Junction Box Size -  Conductors In Back of the Box

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Where a raceway enters the back of a pull box opposite a removable cover, there is a special rule which is used to determine the minimum depth of the box. This dimension is determined based on the largest conductor size in the raceway and not the raceway itself.

The dimension is a look-up in the NEC at table NEC 312.6A Use one wire per terminal.

The result will seem to be a very shallow measurement for a working electrician. Only the locknut size of a raceway on an adjacent wall could increase this required minimum measurement. Ofcourse in real life, the code is the minimum and if you want to install a deeper box on a job, great. But on an exam, stick to the code requirement. This is a code section where going for a larger size than is required makes sense.

Another unusual requirement is the U pull box. Here's a diagram.

So, where two raceways enter the same wall of the box, and they contain the same conductors without splices, then here are the rules:

a measurement 
the distance from the raceway entry in the box to the opposite wall must be at least 6 times the largest raceway plus the trade size in inches of the other raceway.

b measurement
This is the tricky part. The unusual rule is that the distance between the two raceways must be at least 6 times the trade size of the larger raceway. So the b measurement would be the sum of the distance from the corner to the first raceway,
plus the trade size of the first raceway,
plus 6 times the trade size of the largest raceway,
plus the trade size of the second raceway,
plus the distance to the bottom corner in this diagram.

c measurement
Just enough to accommodate the largest locknut would be just fine.

d measurement
6 times the trade size of the larger of the two raceways plus the trade size of the other raceway on the same wall of the box if in the same line.

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