How much of a voltage drop is ok?

[BrettS]

I have a 2015 model S with dual chargers and a HPWC on a 100 amp circuit so I can (and do) charge at 80 watts. When I first plug the charger in it reports 242V and the voltage slowly drops as it ramps up to 80 watts. At that point the voltage has dropped to 235. If my (house) air conditioner starts up while the car is charging it can get down to 233V.

Is this acceptable or something that in should be concerned about?

 

[gregd]

This is not typical, and sounds like you have an undersized feed to the house. I only charge at 40 (amps, by the way, not watts), but the voltage sag is only about a volt or so.

I would have the power company come out for a visit. Then, if they don't find anything wrong, an electrician would be next. Or schedule them both at the same time so they can noodle this together. The worst case scenario is a bad connection somewhere, which could easily lead to a fire. (Electrical math: 7 volts times 80 amps is 560 watts being dissipated somewhere. That's about half a toaster.) In the mean time, I would reduce the charge current as low as you can make it, and still get your charging done in time.

 

[JHWJR]

I can't put numbers on it, but what you report is exactly what I would expect. Voltage will drop as current increases. I think your system is operating exactly as it is supposed to operate.

I will agree with gregd, though. Ask someone who really knows.

 

[Tam]

...233V...

I am no expert but I do expect a drop in voltage too so I agree with JHWJR's assessment: You are fine!

 

[TexasEV]

I have a similar voltage drop under load as the OP. The car will let you know if the voltage drop is too much, by reducing charging from 40 to 30A.

 

[TexasEV]

I was wrong in my post above. My voltage drop is much less than the OP's. I just plugged in and watched the voltage drop from 245V at 0A to 241V at 40A.

 

[Topher]

Ask someone who really knows.

Yes, but start with Tesla, not the electric company. They don't care about anything past the pole or meter, any problem is unlikely to be there, and they will charge you an arm and a leg.

Thank you kindly.

 

[gregd]

Yes, but start with Tesla, not the electric company. They don't care about anything past the pole or meter, any problem is unlikely to be there, and they will charge you an arm and a leg.

Thank you kindly.

Given that both his car and the home AC cause a significant voltage drop, I'm thinking the issue is on the electric company side of the meter, not in his house. At least, was my logic. Getting them to investigate should be free.

 

[Topher]

At least, was my logic.

Not likely. There isn't much that can go wrong to drop the voltage at the pole. (short of a local brown out).

Getting them to investigate should be free.

Ha. It might be if they actually find a problem. If not, you are paying for the crew and truck at exorbitant rates (better then me paying for it).

Thank you kindly.

 

[tnt1971]

My voltage varies by about 10 volts, depending on the time of day. On a really hot day, I see a 20 volt drop. Perfectly normal and it is the power companies way of dealing with spikes in demand.

 

[tga]

The feed from the pole to the weatherhead is often much smaller than you would expect if you look at NEC tables (PoCo's have their own set of rules).

I routinely see a similar drop charging from a friend's dryer outlet (100A service). Almost all of the drop occurs before the main (measuring at the service entrance). I think his overhead feed is 4 ga AL.

 

[Zetopan]

When I first plug the charger in it reports 242V and the voltage slowly drops as it ramps up to 80 watts [amps]. At that point the voltage has dropped to 235.

Let's do some math. 242V minus 235V is a 7 volt drop with an 80 amp load, which indicates a series resistance of 0.0875 ohms. 7V / 242V represents slightly under a 3% voltage drop under full load (actual = 2.89%), while a 3% voltage drop under full load is considered to be an acceptable limit rating for normal wiring. Hence the resistance and voltage drops under load are fine for your Tesla charging.

If my (house) air conditioner starts up while the car is charging it can get down to 233V.

You have not stated the load amperage of your house air conditioner, it should be stated on the nameplate. Your Tesla charging wiring should be on a totally separate circuit from your air conditioner (e.g. using separate breakers and wiring from the breaker box). If that is NOT the case then get an electrician to correct it since it is not safe. If it is the case that totally separate circuits are being used, then you are approaching overloading the primary 240 feed into the house since you are seeing a 3.7% voltage drop, and that is slightly excessive. Not call the fire department excessive, but enough that you should consult an electrician to determine why it is slightly over the limit when both circuits are active. What is the house breaker box rating? 100A is quite light (anything less is into "wimpy" territory) and 200A is quite common.

 

[Snowstorm]

Is the drop measured only at the tesla or does other open circuit 240v circuits in the house also see a drop? If other 240v circuits also see the same drop then it is most likely insufficiency at the house feed level. I do hope you have a 200A supply as 80A charge on a 100A supply means you cannot run much else while your are charging.

I see a 1-2v drop, but I am only pulling 24A on a 14-30 win extension cord. 200A home supply.

 

[BrettS]

Thanks for the replies, guys. I got out my voltmeter and did a bit more investigating today and here is what I found:

First, a bit of background. I have 200 amp service. The service comes up from underground to a small panel with a meter on the outside of the house. The outside panel has a couple of breakers in it for the heat pump and an irrigation well pump as well as a feed for a larger panel in the garage, which feeds the rest of the house. My HPWC is on it's own 100 amp breaker which is in the panel in the garage.

So, for testing I took my voltmeter out to the main panel on the outside of the house and got a voltage reading, then I started to charge the car. As usual, the voltage reading on the car showed a 7 volt drop as it ramped up from 0 amps to 80 amps. But the voltage reading on the panel only showed a 4 volt drop when the car had gotten up to 80 amps. I also did the same test on the panel inside the garage to see if the problem may be the feed between the panels, but I got the same results. Just a 4 volt drop at the garage panel when the car was charging at 80 amps.

So, it seems like the voltage drop may be split... 3 volts are being lost in the HPWC and 4 volts are being lost on the power company side.

Just for 'fun' I wanted to see what would happen if I tried drawing the max amount of power, so I left the car charging at 80 amps and also turned on my oven and my water heater. It's an on demand water heater which can draw a decent amount of power on it's own. Among the three devices and whatever else was on in the house I suspect I was drawing close to 140 amps. With all that on I saw a full 10V drop at both panels. Since I didn't see any difference between the two panels this tells me that the wire between the panels is OK and can handle the power draw, but the wire from the pole to the house likely cannot.

I can see the markings on the wire between the two panels and it shows that it has three AWG 4/0 conductors and one AWG 2/0 ground. The meter part of the panel is locked of course, but I could see a short section of the wires coming in from the pole in there. I didn't see any markings on them, but they are considerably smaller in diameter than the 4/0 wires going between the panels and I think they are even a bit smaller than the 2/0 ground. They definitely appear to be undersized to me.

 

[mspohr]

Thanks for the replies, guys. I got out my voltmeter and did a bit more investigating today and here is what I found:

First, a bit of background. I have 200 amp service. The service comes up from underground to a small panel with a meter on the outside of the house. The outside panel has a couple of breakers in it for the heat pump and an irrigation well pump as well as a feed for a larger panel in the garage, which feeds the rest of the house. My HPWC is on it's own 100 amp breaker which is in the panel in the garage.

So, for testing I took my voltmeter out to the main panel on the outside of the house and got a voltage reading, then I started to charge the car. As usual, the voltage reading on the car showed a 7 volt drop as it ramped up from 0 amps to 80 amps. But the voltage reading on the panel only showed a 4 volt drop when the car had gotten up to 80 amps. I also did the same test on the panel inside the garage to see if the problem may be the feed between the panels, but I got the same results. Just a 4 volt drop at the garage panel when the car was charging at 80 amps.

So, it seems like the voltage drop may be split... 3 volts are being lost in the HPWC and 4 volts are being lost on the power company side.

Just for 'fun' I wanted to see what would happen if I tried drawing the max amount of power, so I left the car charging at 80 amps and also turned on my oven and my water heater. It's an on demand water heater which can draw a decent amount of power on it's own. Among the three devices and whatever else was on in the house I suspect I was drawing close to 140 amps. With all that on I saw a full 10V drop at both panels. Since I didn't see any difference between the two panels this tells me that the wire between the panels is OK and can handle the power draw, but the wire from the pole to the house likely cannot.

I can see the markings on the wire between the two panels and it shows that it has three AWG 4/0 conductors and one AWG 2/0 ground. The meter part of the panel is locked of course, but I could see a short section of the wires coming in from the pole in there. I didn't see any markings on them, but they are considerably smaller in diameter than the 4/0 wires going between the panels and I think they are even a bit smaller than the 2/0 ground. They definitely appear to be undersized to me.

Good job of investigating this issue. It looks like your house wiring is adequate.
From what you say, it sounds like the problem is with the power company feed... But, I'm not sure it is a problem. A 10 volt drop is probably within the power company specs for 140 amp draw. You could talk to the power company and find out what their policy is and they may want to put a monitor on your power feed to verify your numbers.
At my house, I have a more severe problem. My local 25kW transformer feeds 7 houses. Most of them are vacation cabins and only occupied sporadically. I have, however, noticed voltage as low as 225 volts under heavy load. This is almost entirely a voltage drop from the transformer to the house. I have talked to the power company about this and they seem to be OK with the situation. I talked to them about replacing the transformer and they said I could pay $10k to have my own installed.
If the power company is not worried about blowing up the transformer, I'm not, either. When the voltage drops, I don't have to pay for 240 volts since the meter measures actual voltage and current. I only pay for what is delivered to the meter.
I don't think you need to worry about it.

 

[BrettS]

I don't think you need to worry about it.

That may be the takeaway here. I'm pretty well convinced that the wiring inside my house is adequate and that helps me sleep at night knowing that it's not going to overheat and start a fire. Worst case, if the wire coming to the house is undersized and does overheat it would be quite inconvenient, but should be unlikely to damage the house.

I feel like I'm unlikely to really get anywhere talking to the utility company and calling them would only be an exercise in frustration.

 

[mspohr]

That may be the takeaway here. I'm pretty well convinced that the wiring inside my house is adequate and that helps me sleep at night knowing that it's not going to overheat and start a fire. Worst case, if the wire coming to the house is undersized and does overheat it would be quite inconvenient, but should be unlikely to damage the house.

I feel like I'm unlikely to really get anywhere talking to the utility company and calling them would only be an exercise in frustration.

You might try to call the power company. The people on the engineering side tend to be a lot more reasonable and responsible than those on the "customer service" side of the company. Just present the info you have and ask them what they think.

 

[sorka]

My voltage drops about 1 volt. Typically it's 246 to 247 and drops to 245 or 246.

About half of the drop occurs while measuring voltage from any other plug in the house and half at the cutoff switch for the hwpc.