Electrician Talk banner
1 - 3 of 3 Posts

163 Posts
Discussion Starter · #1 ·
So we got these huge electric company trucks to wire up and they have a vent fan in the cab and a pair of vent fans in the cargo area. On one truck the fan in the cab is not working. I climb up to the roof and pop the cover off the not-working fan and one of the working fans. The broken fan had a black and a red wire and the working fan had a green and a red. OK, seems pretty straight forward, right? Even a shade-tree mechanic could figure this out...

My DC voltage tester lights up red when the probe touches (+) and the clip is grounded and it lights up green when hooked up in reverse. I wanted to see if the broken fan had a good ground so naturally I put the clamp on the broken fan's black wire and the probe on the working fan's red wire. The tester lighted up green! WTF???

After considering for some time how I could be getting >12V positive voltage through a negative wire I looked a little closer at the fans and saw that the manufacturers used the RED wires to ground and the BLACK / GREEN wires as (+).


Long story a little shorter, the problem was a loose ground connection.

So, are there any standards of wire coloring in the DC / Automotive world?


licenced and willing
109 Posts
As for cars I have no idea, but in the DC world for telcom there are several "standards".
In -48volt (+ is the grounded conductor) black is neg, red is pos.
AT&T -48volt systems use red for "A" plant and blue for "B" plant (redundant systems).
In +24volt systems I think neg is black and pos is red.

The solar inverters we install are also positive ground but neg is red and pos is white.
1 - 3 of 3 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.