A/C conductors could the rating be wrong ?

[stevenj]

A/C Conductors could the face plate rating be wrong on this unit.

I used 10-3 according the face plate rating of the AC condenser. I also installed a surge suppression (homelite phase to natural 22K amps) in the panel. I also figured the locked rotor amp calculation just to be sure about the conductor size for the A/C condenser. 45 amp breaker also installed per the plate rating and my calculations. The factory and my calculation do match.

Here is the problem or is it a problem?

When this unit starts up. There is a slight dimming of lights in the home sometimes, I mean very sight dimming. I never saw that happen before. This is a new home install with a 40 space Square-D. Could there be a capacitor problem in the A/C unit? I did call the A/C Company; they came and checked out the unit and said ?unit is fine and running in top condition? they were not sure about the dimming somtimes.

Would using 8-3 conductors be better to take out this slight dimming of lights? I figured the motor must be pulling a lot of in-rush current at start-up All connections are on spot with my torque. might use my fluke equipment Monday. is this common with some Ac units ?

 

[ceb58]

A/C Conductors could the face plate rating be wrong on this unit.

I used 10-3 according the face plate rating of the AC condenser. I also installed a surge suppression (homelite phase to natural 22K amps) in the panel. I also figured the locked rotor amp calculation just to be sure about the conductor size for the A/C condenser. 45 amp breaker also installed per the plate rating and my calculations. The factory and my calculation do match.

Here is the problem or is it a problem?

When this unit starts up. There is a slight dimming of lights in the home sometimes, I mean very sight dimming. I never saw that happen before. This is a new home install with a 40 space Square-D. Could there be a capacitor problem in the A/C unit? I did call the A/C Company; they came and checked out the unit and said “unit is fine and running in top condition” they were not sure about the dimming somtimes.

Would using 8-3 conductors be better to take out this slight dimming of lights? I figured the motor must be pulling a lot of in-rush current at start-up All connections are on spot with my torque. might use my fluke equipment Monday. is this common with some Ac units ?

What was the name plate rating for MCA? How long is the run from the panel to the A/C unit?

 

[augie47]

It is doubtful the wire to the unit itself will have much effect on the "dimming". You might discuss the problem with POCO as the size of the service drop/transformer can often have some bearing on this problem.
Many HVAC units have a "soft start" option that can be added which will assist is the dimming is annoying.

 

[kbsparky]

I installed a new high-efficiency heat pump unit last year. Same size as the old one, except it was a 10 SEER, and the new one is an 18 SEER.

Now, when it starts up, the lights blink and my APC box bleeps that the voltage has dipped enough for it to kick in. It never did that before with the old unit.

Nothing else has changed, except my electric bill is lower with the higher efficiency unit.

It saves energy while running, but needs substantially more to get started.

Changing the branch circuit wiring will probably not do anything to deal with this issue.

 

[Minuteman]

Another option is a hard start or booster capacitor. It is simply installed in parallel to the capacitor and increases starting by 300%. They cost about $15 and are available and all HVAC supply houses.

 

[kwired]

Another option is a hard start or booster capacitor. It is simply installed in parallel to the capacitor and increases starting by 300%. They cost about $15 and are available and all HVAC supply houses.

If the probem is voltage drop at the service equipment this may only make the voltage drop problem worse but will shorten the duration. FWIW smaller branch circuit conductors will make the starting current less as they will limit amount of inrush current - they will make the duration of startup last longer though. POCO transformer size and/or long service drops are usually biggest issue concerning voltage dropping when starting motors.

 

[readydave8]

I've rarely seen one here in rural areas that didn't dim the lights, doesn't seem to be a problem in town.

I've tried changing to larger conductors several times which never helped much if at all, when its unacceptable to the customer he gets the HVAC man to put in hard start (as others have said).

 

[dkarst]

Perhaps you can provide some more details like distance from service transformer to home and service size (sounds like 200A, what size conductors, LRA on A/C nameplate?). I did the same calculation for my own home a couple weeks ago http://forums.mikeholt.com/showthread.php?t=128274
and calculated a 8V drop line-line drop. Since then I've refined the spreadsheet and found it is closer to 6V which is still a 3V drop on a 115V incandescent lamp, which is noticeable. If you want to provide some details, I'm sure one of us here will help out.

It sounds like a fairly large unit, (mine is 3 ton and it sounds like it is smaller than yours), and if a slight flicker of the lights is the only thing you are seeing, you may have to live with it. As others have stated, it seems doubtful to me that your conductor size/breaker from the panel to the unit is playing any role here, more likely the drop from transformer to panel.

 

[stevenj]

Thanks for all the help. I read the post and never thought about the poco service drop transformer. I jumped in the truck, went down this morning and did a voltage drop reading at 8am before the demand picks up. Then turned on the lights of the home no dimming at all this time with the AC start up. No voltage drop either. The service drop is 80 feet away. I installed 4/0 Triplex for the home side of the drop to meet the less 3 percent voltage drop, required by the poco.

The local poco was out at 9am doing a phase change on the 25K volt lines down the road. I asked one of the crew when the transformer for the service drop at the location was installed or changed out. Get this 1967. It’s the original transformer; 43 years old. (Amazing) There is 6 homes on that transformer.

So when demand is up in the afternoon , which is the only time I’ve been testing this A\C unit the lights dim a little at start. He said they have been getting complaints. I will test this again this afternoon. I bet I get some voltage drop then. He also said they need new service equipment. The poco is strapped for cash and asking for another rate increase of 22 percent on top of the just approved 7 percent one that was passed. Ouch
.
The crew chief with the poco said that the city should have never approved new homes is this old neighborhood. They just installed a 300K diesel generator the next grid over to pick up the slack. he also told me that for any new A/C installs that it would have to be approved by them, the poco. Then i was kind of floored when he told me they might only approve 18 seer units if they get the commsion to agree. The hvac guys will be mad when they get wind of this.

So I,m sure now this is the problem ( demand at summer time )

 

[stevenj]

The service drop is 80 feet away. I installed 4/0 Triplex for the home side of the drop to meet the less 3 percent voltage drop, required by the poco.

voltage drop is 2.7 V

Panel is 200 amps.

Ac rated load 23

Ac LRA 110.

Found it. it's a 12 seer unit

 

[iwire]

For what it is worth.

If you have flicker now and you increase the size of the conductors from the panel to the HVAC unit the flicker will get worse

To reduce the flicker the conductors between the panel and the source would have to be increased in size or the transformer itself.

 

[gar]

100815-1442 EST

stevenj:

Your problem is the transformer impedance and the line drop from the transformer to the main panel. The amount of load on the transformer should not have an effect on the change in voltage that results from the air conditioner in question turning on. The impedance of the transformer does not change as a result of load. Also the impedance of a transformer should have little change over 40 years. What might change is the resistance of some connections.

If the impedance of the source is 0.01 ohm and I load that with 200 A, then the voltage drops 2 V.

I share a 25 KVA transformer with my neighbor. Only 5 or 6 feet of wire and the transformer are common between us. That is the only mutual impedance. My incandescent lamps flicker noticeably when his air conditioner turns on. Monitoring my voltage near the main panel for a short while now with my Fluke 27 in min-max mode I have 124.5 max and 120.2 minimum. I suspect something close to 4 V change results from his air conditioner, if it cycled during my sample time. I do not currently have any loads to cause that change, and our primary supply is generally relatively stable.

Looking at some house data I have collected with my TED system I do not see near instantaneous voltage changes like 4 volts. The averaging time of the TED system is probably around 1 second. My neighbors peak inrush is probably not more than a couple cycles.

If you want to reduce light flicker, then replace your incandescent lamps with GE dimmable CFLs. Even though these CFLs have limited dimming capability with a phase shift dimmer. Their light output is moderately stable with variations in sine wave excitation down to about 100 V where they just quit.

At my web site http://beta-a2.com/EE-photos.html see photo P14 for the light output vs voltage for the GE and compare this with photo P9 for an incandescent. Use Internet Explorer to view the site.

Do some electrical circuit analysis to see why my above comments apply.

.

 

[dkarst]

I believe gar is spot on with his analysis. I ran the same spreadsheet I posted earlier with 80' of 4/0 and assuming the six homes are fed with of a 50kVA transformer with 3% Z. I calculated 2.5V drop due to your 110 A inrush LRA and 80% of that is due to transformer contribution i.e. you could increase the service conductors much larger and not make much improvement. I believe that is also why gar sees a drop at his home when the neighbor's A/C starts, the secondary (common) side of the transformer is momentarily pulled down.

I would speculate that when you ran down there at 8AM, there was enough natural light that you may have not observed the flicker. Another item is are you seeing the flicker on the same bulb/fixture? ... it is quite dependent on the wattage rating of the bulb.

In my opinion there is likely only a real issue here if the source is so weak the steady state voltage is pulled very low during a high use time, and then you see the drop on top of that.... even then a 2.5V drop isn't going to get anyone very excited to fix.