Power Sags

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Jdoll42

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Here's one for the electrical gurus out there. I have a new house with a heat pump. When the heat pump kicks in, every electrical device in the house has a massive power sag. It's so bad that you can hear it in the microwave, bathroom fans, etc. The lights practically turn off (then immediately back on).

I had the company that installed the heat pump check it out. Their service tech spent several hours out here with several different meters checking everything out. According to him, the system is well within its specs. He did agree that it the power sags are a bit excessive though.

I'm no stranger to residential wiring. In fact, I did all the electrical work on my new house. (I even impressed the electrical inspector, and from what I hear that's pretty hard to do around here.) I went through all of the documentation that came with the heat pump and verified that I'm using the proper gauge wire. I also used some online calculators to double-check that the gauge is suitable for the amperage and length. I've triple checked the connections and verified that everything is tight.

I'm at a loss now for what to check. If anybody has any ideas, please let me know. I can provide pictures and diagrams if necessary. It's beyond annoying now. Help! Thanks!
 

hj

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"sag"?

If everything is affected, then the problem is not with the heat pump, it is in the main power feed, or the distribution panel, and for that you have to contact your electric utility.
 

Lakee911

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How big (in HP) is the compressor for your heat pump? What size wire are you running to it? What size breaker? Is it wired for 240VAC? What is the distance (as the wire is run) from your service panel to the heat pump? What size is your electric service?

If everything checks out, you could talk to your HVAC service guy about a "hard start kit" for the A/C compressor. In a nutshell, it will assist in starting the motor and will cause it to not work so hard. Check with manufacturer first to see if it's possible to add.

Jason
 

pudge565

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possibly need a higher amperage panel not sure but possibly just throwing that out there.
 

Jdoll42

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How big (in HP) is the compressor for your heat pump? What size wire are you running to it? What size breaker? Is it wired for 240VAC? What is the distance (as the wire is run) from your service panel to the heat pump? What size is your electric service?

The front of the unit says the following: Compressor Vac. 208/230, PH 1, RLA 27.2, LRA 150.0, Qty 1. I understand that's 1 single phase, 240v compressor, but any ideas on RLA or LRA? There was no HP listed on the sticker that I could find for the compressor. (It shows the blower is 1HP.)

Regarding the wiring, there is a 100 amp (240v) circuit run on 3-gauge AND a 50 amp (240v) circuit on 6-gauge. It runs about 35-40' max. I believe I purchased 50' of wire and ended up cutting off a good chunk off each end. (Ouch! That was expensive!) The house has a 200 amp service.

If everything checks out, you could talk to your HVAC service guy about a "hard start kit" for the A/C compressor. In a nutshell, it will assist in starting the motor and will cause it to not work so hard. Check with manufacturer first to see if it's possible to add.

My first thought was something along the lines of the compressor needing a start capacitor or something like that. I talked with the HVAC installers and they said that this system already has all that. Of course the manual that I have is next to useless. I'll have to look online and see if I can find any better info.
 

Cass

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What brand is your main panel ?

Is your service in the air or underground?
 
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Mikey

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The front of the unit says the following: Compressor Vac. 208/230, PH 1, RLA 27.2, LRA 150.0, Qty 1. I understand that's 1 single phase, 240v compressor, but any ideas on RLA or LRA?

LRA (locked rotor current) is the initial inrush current, which will be seen when the unit starts up. RLA (rated load current) is the running load.

I'd put a recording voltmeter across the main terminals in the main panel and turn on the AC. The value should stay at 240VAC during the startup if in fact the LRA is no more than 150A as your data plate shows. A very good electrician will have such a device, know how to use it, and understand what we're doing here.

Even the voltage drop at the unit would only be about 3 volts. If the recording voltmeter also has a current probe attachment, you could measure the instantaneous current draw at startup and verify it's 150A or less.

With the data, you could determine that a) if the current draw is substantially greater than 150A, your AC unit is bad; or b) if the voltage at the main panel drops substantially below 240VAC, your service is bad.
 

Lakee911

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I've never seen an AC compressor that uses more than one power feed. That seems kind of odd as to why you would have a 100A and a 50A feeding it, unless the 100A goes to your air handler for electric heat and the 50A goes to your heat pump. Is that the case?

Generally the utility company requests to be involved when large loads (such as resistive heating, or A/C, or motors) are added. There is a chance that the transformer feeding your service is undersized for the given load.

Checking the voltage and current, as Mikey said, would be a good start. This will give you some numbers to go along with the description of "massive power sag."

Do you measure 240VAC from phase (leg) to phase (leg) in your panel, as well as 120VAC from phase to ground?

Have you confirmed, for yourself, that the heatpump is wired correctly for 240VAC service (as opposed to accidentally for 208V)?

Thanks,
Jason
 

Jdoll42

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I'd put a recording voltmeter across the main terminals in the main panel and turn on the AC. The value should stay at 240VAC during the startup if in fact the LRA is no more than 150A as your data plate shows. A very good electrician will have such a device, know how to use it, and understand what we're doing here.

I understand that you would probably not consider this a job for a DIY'er. However, everything you have said makes perfect sense to me. I just don't have a recording voltmeter. Is that something that would be worth my while to acquire? How much $$ are we talking here? (Any suggestions on brand/model?)

I'm not about to go out and get into commercial/industrial electrical work. Residential wiring, however, doesn't scare me one bit. With that said, I'm not going to go out and wire up a bunch of houses just for fun. I'll stick to my own. I'm just not sure how to troubleshoot a weird quirk like this one. I really appreciate the insight and assistance from trained professionals like all of you. If this gets to the point where I need to call a real electrician, so be it. I guess I'm just too stubborn and want to try to do it myself first.
 

Jdoll42

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I've never seen an AC compressor that uses more than one power feed. That seems kind of odd as to why you would have a 100A and a 50A feeding it, unless the 100A goes to your air handler for electric heat and the 50A goes to your heat pump. Is that the case?
Yes, this is a geothermal unit. All of the components are contained within a single cabinet in the basement. There are no components outside (except for a few thousand feet of buried pipe). The 100A runs some of the components, but is primarily for the electric emergency heat coils. I found that if I turn that circuit off, even if it's not calling for electric heat, the system will not run.

There is a chance that the transformer feeding your service is undersized for the given load.
When I talked with the power co-op about this, they looked up the specs on the transformer they placed. They rattled off a bunch of numbers and said that it should be more than sufficient for my house. The way they were talking is that transformer could handle several houses, so just my single 200A service would be no problem for them. (I should have written down the spec's they said over the phone.)

Do you measure 240VAC from phase (leg) to phase (leg) in your panel, as well as 120VAC from phase to ground?
I'm about 99% positive that was the case, at least when I started wiring everything up about a year or so ago. I'll get my meter out this afternoon and double-check that.

Have you confirmed, for yourself, that the heatpump is wired correctly for 240VAC service (as opposed to accidentally for 208V)?
I'll have to get out the installation manual the service techs left here and see what it says. I trust their installation, but then again, somethings not quite right. I assumed that if the tech spent a few hours checking everything out, that he would have verified that part too. Well, we all know what happens when you assume.....
 

JWelectric

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When the heat pump kicks in, every electrical device in the house has a massive power sag.
I'm no stranger to residential wiring. In fact, I did all the electrical work on my new house. (I even impressed the electrical inspector, and from what I hear that's pretty hard to do around here.)
First let me say that just because the installation is code compliant does not necessarily mean that it is going to be adequate for the loads imposed.

The front of the unit says the following: Compressor Vac. 208/230, PH 1, RLA 27.2, LRA 150.0, Qty 1. I understand that's 1 single phase, 240v compressor, but any ideas on RLA or LRA?
LRA or the locked rotary amperage will be the draw on a motor to make it start moving from a standing position.
When the motor is standing and no current is applied the rotor of that motor is the same as locked, it is not moving. The amount of torque required to make the motor start in motion will take the same amount of current as the motor will draw if the rotor was locked down.

When this motor first starts it is drawing 150 amps for a fraction of a second which cause the lights to flicker in most homes. Should this motor be under load when starting then this draw will last longer than usual.

Yes, this is a geothermal unit.
this is enough to tell me that the motor is going to be started under a heavy load that will require a long time to start, maybe three or four of seconds.
I don’t think that there is anything wrong with your installation. I think it is the type of system that was installed that is the major problem.
 

Jdoll42

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First let me say that just because the installation is code compliant does not necessarily mean that it is going to be adequate for the loads imposed.
Agreed. I've always viewed code as more of a "minimum" guideline.

I don’t think that there is anything wrong with your installation. I think it is the type of system that was installed that is the major problem.
I was wondering that. Maybe I'm just nuts. Be that as it is, is there anything that can be done to make this less annoying? Thanks!
 

JWelectric

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Lol! Ok, other than the obvious, is there anything else I can try?

The key to this question is the word "try"

Anything that you do will be a hit-miss type of deal.

One thing that comes to mind is to use a VFD and change out the motor unless it is one that will accept a VFD. This way the draw can be lessened
 

Mikey

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I just don't have a recording voltmeter. Is that something that would be worth my while to acquire? How much $$ are we talking here? (Any suggestions on brand/model?)
Most of the better DMMs have a "Min/Max/Avg" function to record the minimum, maximum, and average values of the parameter under measurement. I'm a fan of Fluke products, which are kind of pricey, but here's a document from Fluke describing that function:

http://support.fluke.com/find-sales/Download/Asset/2674014_C_W.PDF

I've got an older Fluke 112 with this function on it, and have used it occasionally. There's a new model 114 that does this, available for around $100-130 from various sources. The Fluke 117 is an electrician's favorite.

These sample at various rates -- mine samples at 10/second or so, which should be sufficient to see roughly what's going on in your situation. If you're really serious about this, you can spend major bucks on Power Quality Analyzers (~$2500 or so) or several neat toys that work in conjunction with a PC (~$500). The nice thing about these is you get to see the waveform, and some of them have a special "inrush mode" to analyse your problem exactly.

Amprobe also makes something called the "ACD-16 TRMS Pro" which is a datalogger, but I don't know any details other than what you can find on the Web. They're a couple hundred bucks.

I'd start with a Min/Max multimeter and a plug-in current probe to see what the inrush current really is. If it's larger than 150A or lasts for a long time, you've got a load problem. If the sag is seen at your service but the load is only a 150A startup, you've got a service problem.
 
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