DC color code ?

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djohns6

Senior Member
Location
Louisiana
Please help to settle an arguement . At work we use 125v DC for relaying and controls . The company buys two conductor cable for the input source to the devices . The cable has a black and a white conductor , and engineering specs black as positive and white as negative . One of my co workers swears that white should ALWAYS be positive and claims that there are standards that back him up , but I've never seen anything . Is anyone familiar with some code or standard that specs white as positive ? He says it's ANSI or IEEE , etc.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Trade Knowledge says that White is Positive unless an Engineer says it Negative.
Got to love those Engineers!!:D

Ford used red as a negative for some dash wiring in some of their mid 80's cars. They also used red as the positive in the same cars.

I guess that's not as bad as the steel mill I worked in for a while that used black wire for everything and didn't even tag most of it.

I have seen panels with hundreds of wires in them all the same color. Usually red or blue.

Myself, I never trust the color of a bit of plastic to tell me what voltage or polarity lurks within the conductor it insulates. I treat all of them like they are clear.
 

broadgage

Senior Member
Location
London, England
If one side of a DC supply is grounded, then that conductor should be white or light gray.
In this case the O/P states that neither conductor is grounded, therefore IMHO the two ungrounded conductors could be most any colours except white, grey or green.

If only black/white cable is available, then I would use black for negative, and the white wire marked with red for positive.

The use of white for an ungrounded conductor is probably a violation, if used for general building wireing.
If however it is confined to machines and control panels etc. then that could be considered to be an "appliance" and therefore not covered by the NEC.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
No absolute answer to that, other than "it depends".

UL508 has one set of color standards for "control panels", NFPA79 has another for "industrial machinery", ANSI C37 has another for "switchgear" etc. etc. and they don't all match. Then you have the issue of power or control, grounded or ungrounded negative, grounded or ungrounded positive etc. etc. etc.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
There are several different common color coding standards for wiring, along with many used only by a few places, or even just one.

We build some test equipment for a company that uses red/white/blue as phase a/b/c and green as neutral for incoming power connectors. The wiring is all generally black though.
 

rcwilson

Senior Member
Location
Redmond, WA
In process control wiring where black/red twisted pairs are used on 24 VDC wiring, many companies use black = +, red = -. Often the control cables arrive with black/white twisted pairs so white = "-" to keep black = "+".

On the 125VDC switchgear control wiring, to keep everything the same, black = + and white or red = -. (White is supposed to be covered with red tape.)

That always causes issues at the batteries because everyone knows that red on a battery means positive. We've fried a few controls when the polarity was swapped.

Bottom line, it would be nice to have an industry standard, but we haven't found one. Each company and customer does it a little different.

It's similar to the indicating light and pushbutton color discussions that are never resolved. ("Does red mean the valve is open or closed, the motor running or stopped? If red means run then why is the stop button red?)
 
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