3 phase 310.15 adjustment factor afci aim alarm cad ampacity annex d arc flash ballasts bonding breaker cable cable tray circuit breaker code conductor conduit disconnect disconnect switch duct bank egc fault feeder fire fire alarm flat rate gec generator gfci ground ground fault grounding grounding and bonding grounding electrode hot tub hvac led lighting load calculation low voltage ltc medium voltage motor nec nec 310.16 nec 2014 neutral ocpd overload panel parallel conductors power factor pricing protection relay residential roof service short circuit siemens sizing solar switchboard transformer ul508a unbalanced load ups vfd voltage drop