3 phase 12 inch distance 310.15 adjustment factor afci ampacity annex d arc flash arc flash ppe bid bonding breaker cable tray circuit breaker code conductor conduit disconnect disconnect switch duct bank egc equipotential bonding fault feeder fire alarm flat rate gec generator gfci ground grounding grounding and bonding grounding electrode hot tub hvac incendive led lighting load calculation low voltage ltc medium voltage motor nec nec 310.16 neutral non-incendive ocpd panel parallel conductors power factor pricing protection pvc relay residential restricted approach service shock short circuit sizing solar transformer ul508a unbalanced load underground service ups va journeyman vfd voltage drop