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How to Calculate the Number of Fluorescent Lamps in a Final Sub Circuit?

How to Determine the Number of Fluorescent Tubelight and CFL Lamps in a Final Sub Circuit?

The number of fluorescent lamps, tube lights and compact fluorescent light (CFL) is not equal to the number of incandescent lamp having the same power rating. This is due to the rule of chock and ballast in the fluorescent tube circuit. Chock and ballast acts as a controller device and power factor decreased due to the inductance of the chock. Capacitors are used to improve the power factor and overall current of the circuit increases as compared to the incandescent lamps.

Now suppose, We have to install 75 Watts fluorescent lamps instead of incandescent lamp for general lighting.

If the current rating of the final sub circuit is 5 Amperes having supply voltage of 220V (110-120V in US), then the number of flourcent lamps can be find by the following formula.

Supply Voltage x Current Rating of Sub-Circuit / (2 x Power Rating of Fluorescent Lamps in Watts)

220V x 5A / (2 x 75W)

= 7.33

It means, you can use 7 numbers of fluorescent lamps or tube lights each of having 75W on 5A, 220V final sub circuit.How to Find the Number of Fluorescent Lamps in a Final Sub-Circuit

In case of 40W tube lights where the supply voltage is 120V and current rating of final sub-circuit is 10A.

120V x 10A / (2 x 40W)

= 15

Hence, you can install  the maximum of 15 number of florescent tube lights on a final sub-circuit having 10A and 120V AC supply.


  • Final sub-circuits based on max demand i.e. it consumes max power while appliances are in use.
  • Power factor has been consider due to the inductive load (chock and ballast)  in fluorescent lamps circuits.

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