What causes an overhead light bulb to dim when a small dry vac is plugged into a wall outlet in the same room and turned on?

Answer 1

This light bulb may be on the same line as the vac. 1. Even if the dry vac is, let's say 300W, when it starts up it uses at least twice as much energy. If light dims and then goes back to normal, it's perfectly normal. 2. When light bulb dims and stays dim while operating dry vac, unplug it and never use in the same room again; it uses too much energy for the size of the wire and can cause overheating of wiring and fire! == == <><><> If the wiring is modern, check the connections on every device on that breaker or fuse. If you have a meter check the voltage at the receptacle with the vac and the light. <><><> There are two possibilities, depending on the details of the fault:

1) The bulb dims momentarily when you start the vacuum, then returns to normal brightness even while the vacuum is operating.

This is normal, as previously stated. The reason for this is that the limiting factor to current flow in a motor is the counter-voltage created by the running motor. [It actually works as a generator!. If you jam the motor so it cannot turn, the current flow through what is now an unimpeded conductor can be high enough to trip the circuit protective device!] ... and reduce the available voltage to any parallel connected device to the voltage across the motor! In this case, the resistance value of your circuit conductors can be the only significant load on the circuit [low resistance=high current!]

2) The bulb dims and stays dim throughout the operation of the vacuum.

There is something wrong with the circuit ... loose connection, partially open service neutral, partially broken wire, bad plug, dodgy breaker, etc. ... and the defect is part of the load, causing a voltage drop across the defect and your series connected motor! [voltage drop=energy usage=heat at the defect]

In this case, the bigger the load, the worse it looks!