Here's an example of an HVAC transformer that will go from 120 or 240 to 24 VAC.

Obviously, all wires exit the device through the same pipe nipple; the device is intended to be installed by screwing it onto a junction box using that nipple:

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Since installing like that pretty much requires that the high & low voltage wires & respective splices are made together, how can this be used in a code-compliant (not to mention generally safe) way?

It seems clear from other questions here (and other reading) that high & low voltage wires are meant to be kept apart for good reason. (ex1, ex2)

I've seen other types which segregate the high & low voltage wires, but if this transformer was what you had available, what are the options?

  • I would install it in a listed (UL/ETL/etc) device that was designed and safety tested to use the transformer the way it was designed. What are you trying to put this into? Apr 4, 2021 at 12:24
  • @statueuphemism I'm looking into options for using a 24V tstat on a line voltage baseboard; so considering pairing a transformer with a relay like a Honeywell R841D to switch the actual heaters. There are combined devices which have a relay + transformer but the LV current on those is pretty low which is why a separate transformer may be needed. Apr 4, 2021 at 12:28
  • Is this a residential or commercial setting? I helped on a project that replaced the Honeywell controls with 3 parts: A new thermostat (programmable), a 24 volt transformer and a contactor (NOT a relay). The honeywell unit isn't very responsive and can take up to a minute or 2 to turn on....longer if very cold in the room bc it uses a bimetallic strip that needs to heat up to drive the contacts together. The advantage is they are quiet. A real contactor makes a def "clunk" when energized, while that might be acceptable in a commercial setting, probably not in a home. Apr 4, 2021 at 13:13
  • Where would you be mounting the relay and transformer? Apr 4, 2021 at 13:22
  • On those "relay w/transformer" units, the transformer is perfectly sized for the relay. Where you get in trouble is if you're also trying to power a smart 'stat... however those relays usually don't even give you a C-wire. Apr 4, 2021 at 17:48

1 Answer 1


I'd simply take this transformer back and get a different one

The transformer you have was designed for use as a replacement transformer inside HVAC equipment, hence the design with primary and secondary wires in the same nipple. However, this also means it's RU (UL Component Recognized, or Rather Useless to us) instead of being UL listed, which isn't helpful in our situation. I'd take this back and get a different transformer with a full UL listing and a different wiring arrangement; in the Johnson Controls lineup, the Y65T21-0 or Y65T31-0 would be suitable substitutes, depending on the mounting arrangement you're working with. (They also provide an extra "blind" terminal on their secondary side that can be used as a general wire-connection point.)

  • yeah, gazillions of transformers that either install as a junction box cover, or in a KO, will fill the bill. Apr 4, 2021 at 17:49

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