24 VDC, 4-20mA same cable

big john

Senior Member
Location
Portland, ME
There's no particular reason to worry in this case: With steady DC there's no induction and no interference, so there's no reason to worry about cross-talk when mixing these two circuits.

That said, it's always a good idea to shield analog control circuits, especially considering how common and cheap shielded cable is it's just cheap insurance.

I don't know of anything in the NEC that would prohibit this, maybe someone else can chime in.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Are both the 24 VDC power and the 4-20mA circuits Class 2 circuits? If one is and one isn't, in general they can't be mixed. If they both are Class 2 circuits they can be mixed. If neither are, they probably can be mixed.
 

bward

Member
Location
New York
There's no particular reason to worry in this case: With steady DC there's no induction and no interference, so there's no reason to worry about cross-talk when mixing these two circuits.
Beware if the 24VDC power is ever switched on or off, however. That may cause some large spikes of interference.
 

gar

Senior Member
141024-1047 EDT

panhandle444:

Any pair of wires used for a signal should be twisted or coax. This reduces magnetically coupled noise. If electrostatic shielding (electric field -- capacitive coupling) is used, then it is preferable to shield each signal pair from other wires. Neither signal wire should have any current in it that is not related to the signal. Signal wires should be connected to common or ground in only one place to prevent erroneous currents from modulating (affecting) the desired signal. Commonly referred to as a ground loop.

Belden 8723 is two twisted pairs with each pair separately shielded, and also a separate drain wire.

.
 

dfmischler

Senior Member
Location
Western NY
The 4-24mA current loop is pretty noise immune to induced voltage (because it depends on current flowing). Depending on exactly how it is used it may not be an issue at all if it is shielded or not. Having worked for a company that strung many miles of Belden 8723 on customer sites for this kind of stuff, I think it is a safe choice assuming that the current needs of the 24VDC circuit are in range (the datasheet says up to 2.3A per conductor @25C, but the voltage drop on a long run would be horrific if you drew that much current).
 
Top