Type D electrical services are used to power both constant power circuits and lighting circuits. A Type D service contains a panelboard with a main breaker and several slots for plug-in type breakers. It contains a 2-pole lighting contactor and photocell that will switch one circuit on and off for lighting. Type D services are 120/240V only.

Type D services are typically used for traffic signals, where the controller requires constant power and the lights are only powered at night.