lights dim with a/c unit

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090414-1124 EST

How many feet from the transformer to the main panel. 500 ft for some of my neighbors. That is a loop length of 1000 ft. 0000 copper has a resistance of 0.05 ohms per 1000 ft. 0 copper is about 0.1 ohm/1000 ft.

Suppose the inrush is 200 A and 0.1 ohm, then we have my 20 V value calculated above. If the load is a 240 V device, then 10 V would be seen by a 120 V circuit. No change of transformer will solve this part of the problem.

The various important components needed to be evaluated for their contribution to whatever problem may exist.

My loop length is shorter and I do have 0000 copper. I have one neighbor on the transformer. I do not share service wiring with my neighbor except for about 4' from the transformer. I have light flicker from their air conditioner turning on. That is all due to transformer impedance. Some day I will determine how large a drop occurs. Should my transformer be changed? I do not think so. Our combined average load would not justify a change.

A change to a larger transformer, or a lower impedance unit, would be an un-necessary installation cost to DTE. Further a larger transformer would have higher residual losses.

I have no equipment that is adversely impacted by what limited voltage fluctations I have.

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gar said:
. . . No change of transformer will solve this part of the problem. . .
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Replacing the transformer will help mitigate the problem but placing a transformer closer to your service will make a big difference. Three things may be done to help voltage drop or flicker, replace the transformer with a larger one, replace the secondary and/or service drop or lateral with larger cable, or moving the transformer closer. The choice is with the serving electric utility but the outcome is regulated by the state utility commission. :smile:
 
I have another question on this, based on the issue of ONLY the kitchen lights dimming. Forgive me if I'm way off base, I haven't done resi work (other than my own house) since my apprenticeship. But if it is really true that ONLY the kitchen lights dim, and it's not a perceptual problem as mentioned earlier, then wouldn't ALL of the lights dim if it was a problem with the supply xfmr or circuit size?

My take on this is that I have seen similar problems when there is a bad neutral connection somewhere. If, for instance, the kitchen lights are on a dedicated circuit and there is a loose neutral connection somewhere, the load is probably light enough (no pun intended) that under normal circumstances, the flicker caused by that bad neutral connection is not perceptible. But as soon as you have the additional VD caused by the AC unit starting, that bad neutral looms large and is now noticeable.
 
Jraef said:
. . . based on the issue of ONLY the kitchen lights dimming. . .
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My answers were based on the assumption that the location where the dimming was noticed was in the kitchen and the rest of the home was affected as well. I agree with you if this is the only place where the dimming actually occurs. :)
 
wishmaster68 said:
I only noticed to kitchen lights dimming. We had the lights on in the living room, dinning room and breakfast room and none of those rooms dimmed.
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It's possible that there is a small resistance in one phase, and the kitchen light is on that line, but the others aren't.

Temporarily move the kitchen-light circuit to the other phase, and/or the other lights to the opposite phase they're on now.
 
090414-1651 EST

wishmaster68:

You need a more controlled experiment such as the two bulb test that I mentioned above.

I doubt that if you have a large 240 V motor on any normal 200 A service that you won't see dimming of lights on both phases when the motor starts. This assumes the motor comes from the same main panel and service wires.

Note: the 240 V motor should have nothing to do with the neutral. There should be no change in neutral current from the motor starting, and thus no change in netural voltage relative to the transformer center tap.

If only one bulb dims, or more correctly dims more than the other bulb, then look for a high resistance joint on the phase with the bulb that dims the most.

You would have to have a very low impedance source on both phases, as viewed from the main panel, to not see a voltage change sufficient to be detect by incandescent lamps on startup of a large 240 V motor.

Once you can prove that the major variation is common to one phase, then you can make measurements to find out why.

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Jdb3

Jdb3

Years ago, I had the father-in-law of a builder call me that his lights got "birighter" when his a/c came on. I was told no hurry. A week later the called again, that there were sparks coming from behind his electric range. I found that gas piping was also ran to the space behind the range. Arcing had occured from the electric range to the gas pipe. I found a loose connection at the main panel, therefore, when the a/c came on. It was trying to make up for the loose connection, by using the gas pipe for the neutral.
 
090415-0957 EST

JDB3:

The air conditioner that you are describing had to be a 120 V unit, or at least had a large 120 v component or there would be no neutral current.

For a 5 ton air conditioner it will be mostly 240 if not all. If there is a bad neutral and you put a large load on one 120 V phase, then the voltage on that phase will decrease and the voltage on the other phase will increase.

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charlie said:
A 5 ton unit is considered to be a large AC unit. The normal electric utility transformer installation and secondary distribution normally is not sized for larger than 2 to 3 ton units. Rather than spending lots of time trouble shooting the entire system looking for trouble that is not there, call the serving electric utility and complain about the voltage flicker and dip. They will send a trouble man out to do a cursory check and you need to insist on a recording voltmeter for a longer period of time.

The serving electric utility needs to replace the transformer, the secondary, the service drop, or a combination thereof. :smile:
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As with 480's post and Charlie's post, I concur.

It may be a combination, but more likely it may be the length of the run from the transformer/size of transformer. How many dwellings is it serving? How far from the transformer..1, 2, 3 or more houses from the transformer?
 
090416-2057 EST

Tonight I ran a limited test with a 60 W incandescent bulb. I can visually perceive a light change from a 1 V change at 120 V. Without knowing the timing of the event I might not perceive the change while reading a paper. In signal detectability circles this would be described as signal known exactly. A 3 V change is quite noticeable and would be detectable without timing information.

I used a 25 ohm rheostat in series with the lamp and adjusted it for the desired drop. To make the voltage change I short the resistor in series with the bulb. This way there is no drop to a lower voltage during the switch from one voltage to another as there would be with a SPDT break-before-make relay contact.
 
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