24 vdc with 120 vac

[controlnovice]

We have a DCS system (quickly becoming obsolete)that has 24 vdc pulse input signals, driven from DCS system, for the majority of the digital inputs (limit switches, etc).

The wiring for the 24 vdc pulse is run with the 120 vac signals also (120 vac mainly for solenoids to operate valves). These are in bundled wiring.

New DCS systems have a straight 24 vdc digital inputs, no pulse.

Is there an issue with running the 24 vdc with the 120 vac signals? Any interference issues? Any code issues (I can't find any)?

Another option is to change the 24 vdc inputs to 120 vac inputs (but that means more DCS cards as the 120vac inputs are limited to 8 or 16 per card, vs 32 for 24 vdc).

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This is normally the space where people post something insightful.

 

[DRWeig]

Hi novice,

Running them together is usually not an issue unless the 24 VDC circuits are designated as Class 2 power-limited types. In that case, you need separation.

If it's not run as Class 2, as long as all the conductors have insulation rated for the highest voltage, there's no real code issue that I can think of.

If your low voltage wiring is twisted pairs, or shielded, or has some other means of attenuating noise, again -- we find this all the time and it works OK.

Best to separate them, but if you're dealing with an existing situation it might be OK.

Double-check the Class 2 situation, though. And wait for some of the others in this forum to respond too --

Good on ya,

Goober Dave