Control and Power Conductors in Same Conduit

[fiberstress]

Hi,
Need some help interpreting the NEC as well as some help with general engineering sense.

We're interpreting NEC 2014 725.48 that power and control conductors for the same equipment can share a raceway:

725.48 Conductors of Different Circuits in the Same
Cable, Cable Tray, Enclosure, or Raceway. Class 1 circuits
shall be permitted to be installed with other circuits as
specified in 725.48(A) and (B).
(B) Class 1 Circuits with Power-Supply Circuits. Class
1 circuits shall be permitted to be installed with powersupply
conductors as specified in 725.48(B)(1) through
(B)(4).
(1) In a Cable, Enclosure, or Raceway. Class 1 circuits
and power-supply circuits shall be permitted to occupy the
same cable, enclosure, or raceway only where the equipment
powered is functionally associated.

First, are we correct in this interpretation? Second, we have a digital signal on a shielded pair of 18AWG sharing a conduit with 12.5kV shielded power conductors. Would the magnetic field not have a significant impact on the digital signal, regardless of the shielding?

The reason this all came about is we're replacing an existing installation which had this setup already.

Thanks much in advance for your knowledge.

 

[jraef]

What you are missing is the earlier dictate that all conductors in the same raceway must have an insulation rating equal to the highest voltage in the raceway. Don't have access to my NEC to cite the section and paragraph, but this effectively eliminates the possibility of you doing what you propose.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[7anoter4]

300.2 Limitations. (C) Conductors of Different Systems. (1) 1000 Volts, Nominal, or Less.
Conductors of ac and dc circuits, rated 1000 volts, nominal, or less, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.
(2) Over 1000 Volts, Nominal. Conductors of circuits rated over 1000 volts, nominal, shall not occupy the same equipment wiring enclosure, cable, or raceway with conductors of circuits rated 1000 volts, nominal, or less unless otherwise permitted in (C)(2)(a) through (C)(2)(d).
However, (C)(2)(a) through (C)(2)(d) does not include specifically class 1 circuit.

 

[GroovyGuy]

You certainly cannot do this in Canada, and since he CEC is very similar to the NEC, I'd be shocked if you could get away with that south of the 49th as well. The only exception to this rule might be if the cables in question were armoured. Even then, depending upon the nature of the data-signal, the length of the raceway, ampacity, etc., I would still be concerned wrt noise.
Regards,
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[qman5]

What bothers me about the CEC or NEC is that it is not always practical. What about cases where you have the same (e.g. 1000V) insulation rating with foil shielding? In this case, I see no objection to running power and control cables in the same trench/conduit/tray.

 

[GroovyGuy]

Hi Qman,
The NEC & CEC may not always give you the practical solution that you desire, but I believe that the decision here is straight forward; you can't place a 300V instrument cable in the same raceway as a 15kV power-cable.

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[davidbeach]

As has been stated many times here in the past, you can't apply logic to the NEC, and by extension the CEC. The rules simply are what they are. It's simple and straight forward and certainly not open to debate. File a proposal if you wish, but unless and until it is accepted you're stuck with the rules as written.