Lights dimming?? - Electrician Talk - Professional Electrical Contractors Forum

switchleg · 10-20-2009, 12:30 AM

Went out on a service call today. Lights in whole house dim for about a second went the A/C kicks on. Condensing unit is on a 2-pole 30. It is sized correctly. Amperage reading was only around 18 amps. Voltage was fine too. Phase to phase, phase to neutral, phase to ground. I shut down the power and checked all of the connections in the main, sub, disco and cond. unit. Condensing unit is around 5 yrs. old. Could it be a failing A/C? High LRA?

RIVETER · 10-20-2009, 12:39 AM

Hey, Switchleg, how's it going? If you were there when the lights dimmed did you measure the voltage? The A/C, if it is hard to start, can cause a lower voltage problem and the resulting dimming.

zen · 10-20-2009, 01:38 AM

Quote:

Originally Posted by switchleg

Went out on a service call today. Lights in whole house dim for about a second went the A/C kicks on. Condensing unit is on a 2-pole 30. It is sized correctly. Amperage reading was only around 18 amps. Voltage was fine too. Phase to phase, phase to neutral, phase to ground. I shut down the power and checked all of the connections in the main, sub, disco and cond. unit. Condensing unit is around 5 yrs. old. Could it be a failing A/C? High LRA?

i cant say that i know from first hand exp. but the electricians i know say its either a loose neutral or an unbalanced load on the phases. again im not sayin i know just tryn to help.

qckrun · 10-20-2009, 02:10 AM

Quote:

Originally Posted by switchleg

Went out on a service call today. Lights in whole house dim for about a second went the A/C kicks on. Condensing unit is on a 2-pole 30. It is sized correctly. Amperage reading was only around 18 amps. Voltage was fine too. Phase to phase, phase to neutral, phase to ground. I shut down the power and checked all of the connections in the main, sub, disco and cond. unit. Condensing unit is around 5 yrs. old. Could it be a failing A/C? High LRA?

Inrush current, which generally happens when the breaker is not correct sized. Refer to the motor section and correct size the breaker.

brian john · 10-20-2009, 11:04 AM

There are several issues here,

Possible loose neutral which needs attention IMMEDATLY. This can be from the transformer to your branch circuits.

Flicker< which can be something you are dingf (motors starting or possible a neighbor issue, depending on the utility distribution in your area.

Light flicker (dimming and restoration of light to near preflicker level) of lights from motor inrush) is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.

To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.

Ima Hack · 10-20-2009, 01:34 PM

Quote:

Originally Posted by qckrun

Inrush current, which generally happens when the breaker is not correct sized. Refer to the motor section and correct size the breaker.

How does breaker size has any effect on inrush current?

Think about that for a minute qckrun.

brian john · 10-20-2009, 01:45 PM

Quote:

Originally Posted by Ima Hack

How does breaker size has any effect on inrush current?

Think about that for a minute qckrun.

The only way I can think is not flicker, BUT a permanent outage. The breaker is under sized inrush exceeds the CB Instantaneous trip rating and out go the lights!

Ima Hack · 10-20-2009, 01:54 PM

Quote:

Originally Posted by brian john

The only way I can think is not flicker, BUT a permanent outage. The breaker is under sized inrush exceeds the CB Instantaneous trip rating and out go the lights!

Exactly.

switchleg · 10-20-2009, 11:44 PM

Thanks for all of the replies. As far as the loose neutral goes, that is the first thing that came to mind. I did tighten all of the lugs accept on the utility side of the meter. I did not want to pull the meter without informing the POCO. Upon visual inspection, the lugs appeared to be tight. There is no neutral feeding the A/C condensing unit. It's a 240v/30amp circuit.
One other factor to consider, the customer informed me of a lighting strike near the Service Entrance on the pole. No burning was visible to me though. The problem started after the lightning.
Maybe I should schedule the POCO to come out and have a look at their side??

qckrun · 10-21-2009, 12:01 AM

Quote:

Originally Posted by Ima Hack

How does breaker size has any effect on inrush current?

Think about that for a minute qckrun.

Good point... lol I wasn't thinking about it.

Wouldn't the size of the wire be another contributing factor as to why the lights are flickering. Since the amps of the start of the AC will be greater than the usual amps wouldnt that cause flickering of the lights

Magnettica · 10-21-2009, 12:07 AM

Quote:

Originally Posted by switchleg

Thanks for all of the replies. As far as the loose neutral goes, that is the first thing that came to mind. I did tighten all of the lugs accept on the utility side of the meter. I did not want to pull the meter without informing the POCO. Upon visual inspection, the lugs appeared to be tight. There is no neutral feeding the A/C condensing unit. It's a 240v/30amp circuit.
One other factor to consider, the customer informed me of a lighting strike near the Service Entrance on the pole. No burning was visible to me though. The problem started after the lightning.
Maybe I should schedule the POCO to come out and have a look at their side??

The lightning strike puts this into a whole other perspective. If there is damage to the POCO's conductors (like nicks from the lightning strike), then yes that could be the source of the problem, especially if this wasn't happening before. I would definitely be calling the power company if I were you.

switchleg · 10-21-2009, 12:11 AM

Quote:

Originally Posted by Magnettica

The lightning strike puts this into a whole other perspective. If there is damage to the POCO's conductors (like nicks from the lightning strike), then yes that could be the source of the problem, especially if this wasn't happening before. I would definitely be calling the power company if I were you.

Will do.

brian john · 10-21-2009, 07:48 AM

Quote:

Originally Posted by switchleg

Thanks for all of the replies. As far as the loose neutral goes, that is the first thing that came to mind. I did tighten all of the lugs accept on the utility side of the meter. I did not want to pull the meter without informing the POCO. Upon visual inspection, the lugs appeared to be tight. There is no neutral feeding the A/C condensing unit. It's a 240v/30amp circuit.
One other factor to consider, the customer informed me of a lighting strike near the Service Entrance on the pole. No burning was visible to me though. The problem started after the lightning.
Maybe I should schedule the POCO to come out and have a look at their side??

I'd never let that stop me from providing complete FULL service to my customers. POCO who?

Ima Hack · 10-21-2009, 08:32 AM

Quote:

Originally Posted by qckrun

Good point... lol I wasn't thinking about it.

Wouldn't the size of the wire be another contributing factor as to why the lights are flickering. Since the amps of the start of the AC will be greater than the usual amps wouldnt that cause flickering of the lights

Yes, wire size is a big factor. Although I would expect more of a dimming of lights (incandescent) than a flicker when the A/C starts. To me a flicker indicates some type of connection issue.

That being said, fluorescents might flicker when there is serious voltage drop, but I have not seen it personally.

I think brian john expained it best.

brian john · 10-21-2009, 11:52 AM

Quote:

Originally Posted by Ima Hack

Yes, wire size is a big factor. Although I would expect more of a dimming of lights (incandescent) than a flicker when the A/C starts. To me a flicker indicates some type of connection issue.

That being said, fluorescents might flicker when there is serious voltage drop, but I have not seen it personally.

I think brian john expained it best.

Check your IEEE Dictionary.

In most of the testing I have done with residential flicker, there are many contributing factors BUT the utility lateral (service drop) was the one major issue.

RIVETER · 10-21-2009, 03:33 PM

If it is determined to be the A/C "hard start" ;contributing to the possible voltage drop, you could install a Hard Start Capacitor at the compressor.
That may increase the power factor, as well, or maybe I should't bring that up.

waco · 10-21-2009, 06:11 PM

Riveter has a good point, but if the problem is recent, either the condenser is failing or the service legs are dropping too much voltage. I'd use a regular voltmeter to see if the service voltage is dipping on condenser start up. I bet it is.

brian john · 10-21-2009, 06:15 PM

You can also utilize a soft start on the unit.

nutsla · 10-22-2009, 02:23 AM

Quote:

Originally Posted by brian john

There are several issues here,

Possible loose neutral which needs attention IMMEDATLY. This can be from the transformer to your branch circuits.

Flicker< which can be something you are dingf (motors starting or possible a neighbor issue, depending on the utility distribution in your area.

Light flicker (dimming and restoration of light to near preflicker level) of lights from motor inrush) is a byproduct of several factors and generally is difficult to over come.

AC motor loads (in particular compressors) but all motor loads have inrush currents. These inrush currents result in voltage drop in the branch circuit, panel bus, service laterals, utility transformer and possibility the utility HV feeders.

Flicker is in addition user dependent; some people are more susceptible to flicker. For some reason flicker seems to bother women more that men (based upon personal experience).

A Voltage Drop (VD) as little as 3 volts on a nominal 120 VAC system is noticeable (by me), 2.5% VD.

We have had cases were the VD was all a byproduct of primarily single phase 120 VAC loads, in these cases we were able to put the lighting on L1 and all the motor loads on L2, the HVAC compressors were something the homeowner had to live with. This minimizes the flicker.

To test for this VD and resulting flicker to determine the source of the inrush, use a min max amp clamp and min max multimeter at the main service watch the lights(it may take two workers) and correlate to the meters.

Virtually a perfect answer BJ, I have seen all of this before like yourself and there is possibly one way out, but not too cheap either. It may be possible to fit a small soft starter to the unit which will remove the large inrush that is more than likely causing this dimming/flicker. If the supply and circuit conducters were larger then chances are you wouldn't see the flicker so much, but changing cable is a stupid resolution, so the soft starter could be an option...

waco · 10-22-2009, 08:57 AM

Isn't clear if the problem is new, but if it is, then something has changed with regards to the unit or the service or both. Bad L1 and L2 service connections seem most probable.

Problem with PUCOs is that they will charge the HO if they come out and the problem is on the load side and not the line side of the service. That said, voltage "sag" on the line side is their problem, no matter how much they argue otherwise.