I'm bodging together a bench PSU from an old server PSU, about to order a mixed bag of terminal posts for the front panel but then tripped over the problem that I'm not sure what's the correct or most common colour to use for the -ve rail on a split-rail supply.

Obviously there's red for +ve, black for 0v, green for earth... but what for the -ve rail?

Commonly available terminal colours are yellow and blue, and I'm aware that blue is used in -48v telecomms supplies (either as 0v or -48v depending which side of the EU standard you are, headdesk), so it's the front runner at the moment... unless you guys know different?

I thought I'd check here before I make a hideous faux-pas.

  • \$\begingroup\$ Red and black for positive and ground are common, but certainly not a universal standard for something like benchtop use. Any color for a negative supply will be even less standard. Use what you have around, although it would be good to keep away from red, black, and green. \$\endgroup\$ Feb 18, 2014 at 20:34
  • \$\begingroup\$ Id have gone red, white and blue with white an indistinct and colorless 0v. Hang what other folk do. LOL \$\endgroup\$
    – Andy aka
    Feb 18, 2014 at 20:40
  • \$\begingroup\$ Black is colorless too. \$\endgroup\$ Feb 18, 2014 at 20:41
  • \$\begingroup\$ I guess I brought this upon myself, ask a pedantic question, get pedantic comments... :D \$\endgroup\$
    – John U
    Feb 18, 2014 at 20:48
  • \$\begingroup\$ No standard BUT | NOT yellow - to be seen as highest V+. | NOT Orange = ambiguous. | Blue is a good choice. \$\endgroup\$
    – Russell McMahon
    Feb 19, 2014 at 3:14

1 Answer 1


Personally, I would use a "cold" color for the negative such as blue or maybe purple.

enter image description here

One "standard" I can think of that's not incompatible with red=positive and black=0V is the standard PC power supply color code:

enter image description here For more info on wiring colour codes see this wikipedia article.

  • 2
    \$\begingroup\$ Yes, there is no real standard, but blue seems logical. \$\endgroup\$
    – felixphew
    Feb 18, 2014 at 20:46
  • 2
    \$\begingroup\$ @felixphew Or, rather, there are many standards, many of them mutually incompatible (such as the DC power standards allaboutcircuits.com/vol_5/chpt_2/2.html) \$\endgroup\$ Feb 18, 2014 at 20:50
  • 1
    \$\begingroup\$ Or more succinctly: xkcd.com/927 \$\endgroup\$
    – John U
    Feb 18, 2014 at 20:58
  • \$\begingroup\$ +1 - I'd have suggested blue, as not being green or black, which may be more easily mistaken. \$\endgroup\$
    – Russell McMahon
    Feb 19, 2014 at 3:12
  • \$\begingroup\$ Purple (violet) is the standard color for negative supply, as far as I was taught. \$\endgroup\$ Jun 28, 2018 at 19:29

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