HOW TO DETERMINE THE NUMBER OF LIGHTING FIXTURES IN ONE CIRCUIT

How To Determine The Number Of Lighting Fixtures In One Circuit

Many asked, what is the maximum number of lighting outlets for a 15A or 20A circuit breaker, and what is the maximum number of convenience outlets in a 20A circuit breaker? I often hear these questions from friends in the construction field repairing designs for projects. Electrical Plan is one of the design plans required in acquiring a building permit in the Philippines before execution by the owner or its contractor. Some limit the number of lightings in one circuit to 12, and 8 for convenience outlet. Ideally, this is a good practice, but are these the maximum allowed numbers? I'll show you below how to determine the maximum ideal number of lighting fixtures in one circuit.

As a reference for a 15A circuit breaker, we use 2.0mm square stranded wire or #12 AWG for the circuit.

Given Parameters: 

Nominal Voltage: 230Volts

Circuit Breaker rating @ 80% Safety Factor: 12Amperes

Total Computed Load in Watts: ____, to be determined

Formula:

P = I x V

Where,

P is the total computed power in Watts(W)

I is the total current available in Amperes(A), and

V is the given voltage 

P(Watts)= 12A x 230V

                = 2,760Watts

So, the maximum allowable wattage for the lighting circuit is 2,760watts. Assuming we have light bulbs with 50Watt ratings, the total number of lighting fixtures for this circuit is,

No. of Lighting Fixtures= 2,760W/15W, which is 55. 

Now we know the maximum number of lighting fixtures for this circuit is 55 sets. Where can this be applied? I have applied this in a function room and a hallway of a hotel project, also in residential, but the number of fixtures reached only to about 30 sets. The number of lighting fixtures depends on their application and the zone location. For example, you have 15 lighting fixtures for the kitchen, dining, and living area, 10 in combined bedrooms, and 25 at the exterior is good for each lighting circuit. In reality, we don't really need to reach that 55-sets mark, but it is not economical to limit the number of lightings in a circuit due to not knowing the allowable number of sets. The advantage of the zoning arrangement of lighting circuits is to lessen the affected area when trouble occurs to a specific circuit