Conductor Fill

If you’re running electrical wiring in your home, you need to know about conduit fill. This is the amount of space that your wires take up in the conduit, and it’s important to keep track of because you can only fill the conduit so much before it becomes a fire hazard. The NEC (National Electrical Code) has strict guidelines about how much conduit fill is allowed, and you need to make sure that you stay within those guidelines.

To calculate your conduit fill percentage, you need to know the area of the conduit and the area of the wires. The area of the conduit is simply the circumference multiplied by the length. For round conduit, you can use this formula: area = π x (diameter/2)^2 x length. For example, if you have a 3/4” diameter conduit that is 10 feet long, the area would be: π x (3/4”/2)^2 x 10’, or 1.7 x 10^-4 x 10, which equals 0.0017.

Now you need to calculate the area of the wires. This is a little more complicated, because you need to know the diameter of the wires and the number of wires that will be going into the conduit. Once you have that information, you can use this formula: area = (π x (diameter/2)^2) x number of wires. For example, if you have 4 #14 wires going into the conduit, the area would be: (π x (0.064”/2)^2) x 4, or 3.14 x 10^-5 x 4, which equals 0.00013.

Now that you have the area of the conduit and the area of the wires, you can calculate the conduit fill percentage. To do that, simply divide the area of the wires by the area of the conduit and multiply by 100. In our example, that would be 0.00013/0.0017, which equals 0.765, or 76.5%.

The NEC has different guidelines for different types of conduit, but in general, you want to keep your conduit fill percentage below 40%. That will give you plenty of space for your wires and will help to prevent any fire hazards.

Conductor Fill

 Maximum Number of Conductors in Electrical Metalic Tubing Type Conductor Trade Size (in.) THHN, THWN, THWN-2 Size (AWG / kcmil) ½ ¾ 1 1¼ 1½ 2 2½ 3 3½ 4 14 12 10 8 12 9 5 3 22 16 10 6 35 26 16 9 61 45 28 16 84 61 38 22 138 101 63 36 241 176 111 64 364 266 167 96 476 347 219 126 608 443 279 161 6 4 3 2 1 2 1 1 1 1 4 2 1 1 1 7 4 3 3 1 12 7 6 5 4 16 10 8 7 5 26 16 13 11 8 46 28 24 20 15 69 43 36 30 22 91 56 47 40 29 116 71 60 51 37 1/0 2/0 3/0 4/0 250 1 0 0 0 0 1 1 1 1 0 1 1 1 1 1 3 2 1 1 1 4 3 3 2 1 7 6 5 4 3 12 10 8 7 6 19 16 13 11 9 25 20 17 14 11 32 26 22 18 15 300 350 400 500 600 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 1 1 1 1 1 1 1 1 1 1 3 2 1 1 1 5 4 4 3 2 7 6 6 5 4 10 9 8 6 5 13 11 10 8 7 700 750 800 900 1000 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 1 1 1 1 3 3 3 3 2 4 4 4 3 3 6 5 5 4 4

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