I have a brand new (Oct 07) ruud central air conditioner and furnace. The A/C unit is wired to a 220v 30a breaker with number 10 wire. I have also upgraded my home service to 200a. However, Every time the compressor kicks on the lights in the house dim. The A/c unit is wired on the same breaker side of the main panel as the garage (which pretty much has nothing else on it) and the house is wired on the other side of the main panel (which carries the 60a subpanel breaker and a couple of other 20ampers) Can any one tell me what the deal might be??
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A couple of things to check: All wire connections at the unit and panel. All neutral and ground connections in the panel.
Also check the locked rotor amps on the unit information plate. It will be listed as LRA and will likely be 3 to 3.5 times as high as the FLA (full load running amps). Then using a clamp on amp meter check the start up amp draw when the unit comes on. If it is over the LRA, there may be a problem with the unit.
Another possibility is an inadequate ground either at the the house or at the pole or pad transformer. Since you recently upgraded the panel, did you add an additional ground rod. If not, add one and pull a new continiouse wire from the panel to the ground rods (no splices or breaks allowed).
Last, call the poco to check the connections on thier side of the meter. They mau also install a recording meter on your service to monitor the problem and help determine what may be causing it.
"Another possibility is an inadequate ground either at the the house or at the pole or pad transformer. Since you recently upgraded the panel, did you add an additional ground rod. If not, add one and pull a new continiouse wire from the panel to the ground rods (no splices or breaks allowed)"Are you thinking about the neutral. The house Ground electrode system won't have anything affect light dimming. Maybe your thinking of a bad neutral connection. But that would not affect dimming from a 240 load.Now a POCO neutral connection problem on the primary side could cause dimming..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Double check what Dave has suggested, but it is likely normal.
A 4Ton Rheem/Ruud has a 129A LRA (I happened to look at a nameplate yesterday on a spare compressor I have).
If you are typical of residential services, your house is likely serviced by a 50 kVA distribution transformer. Typical impedance of that transformer is 3%.
The calculations can get complex (even needing to account for the power factor and harmonics of your neighbors' houses), but a temporary voltage droop when you hit the 208A xfmr with a 129A LRA reactive load can easily be a few volts.
For example, own house w/200A service has a 50 kVA dist xfmr that feeds 5 houses (POCO assumes load averaging). The feeders to the meter are about 35mohms, so at my meter, adding that IR drop to the transformer drop, when my 4T unit starts, there is approx a 5 volt drop!
If you look up incandescent lamp data, you will see that the lumens are a power function of voltage, so it is very normal for you to see your lights dim, often very noticeably.
Edit PS - re-reading your post you likely have a 3T or smaller A/C, and your Ruud is likely an AC/gas combo vs. heat pump? Would explain why you did not notice your lights dimming last fall immediately.
If you did hte 200A upgrade to avoid lights dimming, you can see from the above that that will not help much.
Edited 7/16/2008 7:26 am ET by junkhound
>>The A/c unit is wired on the same breaker side of the main panel as the garage (which pretty much has nothing else on it) and the house is wired on the other side of the main panel (which carries the 60a subpanel breaker and a couple of other 20ampers).<<
Dave and Junkhound have provided much useful information.
However, a clamp-on ammeter might solve your mystery - I am suspecting that the panel loads may be grossly imbalanced.
Simple test - more complex solution.
Open Main panel - attach clamp on ammeter to L-1 feeder - turn on A/C - read amps on L-1 - move ammeter to L-2 feeder - read amps on L-2. Compare the readings. Repeat test with A/C unit off.
They should be pretty close to each other. Repeat test at sub-panel.
It could be that one leg is loaded significantly higher than the other due to inattentive connection of circuits - and that the more highly loaded leg is the one which has the house lighting circuits on it.
Some shuffling of circuits within the panels could help with your light dimming problem.
Worth looking.
Jim
Edited 7/16/2008 8:10 am ET by JTC1
The AC is 240 and I am 99.9345% certain that the house is also. I think that what he is talking about is that most panels now days has 2 columns of breakers. What is is saying is that the 2 poles on one column feed the AC and the and the top 2 poles on the other column feed the house panel.Un balanced loading would affect this..
.
A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Yes, 240v. Sorry I'm used to working with 3-phase power.Thanks for the responses from every one. The unit is a 3ton. I think It is a balance problem as well. There is sufficient grounding and I've checked the connections between the panel and the Unit and all is good there. Most of the time during the night and day we very little draw on the house side. 6 to 15amps. We are in the middle of a remodel and are confined to a small area of the house. I didn't notice the dimming in the fall because we didn't use the A/c part of the system. in about 3 weeks we'll have use of the kitchen so I will try to rearrange some circuits to balance it out better.
I Had a 30 year old 5 ton unit that when it was installed 30 years ago did the same thing. The HVAC installer put a capacitor on it and never had the problem again. Once that unit died a newer unit was installed. It immediately dimmed the lights in the house when it started. That HVAC installer then installed a capacitor and the problem went away. The HVAC guys have a special name for the device. Something like a hard start unit. It is just a capacitor, doesn't cost much.
No, with a 240 load there is it can't be unbalanced.Likewise for the sub-panel feed. But it is possible that the loads in the sub-panel. BUt with the loads you have that is not likely to be a problem.You need to check the other things..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Let me try and restate my "clear as mud" theory.
OP says in post #1:
Newly installed 200A main SEP.
The 240v A/C breaker is on the top of one column - 240v load so that is automatically balanced. No mention of other breakers in that column.
240v 60A sub panel feed is in the other column of the main panel accross from the A/C, plus a "couple of other 20A breakers." Provided these breakers are all in a vertical row with no skips, they would also seem to be balanced (depending upon how they are actually loaded).
My "unbalanced loading" theory would probably play out in the sub-panel. Fed by a 240v 60A breaker which may or may not have a balanced load on the two legs.
My thought was that perhaps the "normal" sub-panel loading might be say 25A on the red leg and only 5A on the black leg.
In my theory, the general lighting circuits are tapped into the 25A (red) leg.
When the A/C turned on and added an additional balanced load to the red and black legs - there might be sufficient load to produce enough voltage drop to produce dimming of the lights.
OP says in a later post (#7?) that the sub-panel loading is only in the 6-15 amp range ---- this light loading of a 200A service perhaps blows my entire theory.
"No load" and "normal load" voltage readings at the main lugs / breakers of both panels could also be very informative.
I think you agree, at least worth investigating with some readings.
Jim
Never underestimate the value of a sharp pencil or good light.
Maybe I missed it but is this a momentary dimming or the whole time comp is on? If the former I would doubt if it is abnormal unless it is a big dim. Mine lets me know when it kicks on but it is very, very short duration.
yea, it's a short duration. It's not a black out by any means but its a significant bump ....down then back to normal.....I wondered if it has anything to do with the in line dimmer switches.
My theory (which is incomplete and probably holier then the Popes hat)is that because dimmers send back more amperage on the neutral then a non-dimmed balanced load (which I know to be true because of the complex dimming systems we use at work) that when the compressor kicks on the voltage drop from the initial "pull" of the compressor is amplified somehow which results in a dip in the lights.............this all once again leads me to believe it's a leg balance issue.
because dimmers send back more amperage on the neutral then a non-dimmed balanced load
Huh?
Also, now data comes that you have dimmers on the lamp loads. And that this is the first AC has been used since install. Would be nice to include the details in the first post.
Reread what Bill said, there is no neutral current on any balanced load.
Voltage drop from dimmed Lamp current on the neutral is negligible.
Reread what I said about lumens being a power function of voltage.
Now learn circuit analysis, calculate the phase angle the dimmer triacs are firing at and the resultant rms voltage for the normal dimming, then the phase angle and rms voltage when the line voltage is reduced due to LRA during compressor start. Result: greater percieved dimming. At very dim setting, the LRA can latch dimmed lights off with many dimmers.
Conclusion: What you have is normal. If the dimming still bothers you once you know your house is not going to burn down due to a bad connection, then do what others already suggested and add a start capacitor circuit to your compressor to reduce the start current and duration.
I don't know that much but I can see an "unbalanced load" if most of his 120 stuff in use is off one leg and the ac comes on and drops the voltage because of the extra load. The ac load is balanced when looking at it by itself but overall service may have "unbalance".
I would think that if he had dimmers and they were not dimmed it would make absolutely no diffference, probably even if they were dimmed.
I am only asking questions, not answering (:-).
How long is that #10 wire run to the compressor?
Your ammeter will never show the instantaneous draw of that motor on startup, but your eyes will see it in the lights.
The wiring should be sized for the LRA rating of the compressor to minimize dimming without a start capacitor. If the wire run is "long", say 50 feet or more, the wire size should be upped one.
When you factor in low starting voltage because your AC is on when other AC units are on, the amp draw goes up. That means the motor is now drawing more than rated amps due to brownout conditions... which is a problem not figured in when sizing the wire for normal operating voltages.
Make sure the hard start kit has a potential relay that takes it out of the circuit after it has done its job.
I would bet that you need a minimum of #8 wire from the panel to the disconnect, then #10 to the unit from there.
The wiring should be sized for the LRA rating of the compressor to minimize dimming without a start capacitor.
Huh?
LRA may be 3 to 3.5 times the FLA. So for a FLA 240v/ 17a device the LRA can be as high as 60+ amps. Sizing the wire to that amperage is not neccessary. The lock rotor amperage is only a momentary surge current.
The lights are dimming upon start-up, which is the point of maximum current draw, approaching LRA.
While not absolutely necessary, upsizing the wire can reduce the dimming problem... which probably gets worse as more AC units are operating on hot days (lower line voltage = increased amp draw).
Edited 7/18/2008 6:00 am ET by danski0224
While not absolutely necessary, upsizing the wire can reduce the dimming problem... which probably gets worse as more AC units are operating on hot days (lower line voltage = increased amp draw).
That is why I sugested calling the poco and having a recording meter placed on his service. What you are descibing is a brownout condition. Not uncommon in some areas of the country, but the condition normally make the six oclock news and everyone is aware of it,
A recording meter will tell the poco if they need to install a capacitor bank some where on the line, or if the transformer is being overloaded or is fadeing on them. Often the original service loads for a transformer are slowly exceeded as people add additional loads within the homes they are feeding. It may well be a transformer at a substation that is causing low voltage on a whole circuit.
In short there are a lot of things that can be just slightly wrong and the poco may not be aware of it. that is why they should be included in the effoert the find and solve the problem.
This has been discussed before, and IIRC, a quick dimming of lights is normal. If they stay dimmed you may have a problem.
Our four ton unit dims the lights even though I upsized the conductors to #8. I think the starting current on my unit can reach 50A. That is proportionally a lot of current for a residential service.
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.â€
I may be a day late but I relate to your problem.
The last house I did for myself in 03 and am still in has a Rudd a/c and seperate radiant heat. The start of the a/c from day one caused the same 1 second dimming that almost alerts you to the a/c starting. I tried a lot of things with less help than you just got and finally had N-Star replace my transformer with a larger unit. I live in a rural area & have a 600' driveway with the transformer halfway down it, and the power into the house showed a dip at the starting of the a/c.
After they replaced the transformer with a larger unit there was no improvement. Then I called another electrician friend that not only lives a couple of miles away but was an electrical instructor for many years in the area. Before he retired I used him for my electrical work.He made me aware that the power company has not been able to provide the upgrades to the region that is needed for the development that has been nonstop for the last 30 years. I know very little about electrical supply but I think he mentioned the line feed available at the street was too low.
I have no other issues or problems except for that dimming and the capacitor solution seems the best & something I think I'll do soon.
>>The A/c unit is wired on the same breaker side of the main panel as the garage (which pretty much has nothing else on it)
For what it's worth:
"The same side" isn't the same thing as the same leg
As you go down 1 side,
the first breaker is from one ("first") leg
the second breaker is from the other leg
the third breaker is from the first leg
the second breaker is from the other leg
Etc.
That's why 220b breakers are bside by side (right above/below) each other - that way each leg pulls from the separate 110 source.
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Interesting. Makes sense. I wondered about that. Good info for me to know when balancing load. Thanks