480VAC / 24VDC Wiring in same Conduit

[plcguyntn]

I program PLC's (programmable logic controllers) and we usually have a problem whenever an electrician runs our control wiring (24vdc) in the same conduit as our motor wiring (480vac). The 24 vdc is usually going to limit switches, prox switches, photoeyes etc. Is this allowed in the NEC code? And where in the NEC code would I find that where it says that it can or can't be done?

Thanks,
Barry

 

[friebel]

Re: 480VAC / 24VDC Wiring in same Conduit

To: plcguyntn, You are not going to solve your problem by going to the NEC, because your solution is not there. I will tell you what we did in our low voltage situations with PLC's. We would always put our low voltage wiring in separate conduits, and/or cables in separate instrument cable trays. The low voltage cable would be shielded cable, and as you probably know, you only bond one end of a cable shield to ground, and that is generally in the low voltage source cabinet.I worked in large industrial chemical plant, and that was our standard procedure, and you will not find that experience in the NEC.


C's or any low voltage controls.
We would always put the low voltage controls in a separate conduit and/or cable tray and in shielded cable. Now, when you use shielded cable, you will only bond the shield to ground at one end of the cable, and that would generally be back a cabinet where the low voltage source is.

 

[pierre]

Re: 480VAC / 24VDC Wiring in same Conduit

Article 725, Sections 725.26, 725.55
Don't forget to read 725.3.

Pierre

[ January 28, 2004, 01:27 AM: Message edited by: pierre ]

 

[tony_psuee]

Re: 480VAC / 24VDC Wiring in same Conduit

plcguyntn-

Pierre referenced the NEC codes for the field wiring. However, there are also guidelines for identifying the circuits in your PLC panel. If the circuit is not properly identified as Class 2 or 3, how is the installing electrician supposed to know?

Tony

 

[james_mcquade]

Re: 480VAC / 24VDC Wiring in same Conduit

i do not have the code book handy,

the 24 volt wire must have the same insulation rating as the high voltage wiring or greater.

while you can mix the two, we have found that you will always have problems with the interference generated by the high voltage wiring. we have mixed 24 volts with the 110 volt circuits with no problems.

 

[pierre]

Re: 480VAC / 24VDC Wiring in same Conduit

725.55(D)(2) does permit class 2 conductors in the same raceway as power conductors in certain instances. Read that section for details. For plcguy's situation, it would not conform to 725.55(D)(2), because of the voltage rating of the power conductors.

Pierre

 

[don_resqcapt19]

Re: 480VAC / 24VDC Wiring in same Conduit

Are the circuits for PLCs identified as to NEC Class? I've never seen such an identification on a PLC power supply. If they are not identified, then they are Class 1 Remote Control and Signaling Circuits and they can be mixed with power circuits if they are "functionally associated". See 725.26(B)(1). Not that this is a good idea, just permitted by code.
Don