Q. What Code section requires that enclosures containing overcurrent protection devices must be mounted vertically?
A. Section 240.33 of the 2011 NEC requires that enclosures containing overcurrent devices must be mounted in a vertical position unless this isn't practical. Circuit-breaker enclosures can be mounted horizontally if the circuit breaker is installed in accordance with 240.81.
Section 240.81 specifies that where circuit breaker handles are operated vertically, the "up" position of the handle must be in the ON position. So, in effect, an enclosure that contains one row of circuit breakers can be mounted horizontally, but an enclosure that contains a panelboard with multiple circuit breakers on opposite sides of each other will have to be mounted vertically.