There are 2 types of fluorescent lights that we commonly use in our house. One is the 4 feet long ones normally used in the halls and kitchen and the other is the 2 feet ones found in the bathrooms. These 2 types of fluorescent lights uses different kinds of tube and different kinds of ballast. While the tubes are easily differentiated by their length when you want to buy a new one, the ballast looks identical.
The only way you can differentiate them is to look closely at the information printed on the ballast. For a 2 feet fluorescent light you must buy a 20 watts ballast. It will be stated on the surface of the ballast if you care to look.
What If the fluorescent light is fitted with the wrong ballast
If you replace the 40 watts ballast with a 20 watt one, no harm will be done except the lights won’t work. You might be scratching you silly head bald trying to figure out the fault if you don’t realise that you have simple put in a ballast that is not strong enough to kick start the light.
On the other hand, if you use a 40 watts ballast on a 20 watts light, you are going to burn out the tube as well. It won’t explode or anything, but you will see the tube lighting up 2 times brighter than usual for one or two seconds before it just dies out. Now you will have to buy a new tube as well as replace the ballast.
So do take caution when you want to buy a new ballast. Make sure they are the right wattage. 20 watts for a 2 feet light and 40 watts for a 4 feet light.