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I am designing a cable that carries a 4 - 20 mA current loop.

Considerations:

  • The positive lead is connected to VCC.
  • The negative lead is connected to a 120 Ω resistor (other end of the resistor is grounded).
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  • \$\begingroup\$ What colors can you get in a twisted shielded pair? \$\endgroup\$ – Spehro Pefhany Jan 15 '19 at 22:01
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Traditional

Traditional standard colours for DC are red for positive and black for negative. In this case it could cause confusion and result in the black being grounded. To avoid this I would try red and white or red and clear, if available.

Current practice

Industrial sensor leads are now standardised as brown for positive, blue for negative and black for signal. That would leave you with a brown-black pair possibly leading to maximum confusion. If you supplied all three cores and leave the blue unconnected at the sensor end then that confusion would disappear but a new one would be introduced: where does the blue connect to?

enter image description here

Figure 1. Wiring for the ifm TAD981 sensor. Two-wire colour coding shown on left.

Selecting a random 4 - 20 mA sensor from the ifm range shows brown and white.

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I'll bite.

Traditional:

  • Red = Positive = (+) = Vcc
  • Black = Negative = (-) = Resistor.

Not necessarily twisted pair.

Perhaps due to how current-meters and/or voltmeters were connected in prehistoric times.

However, any color combination will will work, as long as you document it.

There are dangers of choosing unwisely - like your poor users constantly hooking things up backwards. For an example of a non-traditional color-code (from another device type), see the color code for a J-type thermocouple
(https://en.wikipedia.org/wiki/Thermocouple#Type_J). I bet dollars-to-donuts that some physicist is responsible for that.

In essence, the "hotter" color tends to be connected to (+). Yes, very technical ;-)

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The easiest jacketed cable that I can get in my locale is 22 AWG twisted-pair in PVC or polyurethane jacket. There are three common pair colors in these single-pair cables:

black & white (most common)

black & red (not as common)

white & red (rare but seen in the wild).

The convention that is common in my locale for signal cables is "White Hot". Even the local electricians respect this convention even though it is contrary to the Canadian Electric Code for power conductors (White is always Neutral). But you aren't wiring power here.

In the above pairings: black is low-side for the first two pairs, white is low-side for the white & red pairing.

Again this is what is considered to be common practice in my locale. This may be different where you are.

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