Replacing a standard plate switch with a dimmer switch should be a relatively straight forward procedure provided that a few golden rules are followed;
The most important thing to remember when fitting a dimmer switch is not to under load or overload it. If a dimmer is under loaded it will not work correctly, usually with erratic, noisy output. An overloaded dimmer will simply blow if the stated load limit is exceeded. The load rating will always be printed on the dimmer packaging.
Most standard domestic dimmers come in 250W and 400W sizes but it Is important to remember that the rated maximum output can vary depending upon the load type connected. Standard toroidal and electronic dimmers work best with incandescent lamps but need to be downrated when a halogen load is connected, usually by 50% but you should always consult the manufacturer’s instructions.
The following lamp types either cannot be dimmed or require special equipment; LED, fluorescent, compact fluorescent (energy savers), metal halide (usually only found commercially) and sodium.
A further consideration when fitting a dimmer switch is the physical size of the unit. A lot of light switches will be fitted into a 16mm deep pattress and this will not be deep enough for almost all dimmers. Some manufacturers provide a plastic collar to lift the dimmer off the wall and out of the pattress. If this isn’t provided or still doesn’t give enough clearance the only alternative is to fit a deeper pattress.
One last thing to remember when working on a 2 way circuit (or intermediate circuit) is that only one dimmer should be fitted to ensure correct operation.