Mike Holt's Workbook to Accompany Understanding the NEC Volume 1 2011 Edition

use for raceways and splices cabinets being used for raceways and splices and enclosure can be used as a Raceway if the conductors don't fill the wiring space to more than 40 percent so you can run wires right through an enclosure just don't fill it up to 40 percent more than 40 percent here's an example that's my house I've done that you can make a splice and enclosure as long as the splice doesn't exceed 75 percent of the area sometimes I get somebody to call me up and say well I can just see the body language in the classes like well how do you know that you didn't exceed 40 percent fill when you ran the wires through or how do you know that you did exceed 75 percent when you made a splice in there I said look when you put the cover on and you put the screws back on if you notice that the metal is not bowing then that is an indication that you're okay see it isn't you can't run wires