How to Find the Number of Florescent Lamps in a Circuit?

Electrical Technology

How to Determine the Number of Fluorescent Tube Light and CFL Lamps in a Branch or Final Sub-Circuit?

The number of fluorescent lamps, tube lights, and compact fluorescent lamps (CFLs) required is not equal to the number of incandescent lamps with the same power rating. This is because of the role of the choke and ballast in a fluorescent tube circuit. The choke and ballast act as control devices, but the inductance of the choke causes a decrease in the power factor.

Related Post: How to Calculate the Number of Incandescent Lamps in a Final Sub Circuit?

Calculating the Number of Fluorescent / Tube Lights in a Circuit

230V/240V Circuits:

Suppose we need to install 75-watt fluorescent lamps instead of incandescent lamps for general lighting.

If the current rating of the final sub-circuit is 5 amperes and the supply voltage is 230V (110–120/240V in the US), the number of fluorescent lamps can be calculated using the following formula:

Number of Lamps = (Supply Voltage × Current Rating of Sub-Circuit) ÷ (2 × Power Rating of Fluorescent Lamp (W))

(230V × 5A) ÷ (2 × 80W)

= 7.18

This means you can use 7 fluorescent lamps or tube lights, each rated at 80 W, on a 5-amp, 230V final sub-circuit.

120V Circuits:

Similarly, in the case of 40 W fluorescent tube lights with a supply voltage of 120V and a branch circuit rated at 10A:

120V × 10A / (2 × 40W)

= 15

Hence, you can install a maximum of 15 fluorescent tube lights on a 15-amp, 120V branch circuit.

Note:

Final sub-circuits and lighting branch circuits are designed based on maximum demand. It means they are calculated to handle the maximum power consumption when all connected appliances are in use.
The power factor has been taken into account due to the inductive load (choke and ballast) present in fluorescent lamp circuits.

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