Dimming lights

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jamesguy10

jamesguy10

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Amsterdam NY
In a restraunt, there is a (single phase service) 200 amp main panel, that feeds a 150amp sub, that feeds a 100 amp sub.

Everytime a large load comes on (AC or walk-in freezer/cooler) the lights in the dining room dim. All loads (lights+equip) come from the 150 amp sub.

I know why its happening, but im not sure how to really remedy this.

Any ideas?
 
This is pretty much impossible to get rid of when a large motor load starts. The same things happens at my parents house when the condensing units start.

The only thing that comes to mind for a solution is a "hard start" kit for the compressor motors, available at Grainger or any refrigeration supplier.
 
ultramegabob said:
larger service, larger wire....
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Increasing the size of the service will not change anything. Increasing the size of the wire will reduce the voltage drop to a degree, but remember that motors draw many times their rated current for a fraction of a second when they start "across the line", and that's impossible to change unless you use a VFD.
 
well, just from experiance, 200 amp single phase service for a resturant sounds kinda small, but I dont know how small this place is...
 
ultramegabob said:
well, just from experiance, 200 amp single phase service for a resturant sounds kinda small, but I dont know how small this place is...
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A table saw could dramatically dim lights on a 200 amp service with minimal other load on it...I've seen it happen. :)
 
If the hard start kit doesnt take care of it, I would upgrade to 400 amp service set 2 200 amp panels and put all the mechanical into one panel and lights in the other, and make sure the power company runs larger wire to the service.
 
Many restaraunts I've done deal with theis problem by oversizing the circuit itslef. They protect the wire size with the breaker, then use a fused disconnect to protect the motor.

At first, it seems wasteful ("Why am I feeding this 40-a load with #2?"), but the outcome is obvious.
 
It's simple physics, you need more electrons... (bigger wires)... Are the lights on the same branch circuit as the motors? if so, separate them. If all else fails install fluorescent lights with electronic ballasts.
 
jamesguy10 said:
In a restraunt, there is a (single phase service) 200 amp main panel, that feeds a 150amp sub, that feeds a 100 amp sub.

Everytime a large load comes on (AC or walk-in freezer/cooler) the lights in the dining room dim. All loads (lights+equip) come from the 150 amp sub.

I know why its happening, but im not sure how to really remedy this.

Any ideas?
Click to expand...

Depends on the wire sizes and lengths you may get excessive voltage drop on some of the wires from the source(transformer) to the load. You may try to increase those or bring a properly sized - for voltage drop - wire directly from the 200A main panel.

Otherwise use a soft starter.

Last solution is upgrading the main service.
 
At start of motor if the impedence of the circuit becomes less than the transformer (exceeding the va rating of the trans.) the voltage supplied by the trans will begin to drop. Maybe the transformer needs to be upgraded to a different size.

I cant see why upgrading the service will help this except for the fact of bigger wire equals less resistance which creates less voltage drop. My experience with this is you have to go alot bigger to make much of a difference.
 
Replacing a service to keep lights from winking is often a lost cause unless you can also get the PoCo to upsize the drop and the pole top transformer(s). Best of luck with that.
 
jamesguy10 said:
Everytime a large load comes on (AC or walk-in freezer/cooler) the lights in the dining room dim. All loads (lights+equip) come from the 150 amp sub.
Click to expand...

It really depends on how bad the lights dim and for how long. If it's really bad there may be a loose connection somewhere in the service.

Last time I had one like this it was an 800 Amp service and the lights would dim when the A/C kicked on. Loose connection at the weather- head. When the lineman laid a leather glove on the connection it started to smoke ( it was hot ).

The thing to figure out is have the lights always dimmed or is this something new?
 
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