I've been trying to understand what wire colors I should use for 24V in the US.
Here are some pages that try to summarize:
https://multilinedesigns.com/control-cabinet-standard-wire-colors/
https://www.eaton.com/ecm/groups/public/@pub/@europe/@Electrical/documents/content/pct_2930585.pdf
I realize there are multiple standards, including NEC, NFPA 79, and UL508A that dictate wire colors.
NEC seems pretty open ended, it mainly says white is neutral bonded to ground.
Here is where I get confused, from NFPA 79:
13.2.3.2 The use of other colors for the following applications shall be as follows:
(1) WHITE with BLUE stripe for grounded (current-carrying) dc circuit conductor
13.2.4.3 Where color-coding is used for identification of conductors, the following color codes shall be permitted to be used:
(1) BLACK for ungrounded ac and dc power conductors
(2) RED for ungrounded ac control conductors
(3) BLUE for ungrounded dc control conductors
There are lots of threads out there where everyone is jumping in saying they see it lots of different ways.
A common one I hear people saying and that I've seen:
24V+: Blue
24V-: White with blue strip
My concern is that there is a misunderstanding here about what the intention of the standard was.
Most of the places where they talk about grounding in these standards is saying how neutral is connected to Earth ground. They don't tend to use ground and "common" or "24V-" interchangeably.
Another common one is probably closer to the intention of the standard:
24V+: Blue
24V-: Blue
I think what they might have intended in the standard was that blue should be used if you have a floating DC power supply, and white with blue stripe should be used if you ground the DC power supply, meaning 24V- is connect to Earth ground.
So if you have a floating DC power supply one interpretation of how you could do it:
24V+: Black
24V-: Black
Control signal: Blue
It seems maddening to have + and - be the same color though, as it seems to easy to mix up the wires.
So what do they really mean here, and what colors should we really use?
And of course in NFPA 79, 13.2.3.2
Exception No. 2: Where the identification of machine power and control wiring is such that compliance with the mandatory color codes is too restrictive for specific applications, it shall be permitted to use additional identification at selected locations as an alternative. This means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means as shall be permanently posted on the inside of the main electrical control panel enclosure in a visible location.
So on some level I guess it doesn't matter really and you can make it whatever you want, as long as you label it? As long as it's not white, grey, or orange, because those are special.
Here are some pages that try to summarize:
https://multilinedesigns.com/control-cabinet-standard-wire-colors/
https://www.eaton.com/ecm/groups/public/@pub/@europe/@Electrical/documents/content/pct_2930585.pdf
I realize there are multiple standards, including NEC, NFPA 79, and UL508A that dictate wire colors.
NEC seems pretty open ended, it mainly says white is neutral bonded to ground.
Here is where I get confused, from NFPA 79:
13.2.3.2 The use of other colors for the following applications shall be as follows:
(1) WHITE with BLUE stripe for grounded (current-carrying) dc circuit conductor
13.2.4.3 Where color-coding is used for identification of conductors, the following color codes shall be permitted to be used:
(1) BLACK for ungrounded ac and dc power conductors
(2) RED for ungrounded ac control conductors
(3) BLUE for ungrounded dc control conductors
There are lots of threads out there where everyone is jumping in saying they see it lots of different ways.
A common one I hear people saying and that I've seen:
24V+: Blue
24V-: White with blue strip
My concern is that there is a misunderstanding here about what the intention of the standard was.
Most of the places where they talk about grounding in these standards is saying how neutral is connected to Earth ground. They don't tend to use ground and "common" or "24V-" interchangeably.
Another common one is probably closer to the intention of the standard:
24V+: Blue
24V-: Blue
I think what they might have intended in the standard was that blue should be used if you have a floating DC power supply, and white with blue stripe should be used if you ground the DC power supply, meaning 24V- is connect to Earth ground.
So if you have a floating DC power supply one interpretation of how you could do it:
24V+: Black
24V-: Black
Control signal: Blue
It seems maddening to have + and - be the same color though, as it seems to easy to mix up the wires.
So what do they really mean here, and what colors should we really use?
And of course in NFPA 79, 13.2.3.2
Exception No. 2: Where the identification of machine power and control wiring is such that compliance with the mandatory color codes is too restrictive for specific applications, it shall be permitted to use additional identification at selected locations as an alternative. This means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means as shall be permanently posted on the inside of the main electrical control panel enclosure in a visible location.
So on some level I guess it doesn't matter really and you can make it whatever you want, as long as you label it? As long as it's not white, grey, or orange, because those are special.
