We have already discussed how to install and wire a photocell switch in a lighting installation and how to size a photocell for a lighting installation. We noted that the photocell switch is an energy saving device used to help conserve energy during the day and switch on the lighting installation during night-time.
Photocell sensors or switches come in various voltage and current ratings. For lighting loads under 5 Amps, it may be possible to wire the photocell switch directly to the lighting load circuit.
However, with large lighting loads, the photocell sensor has to be used indirectly to switch on and off the lighting load using a contactor. For example, let us say you have twenty (24) pieces of 250W High Pressure Sodium Lamps providing lighting to a small sized industrial complex, evidently it is not possible to wire the photocell sensor directly to the lighting loads!
What will be required is a higher current rated 3-phase contactor to power the lights, while a photocell can be wired to energize the contactor coil. During the day, the lights will be switched off and during the night when the photocell activates as a result of increased resistance, the lights come on.
Note that on the photocell sensor, L1 is the live wire, N is the neutral wire and Lo, is the load wire which goes to energize the contactor coil which must be rated for the phase voltage (L1-N or L2-N or L3 -N). Common phase voltage levels are 120V, 208V or 240V.