How to Install and Wire a Photocell Switch in a Lighting Installation

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A photocell switch is essentially a light dependent resistor, LDR. Its resistance decreases with increasing incident light intensity. They are used in many applications for on-off control especially in lighting installations.

In lighting applications, Photocells are placed in streetlights to control when the lights are ON or OFF. During daylight, light falling on the photocell causes the streetlights to turn off and during night hours or darkness to turn on. Thus energy is saved by ensuring the lights are only on during hours of darkness.

How to Wire a Photocell 
A photocell used in lighting application has three terminals labelled as:
1. Load line (Lo)
2. Neutral line (N)
3. Supply or live line (LI)
In most photocells, the load line wire is RED, the neutral wire is WHITE and the Supply line is black. 

This colour code is not universal. It may change for some other brand of photocell.
The picture of the terminals of a brand of photocell is shown below:

Photocell Terminal Markings

Wiring and installing a photocell is pretty straight forward as shown below:
How to Wire a Photocell Switch in a Lighting Installation

As shown above, the load wire (Lo) goes to the lighting installations connected in series while the neutral (N) wire through a breaker is looped to all the lights. The supply line through a breaker supplies the photocell electrical power.