You may have laughed at the electrical drawing the first time you saw it, but now you have to finish the installation, and you still don't have a good drawing. For one thing, the conduit fills are obviously all wrong. Nobody puts 45 No. 12 wires and 15 No. 14 wires in single half-inch raceway. You're out in the field, and you don't have a computer with you. How do you determine what size raceway you need?
The figure shows a flow chart of the general steps. Let's continue with our example of the No. 14 and No. 12 wires, and go through the steps in detail.
The first step is to calculate the size of the wires. Let's say we are using THHN. We go to the National Electrical Code, Table 5 and find that a No. 12 THHN has an area of 0.0133 sq in., and a No. 14 has an area of .0097 sq in. Our 45 No. 12 wires times 0.0133 sq in. give us an area of 0.5985 sq in. Our 15 No. 14 wires times 0.0097 sq in. give us an area of 0.1455 sq in. Our total, then, is 0.744 sq in. Don't forget that the derating penalties exacted by Note 8(a) to the ampacity tables effectively preclude large numbers of power conductors in a common raceway. In this case, the ampacity of the No. 12 conductors would fall to just over 10A (30A30.35510.5A).
What do we do with our 0.744 sq in. of wire? Let's look at the National Electrical Code, Table 4 to see what size raceway we need. We'll assume the raceway is EMT and runs overhead for far more than 24 in. (maximum nipple length). With more than two wires, we can max out at 40% fill.
How do you use the NEC's Table 4 for this type of calculation? The column you want to use is "Over 2 Wires," since we have some 60 wires and want a 40% fill. Accommodating all of our wire will require a 1 1/2 in. EMT.
Now that we've covered how to determine raceway fill, let's cover some basic concerns on conduit selection. Remember that the Code is not a design manual. You may decide to go with a larger raceway if you have a long wire pull or simply want an extra margin of safety and performance from your wiring. Web Kee, whose byline has appeared in EC&M, has done extensive thermography on hundreds of installations. From his perspective, it's almost always better to go with bigger raceway. In some cases, you may want to divide the run into two raceways. If you do this, keep circuits intact. Do not, for example, run two of a motor's phase conductors in one conduit and the third in the other.