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Q. I remember being taught not to use a panel or disconnect as a raceway, but I can’t find a reference to this rule in the Code. I want to be able to justify the extra cost other than just knowing it is the right thing to do. Could you point me in the right direction?

A. Cabinets containing panelboards, cutout boxes or enclosures for switches, and meter socket enclosures can be used as a raceway through which to feed conductors if the conductors don’t fill the wiring space at any cross section to more than 40%.

In addition, splices and taps are permitted if they don’t fill the wiring space at any cross section to more than 75%.