I'm adding a 24V transformer for a powered damper for my HRV, which will run independently of the furnace.

I picked up a suitable transformer, except it's foot mount and I only just learned I probably should have got a hole or plate mount, to enclose all 120V wires in a jbox. Can I mount the transformer close to an existing jbox and run the 120V wires inside & connect in there, or do all wire runs have to be sheathed?

In the area where I need 24V, all I've got to work with is a single romex run to a 2-outlet jbox hanging from the basement ceiling joists.

Should I tap off the furnace 24V transformer in the other side of the basement and avoid all this?

Thanks! It's been quite difficult to learn about applicable codes, foot mount transformers in particular just don't seem to be talked about (despite being sold everywhere)


Yes, according to code all 120 volt wires must be enclosed, splices must be in a box. If your damper is power open, spring close it is probably drawing 12VA. If it is power open power close it is probably drawing 6VA. Your existing transformer is probably 35-40VA. You could get a larger transformer to replace your existing one or you could get the proper mounting transformer. It may work on your existing transformer but it would be pushing near the limit. The other option is to get a large enough electrical box to put the transformer in and wire to that.

  • I know that I'm not really supposed to put the transformer inside a box, because then I'd be mixing low voltage wires with high voltage wires, unless I was to separate them into two subcompartments somehow- which seems pretty iffy. – Patrick Shyvers Oct 24 '17 at 23:38

Return it and get the correct (hole or plate mount) one

The reason why you don't hear foot mount transformers talked about is because they aren't intended to be used on their own -- they're intended to be mounted into equipment (say an air handler or an industrial control panel) where they can be nice and snugly wrapped up in a box that also contains all the wiring connections.

So, you have the wrong part for the job -- return it and get a hole or plate mounted transformer with the same electrical ratings instead. If you're stuck with a foot mount paperweight -- you could take a large NEMA 1 enclosure and mount it + the splices in that, taking advantage of NEC 725.136(D):

(D) Associated Systems Within Enclosures. Class 2 and Class 3 circuit conductors in compartments, enclosures, device boxes, outlet boxes, or similar fittings shall be permitted to be installed with electric light, power, Class 1, non-power-limited fire alarm, and medium-power network-powered broadband communications circuits where they are introduced solely to connect the equipment connected to Class 2 and Class 3 circuits, and where (l) or (2) applies:

(1) The electric light, power, Class 1, non-power-limited fire alarm, and medium-power network-powered broadband communications circuit conductors are routed to maintain a minimum of 6 mm (0.25 in.) separation from the conductors and cables of Class 2 and Class 3 circuits.

(2) The circuit conductors operate at 150 volts or less to ground and also comply with one of the following:

a. The Class 2 and Class 3 circuits are installed using Type CL3, CL3R, or CL3P or permitted substitute cables, provided these Class 3 cable conductors extending beyond the jacket me separated by a minimum of 6 mm (0.25 in.) or by a nonconductive sleeve or nonconductive barrier from all other conductors.

b. The Class 2 and Class 3 circuit conductors are in- stalled as a Class 1 circuit in accordance with 725.41.

...but it's probably simpler and cheaper to use the correct thing to begin with, even with the return involved.

  • Thank you! I was afraid of that, in other words it's pretty much common knowledge the foot mount is not appropriate so it's never discussed. What about the alternative of tapping off the furnace transformer? I swear I read somewhere the other day that wasn't allowed, but can't find it again. It would possibly be the cleanest, with the least opportunity to violate codes, assuming it's allowed in the first place. – Patrick Shyvers Oct 25 '17 at 0:11
  • @PatrickShyvers -- I don't believe there's anything that forbids a furnace from providing 24VAC to other HVAC accessories, but I could very well be wrong (if you find the source for what you ran across the other day, I'd love to hear it!) – ThreePhaseEel Oct 25 '17 at 1:00
  • went down to local supply co and they have hole mount transformers. But the primary & secondary leads both feed through that hole mount! They don't have any of the classic doorbell style with the two screws. Obviously the insulation on the 24V leads must be good enough to bump into 120V leads. If I get cable with sufficient insulation, can I make my 24V connections inside the jbox and just run the cable straight out the side of the box? I figure this is totally electrically safe- but it might look suspicious, it wouldn't be instantly obvious the wires coming out of the jobx are 24V – Patrick Shyvers Oct 25 '17 at 16:14
  • Is this "supply company" one whose facilities are dominated by racks of EMT conduit, reels of THWN-2 wire in every size from 14 to 1000kcmil, NM, SE, MH wires, blue plastic and gray steel junction boxes, etc.? Or racks of air conditioning condensers and reels of freon hose? Or do they have 1000 kinds of capacitor and relay? Or do they have 10,000 kinds of transistor, IC and LED? Those last two types are not what you want. The law of Amazon applies: somebody will sell you anything, even Big Orange has illegal stuff on their racks. – Harper Oct 25 '17 at 18:04
  • Hah, sorry, it was Grainger. The part is 2ETD3 – Patrick Shyvers Oct 25 '17 at 19:45

One way to get around the mixing of voltages is to use only conductor with insulation rated for the higher voltage on the low voltage side. In a given box or container, as long and all the power is AC, you can combine different voltages as long as the insulation value of all the conductors meets the demands of the highest voltage in this box. I use this a lot. I will just grab a couple of rolls of #14 stranded THHN and run my low voltage circuit. It costs a little more but can save me time in labor and other expenses. Hope this helps.

  • If I do this, can I run that #14 right out the side of a jbox to my application? Do I flag the wires in some way to say "hey these are 24V"? Because they will be making exposed connections and so forth, things you can't do with 120V. – Patrick Shyvers Oct 25 '17 at 16:26
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    My understanding is if low-voltage wires travel with mains voltage wires in Class 1 wiring methods, they must be entirely within Class 1 wiring methods. So for instance a doorbell circuit could commingle with mains power in the same conduit, but would need properly installed NM and every connection in a junction box, using devices listed for mains voltage. – Harper Oct 25 '17 at 18:17

My understanding is that once you leave the Class 1 container it is now just like any other low voltage circuit, say just like any other door bell circuit. For that matter you can now convert to 18-2 bell wire.

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