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#100414 - 11/16/06 10:30 PM AC/DC Combined  
George Little  Offline
Joined: Jan 2004
Posts: 1,489
Michigan USA
Does anyone see a problem with installing AC and DC circuits in the same raceway? Assume all wire insulation is rated for the higest voltage in the raceway, all wiring is either Class 2 or Class 3 wiring - not mixed . Should also say that we are talking very low amperage and these are all control wires. Some of you old Navy guys probably have dealt with this. Would like any code references or any other reason why this would be a problem. Thanks.

George Little

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#100415 - 11/16/06 11:50 PM Re: AC/DC Combined  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,099
Class 3 is class 3 (2 is 2), I don't see any difference between AC and DC in the 725 descriptions.
The place I have seen the fight is class 2/3 in the same pipe with article 300 wiring but the conductors are "separated" with cable jackets. I have seen that one go both ways.

Edit to add
The classic example would be a phone cable. That is a class 3 that is DC (10-48vdc) until it rings, then it is AC (100vac).
Back when we had the "princess" phone (Its little, it's lovely and it lights) there was also a class 2 (10-14VAC) circuit on the black/yelow pair for the light.
I know "phone" used to be out of our hands when this was all Ma Bell but it is certainly there now with 3d party installers.

[This message has been edited by gfretwell (edited 11-16-2006).]

Greg Fretwell

#100416 - 11/17/06 08:14 AM Re: AC/DC Combined  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,853
Brick, NJ USA
I can add a little from personal experience..
Did a printing plant move back in mid-80s..
Had some equip from early 40's that was DC. We installed a rectifier and "DC" rated panelboard. I kept all DC in sep. conduits.

Doing a walk thru at the old loc. in NYC, the Shop Foreman showed me some DC tools he had/used. I asked where he plugs them in...he shows me normal 15 amp, 120 volt 3 wire receptacles by his bench...I ask, how do you know which are AC, and DC......????
He replys 'experience'

I installed a non-NEMA oddball twistlock for all the DC 125 volt 'toys'....



#100417 - 11/17/06 10:04 AM Re: AC/DC Combined  
Zapped  Offline
Joined: Oct 2002
Posts: 482
Huntington Beach, CA, USA
I could be wrong on this, but when I was a "pup" in the field I was always told to keep control wires(signal) and supply wires (AC power) seperate to avoid 60hz hum and hamonic distortion from invading the DC lines and distorting the signals...i.e. capacitance, induction, etc., so I've always made a practice of this. Even with shielded cables, such as TV/Cable/Sat, and non-shielded such as phone, solinoid & relay, speakers, etc., the possibility of induction leads me to keep them from crossing paths or feeding through the same conduit.

I've never put the theory to the test completely, but maybe someone here can back me up or shoot me down with some experience with troubleshooting such a matter.

#100418 - 11/17/06 01:30 PM Re: AC/DC Combined  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,099
Zapped, it all depends on what the "low voltage" side is feeding. If this is driving relays or even SSRs the current required to drive the "controlled" load is far beyond anything that could be coupled by running next to power conductors. This started becoming an issue when we started using CMOS "Field Effect" transistors. They respond to microamps so a little noise will be seen. You can relate to this when you see that random number generator we call a digital meter when it is connected to an open circuit. A well designed driver/receiver set will actually present a load on the line (usually 100-150 ohms) that matches the characteristic impedence of the wire so it does not respond to those low current transients. If they are really serious they use a twisted pair like CAT5 which is not going to see that 60 cycle hum at all.
I have seen some poorly designed stuff that does have problems with ingress of noise but if that is true you find the world is full of noise. Wiring practices may make it better but you will still have problems.

Greg Fretwell

#100419 - 11/17/06 01:38 PM Re: AC/DC Combined  
Zapped  Offline
Joined: Oct 2002
Posts: 482
Huntington Beach, CA, USA
Thanks g. That narrows the field a quite a bit.

#100420 - 11/18/06 09:32 AM Re: AC/DC Combined  
jes  Offline
Joined: Jul 2002
Posts: 103
George, if the circuits are definitely Class 2 and/or 3 from listed supplies or equipment then the CODE rules for installation of the conductors or cables together fall squarely under Art 725 (see Section III). However, I have found in the past that the installation instructions or the wiring guidelines put out by the equipment manufacturers will likely be far more restrictive as they are concerned with performance issues of their systems. That part makes it tougher to enforce...

#100421 - 11/18/06 01:40 PM Re: AC/DC Combined  
Dave T  Offline
Joined: Jan 2005
Posts: 155
Waukesha, WI, USA
Interference that could be inducted from on system to another and may interfere with a communication or control circuits and should be considered. One also must assure that the insulation rating of the conductors are rated for being installed next to one another. Placing a conductor with 250v insulation rating next to a conductor carrying 480v is not acceptable.
Identifying the conductors may be something that also should be considered. As an example in the MCCs that I have sold power wire was black, the common control circuit was red, separate source AC that entered and exited was yellow, and where there was DC blue wire was used.

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