There is no allowance in the IEC or SABS/SANS specifications for dimmers to be tested or qualified for more than one dimmer per wall box. It is however often desired to install more than one dimmer per switch box. In these cases it must be kept in mind that a dimmer does heat up the air inside a closed space and two or more dimmers in the same closed space will hence influence each other’s maximum load and temperature ratings. Ideally the additional dimmers should be installed remotely (in the ceiling or roof), or the dimmers should be de-rated.

There is no de-rating factor available from the international specifications, but Shuttle recommends that the maximum dimmer VA rating be de-rated by 1.15 times the number of dimmers inside the same enclosure, as follows:

For example, if two 500W dimmers are being installed in the same wall box, each dimmer’s VA rating is de-rated to 217VA each. Similarly for three 500W dimmers, each dimmer is de-rated to 145VA. The normal dimmer maximum load for non-unity loads must however still be obeyed. Hence in the example of the three dimmers, if one load has a unity power factor, that dimmer’s wattage rating is 145W and if the other two loads have a 50% power factor, each of the corresponding dimmers will have a maximum load rating of 73W.