What can cause lights to dim?

ecrannyMay 10, 2011

I installed a sub-panel in my basement with about 12 circuits, two of them are 15A circuits for kitchen lights, and a 20A circuit for waste disposal. The WD unit is a 3/4hp Insinkerator. When I turn on the WD the lights dim for a few seconds and then return to normal. My understanding is that this is usually caused by a voltage drop, but I am note sure where that could be happening. The light circuits are 14/2 from panel to 3-way switches (about 15 ft) then 14/3 to a set of dimmer switches (another 15 ft) and then maybe 30ft total of 14/2 to individual light. The sub-panel is supplied by three 3awg copper conductors about 20 ft from the main panel. I am not sure if any of this would explain the dimming lights, and I am not sure what is feeding the main panel - from the ground they look like very thin wires (they look thinner than 3awg), about 70 ft to a pole, but the mast looks relatively new, so it seems unlikely that the supply conductors are under-sized.

Any ideas what might be the cause - could it be the lighting circuit run is too long? Could the dimmer switchs be the cause, somehow creating a voltage drop? Should I get the utility company to take a look at the supply?

Thanks in advance for your opinion on this.

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IMO this is a perfectly normal phenomenon.

    Bookmark   May 10, 2011 at 9:24PM
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This is normal while a synchronous motor is coming up to full speed. You can put the lighting and disposal on different legs in the sub-panel to minimize the effect. (Lighting on one leg, motors on the other)

    Bookmark   May 11, 2011 at 9:56AM
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When induction motors(like that used in a GD) start they pull a significant current surge sine the motor is NOT turning initially.
The motor draws its 'locked rotor current' until it starts to move, and then the current quickly drops back to the running current for the load on the motor.
There is also a second start winding drawing power until the motor reaches operating speed.

The start winding is required since single phase motors has zero starting torque without the start winding and a capacity to produce some phase shift in the start winding relative to he 'run' winding.

This short heavy draw causes a voltage drop and lights on the same circuit, or even leg of the service may dim slightly.

The voltage drop is not entirely wiring losses, so increasing wire size can only reduce it so far (and is rarely worth the extra cost).

    Bookmark   May 11, 2011 at 11:44AM
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Thanks for the explanations - I was concerned that I might have to rewire using heavier conductors. I am happy to live with the dimming effect, but anyway I will see how easy it is to move the breakers to different legs.

Thanks agian for the input!

    Bookmark   May 11, 2011 at 4:06PM
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