Wire Colors, Phases, and Circuit Numbers – Part 1: Introduction and Long Division

In the United States, wire insulation colors have been standardized by engineers and installers so that any qualified person can recognize on which phase a conductor is when they know either the insulation color or the circuit number. Although this procedure is not an NEC 2008 requirement, it is done to make installation and troubleshooting easier1.

If we were to look inside a typical 3 phase panel, this is what it would look like:

Circuit Phase Circuit
1 A 2
3 B 4
5 C 6
7 A 8
9 B 10
11 C 12
13 A 14
15 B 16
17 C 18
19 A 20
21 B 22
23 C 24
25 A 26
27 B 28
29 C 30
31 A 32
33 B 34
35 C 36
37 A 38
39 B 40
41 C 42

Each of those circuits and phases will have the following colors assigned to them depending on the voltage of the system:

120/208/240 Volts:

Circuit #’s Phase Insulation Color
1 & 2 A Black
3 & 4 B Red 2
5 & 6 C Blue
7 & 8 A Black
9 & 10 B Red
11 & 12 C Blue
and so on… Table repeats itself every 3 phases and every 6 circuits.

277/480 Volts:

Circuit #’s Phase Insulation Color
1 & 2 A Brown
3 & 4 B Orange
5 & 6 C Yellow

Non-veteran electricians often struggle with knowing which circuit number is on which phase and therefore which color the wire insulation should be.  Some carry around a small card that lists the circuits, phases, and colors, usually up through circuit 84.  There is nothing wrong with doing this, but it is nice to know how to quickly find a circuit’s phase and colors in one’s head.  Most are familiar with the lower circuit numbers, and can therefore do the math.  But once the circuit numbers become higher, say 83 for example, a calculator or long division on a notepad is needed.  Example:

3

It is the remainder which tells us on which phase that circuit 83 is.  Because we are dividing by six, the possible remainders are 0, 1, 2, 3, 4, and 5.  If the remainder is 1, 2, 3, 4, or 5, then just look at the corresponding circuit number in the tables above.  For this example, the remainder is 5, and circuit 5 is on phase C and is blue (or yellow).  This means that circuit 83 is on phase C and its insulation is blue (or yellow).

Let’s look at another example, circuit 33:

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The remainder is 3.  So, we look at our tables above and we see that circuit 3 is on phase B.  This makes the insulation color red (or orange).

So what about remainder 0 when the the number is divided equally by 6?  This means that it is circuit 6.  In short, no remainder = circuit 6.  Notice in the first table at the top of this page that every circuit which is a multiple of 6 is on phase C (6, 12, 18, 24, 30, 36, 42).  Let’s look at the example of circuit 42:

5

Since 6 divides equally into 42, there is no remainder.  Therefore, circuit 42 is on phase C.

There are other ways to find on which phase a circuit number is which require no division.  These will be discussed in upcoming parts of this article.

Continue on to Part 2 of this series to learn an easier way to find the phase of a circuit number.

Table of Contents


  1. For 99% of installations, the only reserved insulation colors by the 2008 NEC are green, white, and grey, with orange being required in a specific situation.  See 2008 NEC 250.119, 200.6, 210.5, 110.15, and 400.22.
  2. Phase B is orange if is it the high leg of a 3 phase 4-wire 240/120V delta system.  See NEC 110.15.   The high leg may not actually be on phase B, however.
  3. Image courtesy of WebMath.
  4. Image courtesy of WebMath.
  5. Image courtesy of WebMath.  

About Ross Finch

Ross Finch is employed by Denver-based electrical contractor Encore Electric.  He holds a Journeyman Wireman's license in the state of Colorado.  He is a graduate of Red Rocks Community College, holding AAS Commercial Electrician and AAS Maintenance Electrician degrees, and has completed the electrical apprenticeship training program at the Rocky Mountain Chapter of the Independent Electrical Contractors.  He presently assist-teaches in the electrical program at RRCC.