Lights Dim when Appliance Used


Old 01-13-05, 09:09 AM
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Lights Dim when Appliance Used

My addition is just about complete, but one of the problems is in the master bedroom. There are three recessed light fixtures, which I believe are 75 watt bulbs each. When I ran my 6.5 HP shop vac (amp draw unknown), the three lights dimmed. I later ran my electric drill (4 amps) and they also dimmed, though very slightly. This is a new circuit on an upgraded service (125 amps). Before I complain to the contractor, I would like to have a little knowledge as to what the cause maybe and to corrobrate that something is not right. It doesn't seem that three lights should dim when a drill is plugged in. If it was an amperage overload, wouldn't the breaker just trip?

By the way, in the bathroom adjacent, which is an existing circuit, the breaker trips when my wife uses a hair dryer. Prior to the addition, this only occurred when the hair dryer and a wall heater were plugged in and used at the same time. What's gone wrong?
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Old 01-13-05, 01:24 PM
Join Date: Feb 2003
Posts: 525
Did the electrician tap off the existing circuit that feeds the bathroom and who knows what else? If the circuit already has a substantial load, plus a goodly feet of wire on it then a motor load could dim the lights. Do the lights dim anywhere else in the house when you use shop tools in the bedroom?
Old 01-13-05, 03:22 PM
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No, they do not dim anywhere else.
Old 01-13-05, 03:34 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Any time you have receptacles and incandescent lighting on the same circuit, the lights will dim when you turn on something plugged into one of the receptacles. It is unavoidable. Incandescent lights dim with even a small drop in voltage. This is one of the reasons many people prefer lighting and receptacles on separate circuits, although you usually have to pay extra for that.

Having said that, I will say that "excessive" dimming could be because of a poor connection somewhere on the circuit. How much is "excessive"? That's quite subjective. A large motor (e.g., a 6.5HP shop vacuum) usually causes the greatest momentary dimming and most people will notice it. Turning on a small hand-held drill should cause only slight dimming and not everybody will even notice it.

Any dimming probably warrants an inspection of all the connections on the circuit. However, depending on how large the loads and how sensitive your eyes and how closely you are paying attention, you may not be able to eliminate it all.

Did anything change on the existing bathroom circuit?
Old 01-14-05, 07:07 AM
Join Date: Feb 2003
Posts: 525
The 6.5 HP motor on the shop vac is highly misleading. For some reason, shop vacs and other shop tools are measured in "peak" HP, which is done by removing anything that will resist the motor and then applying as much voltage as possible until the motor burns up and measuring what kind of momentary power can be obtained. Of course, the vac will never be used in such a manner. The "actual" horsepower of a 6.5 HP hop vac is probably closer to 1.5 HP, which is still a large enough load, especially on inrush, to cause incandescent dimming, especially if the circuit is large and far from the panel or there are any "weak" connections.
Old 01-14-05, 10:27 AM
Join Date: Jan 2005
Location: North Virginia
Posts: 192
Exactly, if the shop vac could pull anything over 2 HP continuously it would be wired for 240V. Realistically, 1.5 HP continous is all you can get on a 120V circuit. The inrush current is going to be very close to the locked rotor amperage and that is quite a bit higher than the average operating load of the vac.

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