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Lights dimming? #174420 02/04/08 03:35 PM
Joined: Jan 2005
Posts: 65
petey_c Offline OP
Here's a question that's probably already been asked. Occasionally when I'm working residential, a customer will ask, "Why do the lights dim momentarily when my A/C (or other heavy load device) turns on. I try and give a non-technical or tech-light explanation. It's usually the "electricity is like water in a plumbing line. You're taking a shower and someone flushes the toilet. You experience a momentary loss of "pressure."" Any thoughts? pete

Work Gear for Electricians and the Trades
Re: Lights dimming? [Re: petey_c] #174427 02/04/08 07:31 PM
Joined: Jul 2007
Posts: 1,335
sparkyinak Offline
It can be several things. It is all dependent on the problem.

* Too many things on the circuit and it is trying to trip the breaker but the surge is over before it can. in layman's term: It is like trying to stick 10 pounds of crap in a 5 pound box.

* The service capacity is too much at the transformer and can not carry the load. In layman's term: It is like the little engine that coundn't.

*They have a serious problem with their service neutral. There is potential fire or electrical damage to equipment due to higher the normal voltage. In layman's term:
it is like there is potential fire or electrical damage to equipment due to higher the normal voltage.

"Live Awesome!" - Kevin Carosa
Re: Lights dimming? [Re: sparkyinak] #174435 02/04/08 08:55 PM
Joined: Nov 2005
Posts: 823
JoeTestingEngr Offline
Just explain to them how a motor takes much more current to start and get up to speed than to run at that speed. Have them think about how little fuel is required to hold 60MPH on the highway. It will be intuitive to them that it takes more when they go from zero to sixty. Then have them consider what it would take from under their hood if their car could even come close to zero to sixty in under three seconds. Then just explain that the huge current draw causes a voltage drop that causes the lights to dim even though they draw little current.

Re: Lights dimming? [Re: JoeTestingEngr] #174445 02/05/08 06:45 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline
Just tell them the truth- the power company are a bunch of cheapasses and undersize the aerial drop from the powerline transformer to their house, and then when big motors like a heat pump start up, they draw more power than the rest of the house combined for a fraction of a second. So when the big motor on heat pump starts up, the in-rush of current drops the voltage on the line into the house a few volts. It takes surprisingly little voltage drop to make a notible dimming of lights.

You can even point out that a typical 5hp motor has inrush of 168A, even if it only draws 20A while it's running. Shoot, if the pole pig is only 25kVA, that in-rush would completely saturate the transformer and dim *everyone's* lights.

Re: Lights dimming? [Re: SteveFehr] #174448 02/05/08 07:01 AM
Joined: Jan 2005
Posts: 65
petey_c Offline OP
Steve, Yeah, the PoCos are cheapasses... I've seen people's eyes start to glaze over when I start anything remotely technical (tech-lite). They seem to relate better to the sudden inrush of hot/cold water whilst they're taking a shower, than current. Had the pleasure of spending several months as a guest of the Navy out in Chesapeake at the Residence Inn off of Greenbriar. Nice area, considered getting my Va/NC license ('cause I was bored), but couldn't get the paperwork sent to me. Wife wouldn't be able to find it. Thanks for all the replies

Re: Lights dimming? [Re: petey_c] #174626 02/09/08 10:06 PM
Joined: Feb 2002
Posts: 2,233
harold endean Offline
Another thought, Ask the homeowner (HO) if the dimming lights are in just one room or in the whole house. Sometimes the problem is in the branch circuit. If the lights dim in the whole house the problem could be in the main service panel, the meter, the connections at the head or out on the street. It might take some time and patience to figure out just where the problem is located.

Re: Lights dimming? [Re: harold endean] #174645 02/10/08 11:17 AM
Joined: Jan 2006
Posts: 558
Rewired Offline
If they are having a problem with the lights flickering when an A/C starts, assuming there is no actual problem with the wiring or the service etc, you might want to try putting a " kick start" hard starting device in the A/C unit. I did on my parent's A/C (for other reasons) and now there is little or no flicker in their lights.


Re: Lights dimming? [Re: Rewired] #174693 02/11/08 09:27 PM
Joined: Jul 2007
Posts: 265
wire_twister Offline
Do you have a link for the "Kick start" device, I might want one for my house. 15kva pole pig + 5 ton heat pump = dim lights for split second.


Life is tough, Life is tougher when you are stupid
Re: Lights dimming? [Re: wire_twister] #174708 02/12/08 06:42 AM
Joined: Mar 2005
Posts: 1,213
SteveFehr Offline

It's a capacitor and a relay wired in parallel with the motor. Don't most condensors come with one of these already built into it?

Re: Lights dimming? [Re: wire_twister] #174710 02/12/08 09:05 AM
Joined: Jul 2004
Posts: 783
LarryC Offline
They are also known as hard start kits. Talk to your AC&R technician to verify that there is not another reason for the large current draw when starting.

Larry C

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