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Charger Voltage Question

Judders

Member
Sep 27, 2019
55
22
Temecula, Ca
Installed the home wall charger on a 60 amp circuit with 444/6 wire for a 45 foot run. 60 amp only calls for 6 gauge I went a size up do to local availability and the ability to run 80amp down the road if we desired. The neutral is properly grounded and capped off as its not used.

Install was a breeze minus maneuvering that wire, hooked it up the loaner tesla model s and it read the 48 amp setting and 239 volts charging roughly 34 miles. I had a loaner temporarily waiting for our order 2014 model s.

So here's the issue, Just brought home the new model 3 today (a day later) and its reading 234 volts and 48 amps. So that puzzles me. I broke down the charger to double check the connections and rechecked the breaker at the panel. All looked well.

Any thoughts on why there such a difference and is that of concern? Im assuming 234 is fine as the amperage is fine but I'm not a full time electrician, a very good and safe handyman but that's above my pay. By the gauge and length wire I ran it should only have a.5 volt drop over the run. I am confident the wiring and breaker is solid. I am using the top entry wall mount box. I am not sure if it is the tesla just reading different being new or if there is something up. The model 3 charge at about 42 miles per hour on that 234 48amp reading.

Any ideas on the change in voltage??
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
10,418
12,663
San Diego
234 volts

239 volts

It is probably the utility voltage that is changing. Or, other loads in your house (like AC) are causing drop along your service entrance wiring.

You should have observed the open circuit voltage as well for both cars (can’t now, of course).

You wanted to see the change in voltage (open circuit to loaded) be the same between the two cars, to be sure there is no issue (however, there is probably not an issue with your wiring).

I understand the concern and definitely worth double checking. The easiest thing would be to note what the open circuit voltage (really can only do 2A - car stays at this load on the Wall Connector for a few seconds after you lower the charge threshold to below current charge level, if HVAC is off) to 48A load voltage difference is. It should be less than maybe 6-8V. Most of this would likely be across your service feed line (you can measure the voltage at your main breaker output while under load, and when not under load, if you really want to know where the drop is).


Another possibility, of course, is that the Model S and Model 3 don’t read the same voltages in the car, due to implementation differences. No idea.

For the safety of the install, you just want to make sure your connections are all right and voltage drop is as expected. Not really a need to compare between cars.
 
Last edited:

ewoodrick

Well-Known Member
Apr 13, 2018
5,285
3,735
Buford, GA
Very possibly just the slight voltage meter differences. That's only 2% difference.
If you are worried, use your voltmeter (which if you installed yourself, you surely have) and see what the voltage at the panel and the plug is when compared on the same device.
 

Bill25cycle

Member
Mar 31, 2011
103
69
Installed the home wall charger on a 60 amp circuit with 444/6 wire for a 45 foot run. 60 amp only calls for 6 gauge I went a size up do to local availability and the ability to run 80amp down the road if we desired. The neutral is properly grounded and capped off as its not used.

Install was a breeze minus maneuvering that wire, hooked it up the loaner tesla model s and it read the 48 amp setting and 239 volts charging roughly 34 miles. I had a loaner temporarily waiting for our order 2014 model s.

So here's the issue, Just brought home the new model 3 today (a day later) and its reading 234 volts and 48 amps. So that puzzles me. I broke down the charger to double check the connections and rechecked the breaker at the panel. All looked well.

Any thoughts on why there such a difference and is that of concern? Im assuming 234 is fine as the amperage is fine but I'm not a full time electrician, a very good and safe handyman but that's above my pay. By the gauge and length wire I ran it should only have a.5 volt drop over the run. I am confident the wiring and breaker is solid. I am using the top entry wall mount box. I am not sure if it is the tesla just reading different being new or if there is something up. The model 3 charge at about 42 miles per hour on that 234 48amp reading.

Hi - First off the voltage at any given charging point isn't guaranteed to remain the same all the time. The juice at my house for instance is horrible - depending on whether it is winter (high) or summer (very low) - due mainly to my utility's incompetence as far as adjusting their voltage regulators.

So when you check your voltage at the car - see what it is at the house. I'm assuming all your wiring is copper, and that connections have been made up wrench tight. If aluminum there are other issues.

One point about the 444/6 being used at 80 amperes continuous - this would demand 100 amp wiring, and although #4 copper is used for 100 ampere residential services it IS NOT sufficient for an EV feeder. it being rated at 85 amperes at 75 degrees centigrade and 95 amperes at 90 degrees centigrade. Now if you set your HPWC to a maximum of 72 amperes, most inspectors would pass this, especially if you pigtailed in a #3 Copper connection at each end and used '90 deg C' rated splices on the pigtails, since breaker terminations are 75 deg C.

Incidentally the HPWC manual states that, at 80 amperes, #3 copper is required at the HPWC lugs.
 
Last edited:

woodguyatl

Member
Oct 3, 2018
446
359
GA

My house voltage varies about 8 volts over the course of the day. I typically drops in the late afternoon through mid evening when neighborhood usage is highest. The drop you are seeing is within normal variability.
 
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Judders

Member
Sep 27, 2019
55
22
Temecula, Ca
Thanks all, I will double check it with the volt meter. Just noticed it last night when we went to charge the new tesla, took my kids to their football game down the road which we walk to and by the time I got back to check all the wires my battery was already full. So we will see today when we charge again.

We did have the house fan on but not any hvac etc. The night before it was the same situation is why it intrigued me really no difference in total loads on the system. However the model s was at about 40 miles range vs 206 miles range on the model 3, maybe the soc had a play in that. Thinking that could be the difference and the 3 was not needing as much juice to get to 90%.

I we do regularly run the pool pump and both acs in the summer and it never even phases the 200 amp panel. I know my panel has plenty of power with a load calc too so just curious. I will be staggering the pool pump run times and regular evening charging time just to be sure. I would like to maximize that 44 miles per hr charge as much as possible as there are times i get home in the morning and my wife will take the car a few hours later.
 

eprosenx

Active Member
May 30, 2018
2,074
2,517
Beaverton, OR
Installed the home wall charger on a 60 amp circuit with 444/6 wire for a 45 foot run. 60 amp only calls for 6 gauge I went a size up do to local availability and the ability to run 80amp down the road if we desired. The neutral is properly grounded and capped off as its not used.

Install was a breeze minus maneuvering that wire, hooked it up the loaner tesla model s and it read the 48 amp setting and 239 volts charging roughly 34 miles. I had a loaner temporarily waiting for our order 2014 model s.

So here's the issue, Just brought home the new model 3 today (a day later) and its reading 234 volts and 48 amps. So that puzzles me. I broke down the charger to double check the connections and rechecked the breaker at the panel. All looked well.

Any thoughts on why there such a difference and is that of concern? Im assuming 234 is fine as the amperage is fine but I'm not a full time electrician, a very good and safe handyman but that's above my pay. By the gauge and length wire I ran it should only have a.5 volt drop over the run. I am confident the wiring and breaker is solid. I am using the top entry wall mount box. I am not sure if it is the tesla just reading different being new or if there is something up. The model 3 charge at about 42 miles per hour on that 234 48amp reading.

Any ideas on the change in voltage??

Lots of good comments here so far, but a couple of notes:

When I first plug in my model 3 it closes the contactor in the Wall Connector and shows the voltage on the screen before it ramps up the charging current. So if you look immediately after plugging in you can get the unloaded circuit voltage and then watch as it ramps to see where it stabilizes out. If you are getting a lot of drop there (which five volts is not really a lot at 240v) then the question is where are those being lost? They could be lost upstream if your main panel... A poor connection somewhere on the link back to the service transformer or undersized wiring coke cause this (outside of your control).

As others have mentioned, utility voltage fluctuates during the day. It all depends on how far you are from the substation you are and how good your utility is. My utility is fantastic and I am close to the substation, so I see very little voltage fluctuation, but it still does happen!

Fun fact: The utility transmission grid varies a bunch in voltage due to load and generation on the grid. We depend heavily on “online tap changers” or other voltage control units at the serving substation to bring the voltage into a good usable range. If you ever look at voltage graphs of delivered voltage from the grid you will see them operating constantly during the day. They typically change in two volt increments up or down.

Nice job on the install. I would not be worried! (Just to verify: You used copper wore right?)
 

tccartier

Supporting Member
Supporting Member
Oct 27, 2015
974
990
AZ.
Thanks all, I will double check it with the volt meter. Just noticed it last night when we went to charge the new tesla, took my kids to their football game down the road which we walk to and by the time I got back to check all the wires my battery was already full. So we will see today when we charge again.

We did have the house fan on but not any hvac etc. The night before it was the same situation is why it intrigued me really no difference in total loads on the system. However the model s was at about 40 miles range vs 206 miles range on the model 3, maybe the soc had a play in that. Thinking that could be the difference and the 3 was not needing as much juice to get to 90%.

I we do regularly run the pool pump and both acs in the summer and it never even phases the 200 amp panel. I know my panel has plenty of power with a load calc too so just curious. I will be staggering the pool pump run times and regular evening charging time just to be sure. I would like to maximize that 44 miles per hr charge as much as possible as there are times i get home in the morning and my wife will take the car a few hours later.

Just keep in mind that the difference may not be within your house usage or loads, could be in the line voltage coming into your house at any given time.
 

qdeathstar

Completely Serious
May 17, 2019
3,437
2,454
VB
measure your voltage at your the top of your meter base under load. You want the difference between what you measure there and what you measure at the car to be less than 3 percent. More than three percent indicates an issue. At a nominal 240v that limit would be 233v...
 
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eprosenx

Active Member
May 30, 2018
2,074
2,517
Beaverton, OR
measure your voltage at your the top of your meter base under load. You want the difference between what you measure there and what you measure at the car to be less than 3 percent. More than three percent indicates an issue. At a nominal 240v that limit would be 233v...

Notionally this is correct, but NEC actually recommends (it is not a requirement) no greater than 3% voltage drop on “feeders” and 3% on branch circuits and a combined total of not more than 5%.

So if your utility has a 5% plus or minus target (most try to be better than this) and you have engineered your electrical system to the max of 5% drop then worst case could be a perfectly in-spec value of 216.6 volts!
 

qdeathstar

Completely Serious
May 17, 2019
3,437
2,454
VB
Notionally this is correct, but NEC actually recommends (it is not a requirement) no greater than 3% voltage drop on “feeders” and 3% on branch circuits and a combined total of not more than 5%.

So if your utility has a 5% plus or minus target (most try to be better than this) and you have engineered your electrical system to the max of 5% drop then worst case could be a perfectly in-spec value of 216.6 volts!

yes, which is why the measurement a the meter base is required.
 
Last edited:

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,126
Canada
Some interesting reading on the subject of power line frequency. In North America it is 60 Hz, and maintained to within about 0.5 Hz of that at all times, and maintained to a daily average of 60 Hz to a much more accurate degree (because some systems rely on this for accurate time keeping). Frequency can vary with load, so corrective action like load shedding takes place to maintain the frequency if necessary. Some of this may result in the voltage fluctuating, but the voltage has a much larger margin of allowable error.

Utility frequency - Wikipedia

The 2nd sentence there actually mentions Tesla! (The original one from 100+ years ago, you know -- Nikola Tesla :)).

"Tesla demonstrated the concept of clocks synchronized by line frequency at the 1893 Chicago Worlds fair."​
 

Sophias_dad

Supporting Member
Supporting Member
Jul 29, 2018
1,469
1,495
Massachusetts
Installed the home wall charger on a 60 amp circuit with 444/6 wire for a 45 foot run. 60 amp only calls for 6 gauge I went a size up do to local availability and the ability to run 80amp down the road if we desired. The neutral is properly grounded and capped off as its not used.

Just an FYI.... If that's romex, I don't think you'll be allowed to use an 80 amp breaker(which even then would equate to a 64 amp charge rate due to the continuous-loading rule of 25% excess capacity)...According to the 60C limit of romex, you are limited to 70 amps on the breaker but again, due to the 25% continuous-load overhead, the HPWC would need to be limited to 56 amps

If its individual conductors in conduit, you'd likely be limited to the 75C rating of the connection at the HPWC, and be limited to an 85 amp breaker which you could load up to 68 amps continuous. You might be able to round-up to the next regularly sized breaker.... I'd have to check the rules for it....
 

ajdelange

Active Member
Dec 10, 2018
1,077
541
Virginia/Quebec
Here is a histogram of recorded voltages delivered to me by Dominion Energy during August of this year. The voltage was, for the vast majority of the measurements between 243 and 248 V (5 V range) but there were a few observations as low as 239 V. The standard deviation was 2 V.

VoltHist.jpg


Thus your observed 5 volt difference while large is not all that surprising. I would do as the others here have suggested and check the open circuit voltage, the voltmeter in the car against a good Fluke and the torques where you landed the conductors in the HPWC. IF you have a camera take some IR pictures at the panel and at the HPWC to look for hot spots.

Though it's not related to the question at hand I checked the month's frequency stats too. Average frequency was 60.0001 Hz and the standard deviation was 0.015 Hz. As all the generators in the network have to be in phase there is lots and lots of flywheel effect and frequency is very stable for that reason. Any generation station in the network with a GPS receiver knows absolute time within a few 10's of nS.
 

ajdelange

Active Member
Dec 10, 2018
1,077
541
Virginia/Quebec
Forgot to mention that as you will have noticed the car ramps up the charge current. As it does this it monitors the voltage. The slope of the voltage vs current curve is the resistance between the source and the charger. If that's too high the car concludes that there is a fire hazard and shuts down. I should say that this is something I have concluded and is not something I have been able to verify.
 

darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,126
Canada
Though it's not related to the question at hand I checked the month's frequency stats too. Average frequency was 60.0001 Hz and the standard deviation was 0.015 Hz. As all the generators in the network have to be in phase there is lots and lots of flywheel effect and frequency is very stable for that reason. Any generation station in the network with a GPS receiver knows absolute time within a few 10's of nS.

Awesome data. As I was reading the first bit I was thinking “I need to ask if there’s frequency data as well” ... and then there it was :)

See my prior post. They really prioritize frequency over voltage so, in a way it is indirectly related. You see more variation in voltage because they care more about keeping the frequency so stable, and in a tight range.
 

ajdelange

Active Member
Dec 10, 2018
1,077
541
Virginia/Quebec
FreqHist.jpg
The reason that the frequency is so stable is because that is set at the generating plants and it is absolutely necessary that all generators tied to a grid run at very close to the same frequency. If a generator is put on line that is off in frequency by even a little bit huge reactive currents flow which can cause damage to the generator and the equipment it is connected to. Utilities take great care to insure that a generator is turning at the correct frequency and is at the right phase before the contactor connecting it to the grid is closed.

They also take care that the voltage is close to grid voltage but the voltage that you see at you panel depends on myriad factors beyond the voltage of a particular generator at a particular power plant. What you and your neighbors do can have a noticeable effect on the voltage you see at a panel in your house. If you turn on a big heat pump more current is drawn, voltage drop across the wiring between you and the transformer increases and you see lower voltage to the extent, sometimes, that you see the lights dim. And the voltage on the line that feeds the transformer drops a wee bit too. But if all your neighbors turn on their heat pumps at the same time you do that high voltage drop is no longer wee. Even neighbors who did not turn on their heat pumps will see lower voltage. If every one in the town turns on his heat pump (this happens in the afternoon on hot days) the voltage at the substation will drop and the utility may change taps on a transformer to boost it back up. Etc. Thus voltage is going to wander and there is nothing you can do about it at your home except install an autotransformer that changes tap location automatically or use other regulating equipment. This is rarely done in homes but is done in industrial settings.

Now all this extra load does, of course, slow down the generators by increasing the torque on their shafts. There are plenty of monitors on them to sense, for example, that the generator is starting to deliver VARS in addition to watts and their outputs are used to adjust the prime mover in order to remove those thus insuring that the shaft is turning at exactly 60*60/P rpm (P is the number of pole pairs).

If the generator is solar then it locks a local oscillator to the grid frequency and uses that oscillator to control its inverter(s).

Thus the stability of grid frequency depends on the grid and is under its control whereas the stability of the voltage observed at your house depends on things the grid cannot control for and is less stable.
 
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darth_vad3r

Well-Known Sith
May 6, 2019
1,574
1,126
Canada
Thus the stability of grid frequency depends on the grid and is under its control whereas the stability of the voltage observed at your house depends on things the grid cannot control for and is less stable.

This is exactly my understanding. Thanks for the description and chart.
 

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