wiring a control relay

[gsjhand]

Hello

Is there any code requirements for using 24 volts and 120 volts on two separate set of contacts of same control relay? I want to keep them separate but I am being questioned by some guys working along with me.

 

[jraef]

Well, you MUST keep them separate obviously, but separate sets of contacts is separate. There are rules about the wiring, i.e. all wire insulation in a common space must be rated for the highest voltage in the space, but that's rarely an issue.

There are code issues about separation and isolation with regards to medical equipment, but "low voltage" in that realm is 10V or less.

There are UL listing issues with regards to "limited energy circuits", wherein the 24V may need to be sourced from a "Class II transformer", meaning no more than 100VA available on the circuit in order to avoid having to have the devices it feed be UL listed. But I still don't think that precludes connecting the circuits on the same relay.

So in a word, no.

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[dpc]

I agree with jraef. There is no NEC issue as long as the 24V circuit is adequately insulated. But meeting the NEC is not necessarily the same as good design practice. The NEC is mostly concerned about preventing fires, not making sure everything works perfectly.

 

[waross]

I used to see 24 V and 120 V mixed on relays in garage door and gate operator circuits.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DRWeig]

UL panel shop here. No problem, if you do what jraef said above. Maintain separation between conductors at >40V and those at <=40V after leaving the relay terminals. The relay contact terminals, if rated and listed for 120V, will have adequate separation. The separation distance for Class 2 power-limited circuits is 50 mm, just like the NEC says. There is, however, an exception allowed at the termination of multi-voltage components.

Have a look at the contol board in a modern residential A/C and heating system. The 24V control circuits terminate to the same board as to 120V fan power wiring. Not a problem.

If you want to review detailed rules, get a copy of UL508A or UL508 standards.

Best to you,

Goober Dave

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