the smoke detectors need their own separate circuit. If the fan should happen to trip the breaker, then the detectors will have to rely on the batteries.

I would run a separate circuit for the fan. Get a good contactor for the fan that has a 100% duty cycle, that is, the contactor coil can stay powered on all the time. It will be on all the time so the contactor stays closed, and so the fan motor can be powered on as needed from a wall switch etc in series with the contactor.

Then drive the contactor coil with power routed through the NC contacts on the SM120X switch. This power should also probably come from the fan circuit.

The power for the SM120X coil will come from the smoke detector circuit.

Under normal operation the SM120X switch would be closed, power goes through the SM120X to the contactor coil, which stays energized and closes the contactor, letting power go to the fan motor as desired through the wall switch.

In a smoke alarm, the SM120X triggers, opens the NC switch and de-powers the contactor coil, which opens the contactor, which in turn cuts power to the fan motor and absorbs the kickback.

  • 1
    Residential smoke detectors do not need their own circuit -- in fact, it's best to put them on a hall lighting circuit or such, so you know if the circuit ever trips Jun 26 at 3:46

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