How many 125 watt bulbs can be used on a 15amp circuit?

Assuming that your voltage is 120 volts the maximum wattage on that circuit is W = A x V = 15 x 120 = 1800 watts. Code only allows circuits to be loaded up to 80% capacity. 1800 x .8 = 1440. 1440/25 = 57 lamps.

Assuming that your voltage is 240 volts the maximum wattage on that circuit is W = A x V = 15 x 240 = 3600 watts. If the question is from a 240 volt country you will have to find out what the local electrical regulations are for circuit loading.