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Voltage Sag Effects on MS Charging

AB4EJ

Member
Feb 25, 2015
771
377
Tuscaloosa, AL
I recently moved to a house that has a heat pump. My plan is to install a NEMA 14-50 outlet in the garage for charging. (I plan to get a Model S this spring). I just noticed that when the heat pump starts, the lights in the house dim a bit (for perhaps a quarter of a second), indicating a voltage sag. This is probably not abnormal due to high inrush current on the heat pump compressor. I suppose this will probably also happen in the summer when the compressor comes on for A/C.

The heat pump is on its own circuit at the main panel, suggesting that the voltage sag is on the power company side. This is not surprising - I am out in the country, so our house is served by its own power company transformer. The voltage sag could be in the transformer and/or the (underground) wiring between the transformer and the house.

Here is my question: on this forum I have seen some discussions about the MS charging system automatically backing off charging current when it detects a voltage drop. Is it likely that the voltage sags caused by the compressor start up will cause the Model S to drop its charging current, leading to very long charging times? (If so, I suppose I could tell the power company I want this fixed, but I think they would want me to pay for it - $$$$). What do you think?
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,710
22,716
Texas
In my area you can't fix the voltage sags (thank you deregulation), and yes, sometimes the rate drops to 30 amps. It's really not a big deal.
And yes, if I start my air compressor, charging will stop and then start at 30 amps. The Clipper Creek charger (for the Leaf) just stops and won't start again until you reconnect, and sometimes the breaker needs to be cycled (the breaker doesn't actually trip, but the Clipper Creek doesn't remove the error automatically).

I've never had the A/C compressor cause a reduction in charging amps.

Note that whether you charge at 30 or 40 amps, the Model S will still be full in the morning.
 

JeffS

Member
Oct 7, 2015
239
48
Wisconsin
Start-up draws of electric motors cause those sags especially in situations like you described. The MS doesnat hit current hard like a heat pump does. It gradually ramps up when charging starts. After it is pulling as hard as it wants to pull, you can watch on the das as voltage varies. The car just rolls with it.
 

msnow

Active Member
Jul 14, 2015
4,951
2,236
SoCal
I used to have this happen to me but I think the power company eventually fixed it on their end but there's another thread in this forum about dialing down from 40 amps to 38 amps which would stop it from dialing all the way to 30 amps. I tried it and it worked. Also the charging time increase was insignificant. See if it works for you.
 

jerry33

(S85-3/2/13 traded in) X LR: F2611##-3/27/20
Mar 8, 2012
19,710
22,716
Texas
I used to have this happen to me but I think the power company eventually fixed it on their end but there's another thread in this forum about dialing down from 40 amps to 38 amps which would stop it from dialing all the way to 30 amps.
That helps because often the sag is never seen. However, I've had it drop from 33 to 30 on occasion, so it won't prevent all reductions when you have a poor electric provider.
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
The heat pump is on its own circuit at the main panel, suggesting that the voltage sag is on the power company side. This is not surprising - I am out in the country, so our house is served by its own power company transformer. The voltage sag could be in the transformer and/or the (underground) wiring between the transformer and the house.

It depends on many different factors.

How old is the heat pump? You could be seeing the effects of a dying motor-start capacitor or bad bearings, etc. that causes a huge inrush current.

You're likely not going to get the power company to move for you, unless you are in a situation where something is undersized by quite a bit. If you knew a bit more about your situation (transformer size, service entrance conductor size, etc.), I might be able to help with some advice.

Also, different power companies work differently with you. Investor-owned utilities will likely demand that you pay for everything. Co-ops and municipal departments offer better options. You may want to see how the Tesla reacts before you call them in.
 

AB4EJ

Member
Feb 25, 2015
771
377
Tuscaloosa, AL
It depends on many different factors.

How old is the heat pump? You could be seeing the effects of a dying motor-start capacitor or bad bearings, etc. that causes a huge inrush current.

You're likely not going to get the power company to move for you, unless you are in a situation where something is undersized by quite a bit. If you knew a bit more about your situation (transformer size, service entrance conductor size, etc.), I might be able to help with some advice.

Also, different power companies work differently with you. Investor-owned utilities will likely demand that you pay for everything. Co-ops and municipal departments offer better options. You may want to see how the Tesla reacts before you call them in.

Good input. I think the HVAC is about 5 years old, but it would be a good idea to have it inspected & check for this.

Alabama Power is investor-owned, so I expect your comment that they will want me to cover the cost of any upgrades is right on. I will post a photo of the transformer in a bit, and see if I can determine the size of the conductors. Also, the run between the panel and the transformer is fairly lengthy - I will estimate the length of the cable.
 

Yitt

Member
Oct 17, 2015
39
25
Charlotte, NC
I used to have this happen to me but I think the power company eventually fixed it on their end but there's another thread in this forum about dialing down from 40 amps to 38 amps which would stop it from dialing all the way to 30 amps. I tried it and it worked. Also the charging time increase was insignificant. See if it works for you.

Thanks! Mine was bumping down to 30A at my home and I did not know why. I checked out FlasherZ's FAQ (thank you too!) and realized it was going to be a while before I got motivated to do a full troubleshoot. Since the car fully charges overnight at 30A anyway, this wasn't TOO pressing an issue.

Anyway, last night I manually set it at 38A and I think it stayed there. So, great solution. Hopefully it will stick.
 

AWDtsla

Active Member
Mar 3, 2013
4,265
3,959
NE
In my area you can't fix the voltage sags (thank you deregulation)
While I think deregulation was a terrible thing for electricity, that's not to blame for voltage sag. The distribution part is still regulated, and they own your local pole transformer.
 

dhanson865

Active Member
Feb 16, 2013
4,552
6,955
Knoxville, Tennessee
I recently moved to a house that has a heat pump. My plan is to install a NEMA 14-50 outlet in the garage for charging. (I plan to get a Model S this spring). I just noticed that when the heat pump starts, the lights in the house dim a bit (for perhaps a quarter of a second), indicating a voltage sag. This is probably not abnormal due to high inrush current on the heat pump compressor. I suppose this will probably also happen in the summer when the compressor comes on for A/C.

If resistant heat kicked in that will cause more of a sag than the heat pump. Don't assume that AC will be as bad as heat.
 

Ingineer

Electrical Engineer
Aug 8, 2012
1,507
3,699
I'd posit that the momentary sag of compressor start-up isn't going to trip the algorithm for reducing charge current. But no speculation needed; wait until you have your car and see if it's even an issue. If it is, you can always install a soft-start device on the compressor. (providing the power company won't do anything.)
 

AB4EJ

Member
Feb 25, 2015
771
377
Tuscaloosa, AL
I'd posit that the momentary sag of compressor start-up isn't going to trip the algorithm for reducing charge current. But no speculation needed; wait until you have your car and see if it's even an issue. If it is, you can always install a soft-start device on the compressor. (providing the power company won't do anything.)

That's a good idea - will keep it in mind. BTW, I like your "Reddy Kilowatt" character.

- - - Updated - - -

It depends on many different factors.

How old is the heat pump? You could be seeing the effects of a dying motor-start capacitor or bad bearings, etc. that causes a huge inrush current.

You're likely not going to get the power company to move for you, unless you are in a situation where something is undersized by quite a bit. If you knew a bit more about your situation (transformer size, service entrance conductor size, etc.), I might be able to help with some advice.

Also, different power companies work differently with you. Investor-owned utilities will likely demand that you pay for everything. Co-ops and municipal departments offer better options. You may want to see how the Tesla reacts before you call them in.

Here is the transformer - unfortunately, I don't know the kva rating of it.
DSC_3989.jpg
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
That's a good idea - will keep it in mind. BTW, I like your "Reddy Kilowatt" character.

- - - Updated - - -



Here is the transformer - unfortunately, I don't know the kva rating of it.

Sometimes the power company will paint and/or use a decal to denote the ratings on the outside of the can. Mine has a big "37.5" sticker on it, for 37.5 kVA. If the other side has no label, anything I offer will just be guess. It doesn't appear to be very big, though - and is dedicated for your home.
 

cinergi

Active Member
Sep 17, 2010
2,176
40
MA
Here is the transformer - unfortunately, I don't know the kva rating of it.

That looks like a 50 or 75! The entire canister is blank?
Definitely would want to know how far you are from that transformer.
I can't imagine any circumstance where that transformer is too small. Either it's faulty or the house's distance from it is at play. Possibly poor connection at/with the meter or panel.

I recently went through getting the 50 kva that was serving 26 homes (!) split up (as I was seeing severe sag and amperage cut-backs)... a new 50 kva was installed on the pole right outside my home (<100 feet) and, despite it serving multiple homes, I never notice anything. I draw 80 amps without issue (maybe a 7 volt drop at the car). Even when the car stops charging, I barely notice any change in lighting intensity in the house. When the real power company technicians came out to investigate, they couldn't believe their eyes. But it took over 6 months of back-n-forth with various people including supposed "neighborhood engineers" that insisted the design was fine and people coming out to measure voltage at the meter which involves disconnecting it (well, dummy, you won't notice anything because there's NO LOAD when you disconnect the meter).
 

FlasherZ

Sig Model S + Sig Model X + Model 3 Resv
Jun 21, 2012
7,024
1,013
Given the size of the pole compared to the transformer, my guess is actually somewhere between a 10 and 25. Initially I thought "wow, that's big!" but then you see it's zoomed in quite a ways. That pole appears to be one of the 6 or 8" poles, when put in context of the other fixtures on it (lettering, insulators, etc.)

And I concur with you that power companies tend to be lazy. :)
 

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