A/C unit comes on and lights dim for brief moment. I have tighten the connection related to a/c unit including breakers and disconnect and neutrals.

Can other applinces have an effect on the a/c unit if they are turned off. Can loose connection for other applicances effect lights when a/c comes on?

The AC may be causing the dimming lights due to a loose connection anywhere beginning at the utility system, your meter, and panel.

You may be able to check conections in the panel, however, if the problem is in the meter and/or the utility system, you may need an electrician and/or the utility to check those connections.

AC units draw a lot of power on startup. Depending on your panel rating, it is possibly just too much momentarily. I have a 150 AMP panel and the AC causes a very brief dimming, less than 1 sec.

The A/C unit drawa quite a bit of power for the first few cycles when the motor kicks on. It's that inrush of current that makes the lights dim. Are the lights and the A/C on the same circuit? Putting the A/C on a dedicated circuit should solve the problem. If not, don't worry about it too much. As long as you don't have electronics on the circuit nothing really to worry about.

A well designed system in good condition should never result in lights dimming from an air conditioner starting.

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Originally Posted by tkrussell

A well designed system in good condition should never result in lights dimming from an air conditioner starting.

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Could a loose neutral on other circuits cause the light to dim? The a/c is installed properely and the electrical to it is in excellent shape!

A neutral of other circuits should not be affected by the AC, but a loose neutral in the panel, meter or utility could.

All connections from power company meter to panel to disconnect to a/c unit are secure! Change the breaker and the disconnect. Lights are still dimming. Only briefly but enough to notice! All connections in panel are tight so I don't understand . Didn't know if neutrals in other parts of the house would cause the lights to dim if they were loose. Thanks for your help

What type of AC do you have? What type and size circuit?

If possibilities all I mentioned are fine, there could be bad contacts on the Main Breaker.

Could be the utility line is too small, and/or too long.

I have found both of the problems I just mentioned causing dimming also. Any chance you could post some photos of the meter, panel, AC disconnect, and perhaps the utility feed, if at all possible.

Also, the distance of the utility feed to a utility transformer.

To check the breakers, a simple test is to measure the millivolt drop across the breaker contacts. However, you need special test instrument to measure millivolts.

Is the main breaker a plug in ata meter/disconnect, or bolted to bus bar in the panel?

There are so many points of possible failures. We will keep going until we find something.

What size service do you have? Do you know what the A/C draws?

I see you changed breaker and disconnect, I assume this is for the AC?

The AC is not, or should not , be the problem.

The dimming is by a voltage drop that is occurring, caused by the high current the AC requires. All AC equipment does this, the starting current for a refrigeration unit, every single one, is much higher, 6 to 10 times, the running current.

And since the AC is connected to the same feed/wire/equipment, at the source, the source is the problem.

I am assuming this is a 240 volt unit. If so, the problem is a connection in the live circuitry.Most do not require the use of a neutral.

Mike asked good questions. Read off all of the nameplate data on the AC, and what size amp service do you have.

All connections are good power company has check voltage from their transformer and at the meter base. The connections are tight at meter both line and load side. The connections at panel are also good and the voltage is good. 120/240 a/c is 240 max amp 40. Both disconnect and breaker in panel is 40 a 2 pole. And I already have used a big hammer. a/c unit is about 4 years old and we just put in new furnace upstairs with remote control. Since there isn't any wiring that inter connects the furnace and the a/c unit didn't think it should cause the problem. Lights didn't dim before we but in new furnace. a/c guy doesn't have a clue. He said he put hard start controller on a/c unit and it didn't help but he didn't leave it. Thanks for your help

We need to know the size of the main breaker. It makes a difference if you're using 40% of the power, or only 20%

AC nameplate info? Did AC tech measure the running and starting current?

AC is old and furnace is new? Data on the furnace?


How is the furnace connected?

You started by saying the AC dimmed the lights, now the furnace is involved?

Can you quantify the dimming?

Distance of utility line? Did utility do a load test at meter?

Have all connections and breaker contacts checked? This requires an electrician with specialized instruments.

Photos?

Service is 200 amp single phase 120/240 all connections are tight. I mentioned the furnace because I didn't know if it could have influence.

Another take on this problem.
Any breaker panel can cause this problem if not arranged correctly when the branch circuit wiring comes in.

For Single Phase Services;
Where the panel Line feeders (the two big Hot wires, each are a separate Phase)are hooked up (landed) on the bus bars or the Main Breaker is connected to the bus bars in the panel is at one end (top or bottom). The bus bars are arranged as two long bars to which the branch circuit breakers are attached.
The bus bars are arranged so that the two (or three in 3 phase power) present access points to the two phases alternately going up or down the panelboard. Or said a different way this means that every other breaker is the same phase. So 1&2 are the same phase as 5&6; 3&4 and 7&8 are the opposing phase.

When new panels are connected in the field, Good Practice always dictates landing the lighting loads at the end of the bus bars closest to the Main Breaker. Large loads and subpanels should be landed at the other end of the panel.

This is because of voltage drop across the bus bars in the panel. Without going into a long discussion of voltage drop, what is happening is that when a large load (motors, resistance heat, A/C etc) is turned on, a voltage drop is seen by loads farther away from the Main Breaker.

Look at the arrangement of the circuit breakers in your panel, you must know where the incoming (Big) feeder wires land, at the top or at the bottom. If you have a Main Breaker in the panel it should be obvious, If your Main is not there (could be located outside at the meter) then you will need to (carefully) remove the panel cover and look.

It may be necessary to rearrange your circuit breakers. Sounds easy, but it's more complicated than you might think. Phase relationships MUST absolutely be respected.

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Originally Posted by shuntripper

Another take on this problem.
Any breaker panel can cause this problem if not arranged correctly when the branch circuit wiring comes in.

For Single Phase Services;
Where the panel Line feeders (the two big Hot wires, each are a separate Phase)are hooked up (landed) on the bus bars or the Main Breaker is connected to the bus bars in the panel is at one end (top or bottom). The bus bars are arranged as two long bars to which the branch circuit breakers are attached.
The bus bars are arranged so that the two (or three in 3 phase power) present access points to the two phases alternately going up or down the panelboard. Or said a different way this means that every other breaker is the same phase. So 1&2 are the same phase as 5&6; 3&4 and 7&8 are the opposing phase.

When new panels are connected in the field, Good Practice always dictates landing the lighting loads at the end of the bus bars closest to the Main Breaker. Large loads and subpanels should be landed at the other end of the panel.

This is because of voltage drop across the bus bars in the panel. Without going into a long discussion of voltage drop, what is happening is that when a large load (motors, resistance heat, A/C etc) is turned on, a voltage drop is seen by loads farther away from the Main Breaker.

Look at the arrangement of the circuit breakers in your panel, you must know where the incoming (Big) feeder wires land, at the top or at the bottom. If you have a Main Breaker in the panel it should be obvious, If your Main is not there (could be located outside at the meter) then you will need to (carefully) remove the panel cover and look.

It may be necessary to rearrange your circuit breakers. Sounds easy, but it's more complicated than you might think. Phase relationships MUST absolutely be respected.

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Thanks you I will try to reagrange the breakers

While having large loads at the top of a panel and lighter loads at the bottom, maybe a field practice, it has no bearing on voltage drop.

The 12, 18 or even 24 inch long busbar of a panel is just that, a long metal bar. ANY voltage drop issue caused in a panel would be at the bolted connection of a Main Breaker to that bar, or where the input lug bolts to the busbar, and that connection would be loose.

Anytime having a large load at the bottom of a panel or far away from the input, causes voltage drop, would indicate a mechanical problem n that panel.

After your done moving breakers around, and you find it does nothing to solve your problem, would you take some time ans answer the several questions I have already asked.

a/c load has been check by a/c guy. The a/c comes on and the lights dim. All the connections in panel are tight and in good condition. Power company has checked the incoming voltage both at transformer and meter center. All the connections in meter center are tight and in good condition. Furnace is only a 110v and is thermostat controlled dimming only occurs where compressor kicks on. All voltage amps have been check and all contact points have been tighten. I didn't move breakers around because I didn't under stand how you could have a voltage drop on a 18 inch buss bar. I will meet a/c and electrician on Tuesday.

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Originally Posted by tkrussell

While having large loads at the top of a panel and lighter loads at the bottom, maybe a field practice, it has no bearing on voltage drop.

The 12, 18 or even 24 inch long busbar of a panel is just that, a long metal bar. ANY voltage drop issue caused in a panel would be at the bolted connection of a Main Breaker to that bar, or where the input lug bolts to the busbar, and that connection would be loose.

Anytime having a large load at the bottom of a panel or far away from the input, causes voltage drop, would indicate a mechanical problem n that panel.

After your done moving breakers around, and you find it does nothing to solve your problem, would you take some time ans answer the several questions I have already asked.

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I respectfully disagree, you have misstated my point and my assertion was that light loads closer to the lugs is good practice and I stand by it.
I have solved this problem more than a couple of times by rearranging CBs, I'm not saying that it is the solution in every case.
Having a large load at the TOP of the panel was the potential problem as stated by me, not the BOTTOM as you just said. The lighter loads farther away, on the other side of the larger loads from the main/lugs are affected by the voltage drop form large loads. (Top fed Panel this example) Voltage drop along the bus bars is definitely a real phenomenon, and the OP has reported that all the connections have been checked and are tight.