The Guard Dog RB-122E Low Water Cutoff is UL listed, it requires 120V AC and is capable of switching 24VAC on the control side of a boiler.

It is a self-contained metal enclosure with knockouts for its hard-wired power supply. When used for 24V control, per "Option 2" on page 6 of the manual, you connect it in series with the boiler's control circuit. That circuit is outside the Class 1 enclosure, and unavoidably outside as it includes the thermostats, gas valve, etc.

This manual sidesteps the question of how to connect its terminals B and C, which are inside the Class 1 enclosure, to the boiler's control circuitry, which is not. The way it's done in practice is to run thermostat wire through the second knockout, like this installation that passed inspection:

enter image description here

Is this ok? Given the UL certification there must be some reasonable way to follow these instructions? I would expect to see separation of the high and low voltage circuitry inside the enclosure, but as you can see it's quite the opposite. In this case the supply and control wire nuts are touching, as if to illustrate the question.


1 Answer 1


The wiring scheme is approved by UL, but using the second knockout as the photo shows is not. The sharp metal edges will cut through the thermostat wire insulation. The knockout plate should be removed. A knockout hole metal fitting with a gland nut that squeezes a rubber grommet tightly around the thermostat wire may be easier to find locally than a slitted rubber grommet that covers the entire hole. It’s important to choose something that covers the entire hole to contain the hot metal splatter and dripping burning plastic if there is ever a component failure.

The 24v wire and the 120v wire should be separated, it’s not clear if the 120v wire leads have excess length causing poor spacing. As a practical matter, it will be adequate to keep the tstat wire at least 1/2” from the power wire. It’s not a duplex outlet, don’t fold up 6” of wire in the box.

If there was originally a piece of fish paper (fire retardant treated cardstock) in there as a divider between the circuits then there has to be one, but if there wasn’t one originally then it isn’t required and shouldn’t be added. (Some similar devices use them but a brief look at the product literature doesn’t suggest this device uses one).

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    Most of his answer is good advice on how to improve this specific installation. But the answer to the question seems to be in the last paragraph. You seem to be saying basically, separation is achieved by the installer making best effort, and somehow this device is approved whereas in other cases such as an energy monitor inside a breaker panel, nothing is supposed to leave.
    – jay613
    Commented Nov 23, 2023 at 3:49
  • I think if I wanted to do this by best effort, I would make the 24V connections outside.
    – jay613
    Commented Nov 23, 2023 at 3:55
  • @jay613 - not best effort, UL approvals assumes a workmanship standard, a low water cutoff isn’t a consumer product. 24v connections outside a box next to a boiler get damaged. The breakdown voltage of air is greater than 10kV/inch. Keep the wires away from each other for insulation failures, the breakdown voltage of the arc path down one wire nut and up another exceeds the inflation breakdown voltage. A box is to trap sort circuit metal spray and dripping burning insulation. The fish paper divider is enough for metal spray when there’s nothing to burn, melt, and drop onto the control wiring. Commented Dec 25, 2023 at 15:46
  • Oliver your logic makes some sense but by that logic there should be far more devices permitted to be installed this way. But they rarely (never?) are. Transformers, relays, energy monitors ... whenever they include LV wires exiting a box they are designed to be installed through a knockout with separation inside the device and all low voltage wiring outside. In all cases they could be designed without the knockout and installed professionally with appropriate air gaps inside the box but that's not allowed. I do not see what is different here, though I do accept the UL listing makes it ok.
    – jay613
    Commented Dec 25, 2023 at 16:05

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