Transformer Upgrade Required?

[gnuarm]

A neighbor is installing two EVSE and has upgraded his service from the power company. I'm having a hard time understanding why that would be. I'm sure his service is 200 amps (all these houses were built about the same time and my smaller house is on 200 amps). The closer neighbor who told me about it is getting a Mustang Mach-E and wants to install a fixed charger which uses a 60 amp circuit. I guess he is thinking he may need to up his service as well.

Is it customary to upgrade the service from the utility when adding more than one EVSE? I have a model X which can charge at up to 72 amps on a 90 amp circuit. I'm wondering if my 200 amp service would need to be upgraded for that. The only appliance I have that isn't standard in a house is a hot tub on a 50 amp circuit. It's not even used now. Maybe I should have it removed.

 

[UncleCreepy]

There is no general answer to your question. You'll need to have an electrician do a load calculation.

I have two HPWCs installed in my garage and while I can use both at the same time, I'd just activate load sharing if the total power drain was too high. In my situation (ymmv) I barely ever charge both cars at the same time.

 

[RTPEV]

I have two HPWCs installed in my garage and while I can use both at the same time, I'd just activate load sharing if the total power drain was too high. In my situation (ymmv) I barely ever charge both cars at the same time.

Same...we easily share one EVSE between two EVs. Granted, I have a dual headed EVSE that can split power between my two vehicles (or charge them sequentially), but we rarely use that capability since both of our EVs are long range (250+ mile) capable. We only really used the dual capability when I had a 3.3kW Nissan LEAF and my wife had a 3.3kW Chevy Volt. Both needed to be charged every day and both took many hours to recoup daily driving. Fast forward 7 years and both our cars can recoup daily use in about 90 minutes, but we only have to plug them in every 4-5 days anyway, so it's no big deal at all to stagger which day each is plugged in.

 

[gnuarm]

Same...we easily share one EVSE between two EVs. Granted, I have a dual headed EVSE that can split power between my two vehicles (or charge them sequentially), but we rarely use that capability since both of our EVs are long range (250+ mile) capable. We only really used the dual capability when I had a 3.3kW Nissan LEAF and my wife had a 3.3kW Chevy Volt. Both needed to be charged every day and both took many hours to recoup daily driving. Fast forward 7 years and both our cars can recoup daily use in about 90 minutes, but we only have to plug them in every 4-5 days anyway, so it's no big deal at all to stagger which day each is plugged in.

Yeah, I tried to explain to my neighbor (the one who is getting the Mustang) that you don't need the biggest, fastest charging you can get and the 32 amp cabled EVSE would do fine. He very quickly started talking about coming home and needing to charge from 0 to 100% in an hour. I had to give up trying to talk sense to him.

I keep thinking of getting an EVSE, but my parking isn't as convenient and I'd likely put it out in the yard on a pole. If the cable was long enough, I could use a swivel arm and let it dangle to the car when charging keeping the box under the porch roof. Or just add a 240VAC outlet and use my cable. I bought a gen 1 cable so I could get a full 40 amps from a 50 amp circuit.

I was talking to the power company the other day and they are going to institute an EV charging plan where they give you a fixed $7 per month credit for promising to not charge during peak times. Hard to get more reasonable than that. At one time I was on ToU. The EV charging never made a dent in my bill though. To keep from getting whacked by peak charges, I had to turn off heat and AC during peak hours which was not great. The heat pump would run the resistance heat when it came back on, not ideal!

It's still early days for EV charging.

The woods are lovely, dark and deep.
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.

 

[UncleCreepy]

Yeah, I tried to explain to my neighbor (the one who is getting the Mustang) that you don't need the biggest, fastest charging you can get and the 32 amp cabled EVSE would do fine.

That is probably true for you and your neighbour. However for those folks who live further up north, it can make a lot of sense to get the most powerful charging available. I have never tried, but I'm pretty sure that if I tried charging my car on a 120 V circuit at below -20 °C, all the energy would be used to heat the battery. I know nobody was talking about 120 V/12 A here, I'm just trying to make a point that there are situations where more powerful charging makes a lot of sense. Just for reference, if I charge at 11.5 kW in these temperatures, the heat generated by the charging is not enough to keep the battery sufficiently warm. The car would still have to heat the battery every now and then. That's not really a problem at 240 V/48 A, but the lower the power, the more energy goes to waste.
Just wanted to add this for users in colder climates. Obviously this is not an issue in Florida.

 

[pb2000]

If the load calculation done by the electrician exceeds the capacity of the service, you really don't have a choice but to upgrade. In Europe, they just stick a current transformer on the incoming feed and program the charger not to exceed the service capacity, but I don't know if that's an option on this side of the pond.

 

[gnuarm]

That is probably true for you and your neighbour. However for those folks who live further up north, it can make a lot of sense to get the most powerful charging available. I have never tried, but I'm pretty sure that if I tried charging my car on a 120 V circuit at below -20 °C, all the energy would be used to heat the battery. I know nobody was talking about 120 V/12 A here, I'm just trying to make a point that there are situations where more powerful charging makes a lot of sense. Just for reference, if I charge at 11.5 kW in these temperatures, the heat generated by the charging is not enough to keep the battery sufficiently warm. The car would still have to heat the battery every now and then. That's not really a problem at 240 V/48 A, but the lower the power, the more energy goes to waste.
Just wanted to add this for users in colder climates. Obviously this is not an issue in Florida.

So what are the conditions when you need more than 7.7 kW to charge your battery? How much power is used to heat the battery when the temp is -20 °C? Unless you can provide numbers, your point is not really valid.

 

[gnuarm]

If the load calculation done by the electrician exceeds the capacity of the service, you really don't have a choice but to upgrade. In Europe, they just stick a current transformer on the incoming feed and program the charger not to exceed the service capacity, but I don't know if that's an option on this side of the pond.

I'm just trying to imagine what could be connected that would require a service upgrade. 200 amps is a LOT. I read something that said the total of the circuit breakers should not exceed 120% of the service. Or maybe that was the panel rating.... I forget. So a 200 amp panel would be good for 240 amps of breakers. That's a lot of breakers.

240V
60A - Heat pump
40A - Heat pump
30A - hot water
40A - stove
30A - dryer
20A - well
220A total

120V
20A - septic pump
20A - pair outlets
15A - pair outlets
15A - outlets
15A - outlets
15A - outlets
15A - outlets
15A - outlets
15A - outlets
20A - outlets
20A - outlets

Not sure how to add up the 120V circuits since they should be on both sides of the 240V, and so only count half. So 110A.

That's a lot of amps and way over the +10% rule I saw. I wonder if that's a problem. The lighting/outlet circuits carry very little current in the real world. I seem to recall the hot water heater actually draws only 18A. Stoves typically don't use nearly as much power as the circuit is rated for, but if everything is turned on, my stove draws 49A on a 40A breaker. That's odd.

I can always put the charger on the hot tub circuit I suppose with an A/B switch (hot tub hasn't worked for years). I'm also not sure the heat pump really needs both a 40A and a 60A circuit. When they installed the current system, they took a circuit breaker I had put in for the hot tub, but not yet connected. Later they put in another breaker for the hot tub, but I don't know if they paid much attention to what the heat pump actually required, or if they simply used what was there.

I'll use an electrician to install an EVSE anyway. I guess I'll see what he comes up with.

 

[UncleCreepy]

So what are the conditions when you need more than 7.7 kW to charge your battery? How much power is used to heat the battery when the temp is -20 °C? Unless you can provide numbers, your point is not really valid.

Heating the battery takes about 7 kW in a dual motor Model 3. If you drive short distances and the battery doesn't get warm you'll have to start with a cold battery.

I can't provide you with exact numbers as I don't have any information about mass, heat capacity and thermal conductivity of the battery. But here's what I observed:

When I plug in the car in very cold temperatures, it draws a little over 7 kW and it gains 0 km/h while I can hear the pumps running. During this phase, the battery heats up. Eventually, depending on the temperature, the car will start charging, so the power draw goes up to 11.5 kW with the motors still providing heat for the battery. During this phase, I typically see about 15-20 km/h charging speed. Once the battery is warm enough, all the available power is used to charge the battery and I get about 70 km of charge per hour.

What I am saying is that I have noticed that at some point the BMS decided that the battery is getting too cold again and it once again turns on the heat, thus reducing the available power for actual charging.

Even without numbers, it should be obvious that this effect gets worse with less power available. At least I'm still getting something while the battery needs to heat up whereas if I used a weaker charger with less than 7 kW, I would lose range while heating the battery. Of course sooner or later the battery would start charging, but it would also take significantly longer if the battery needs to be reheated while charging.

I am not saying that it is impossible to charge at less than 11.5 kW, but it gets inefficient when it's -20 °C or colder outside due to the necessity to keep the battery warm while charging.

Does that make sense?

 

[pb2000]

I'm just trying to imagine what could be connected that would require a service upgrade. 200 amps is a LOT. I read something that said the total of the circuit breakers should not exceed 120% of the service. Or maybe that was the panel rating.... I forget. So a 200 amp panel would be good for 240 amps of breakers. That's a lot of breakers.

It's based off of square footage of your house and then various appliances like you listed, although it would go off the nameplate, not the breaker. I agree with you that 200A is a lot, but if you put in two EVSEs that can each pull 48A, that's 60% of your continuous load capability on a 200A panel (the 80% rule applies to your main breaker too).

 

[jdr93]

A neighbor is installing two EVSE and has upgraded his service from the power company. I'm having a hard time understanding why that would be. I'm sure his service is 200 amps (all these houses were built about the same time and my smaller house is on 200 amps). The closer neighbor who told me about it is getting a Mustang Mach-E and wants to install a fixed charger which uses a 60 amp circuit. I guess he is thinking he may need to up his service as well.

Is it customary to upgrade the service from the utility when adding more than one EVSE? I have a model X which can charge at up to 72 amps on a 90 amp circuit. I'm wondering if my 200 amp service would need to be upgraded for that. The only appliance I have that isn't standard in a house is a hot tub on a 50 amp circuit. It's not even used now. Maybe I should have it removed.

when i added the tesla charger the electric company said the primary issue was whether there was a 200 amp service or greater, but the other issue was the size of the transformer.

 

[gnuarm]

Heating the battery takes about 7 kW in a dual motor Model 3. If you drive short distances and the battery doesn't get warm you'll have to start with a cold battery.

I can't provide you with exact numbers as I don't have any information about mass, heat capacity and thermal conductivity of the battery. But here's what I observed:

When I plug in the car in very cold temperatures, it draws a little over 7 kW and it gains 0 km/h while I can hear the pumps running. During this phase, the battery heats up. Eventually, depending on the temperature, the car will start charging, so the power draw goes up to 11.5 kW with the motors still providing heat for the battery. During this phase, I typically see about 15-20 km/h charging speed. Once the battery is warm enough, all the available power is used to charge the battery and I get about 70 km of charge per hour.

What I am saying is that I have noticed that at some point the BMS decided that the battery is getting too cold again and it once again turns on the heat, thus reducing the available power for actual charging.

Even without numbers, it should be obvious that this effect gets worse with less power available. At least I'm still getting something while the battery needs to heat up whereas if I used a weaker charger with less than 7 kW, I would lose range while heating the battery. Of course sooner or later the battery would start charging, but it would also take significantly longer if the battery needs to be reheated while charging.

I am not saying that it is impossible to charge at less than 11.5 kW, but it gets inefficient when it's -20 °C or colder outside due to the necessity to keep the battery warm while charging.

Does that make sense?

The problem is it is inefficient no matter what the charger capacity. Having to burn 7 kW to heat the battery is HUGE! Good thing I don't live where it gets down to -20°C ever.

 

[gnuarm]

when i added the tesla charger the electric company said the primary issue was whether there was a 200 amp service or greater, but the other issue was the size of the transformer.

Did you need to upgrade the transformer? I'm wondering if they make people pay for that. The wire from the transformer is owner responsibility... at least if it's above ground. I expect they take responsibility for the transformer and everything upstream. That doesn't mean they don't make you pay for an upgrade though.

 

[gnuarm]

It's based off of square footage of your house and then various appliances like you listed, although it would go off the nameplate, not the breaker. I agree with you that 200A is a lot, but if you put in two EVSEs that can each pull 48A, that's 60% of your continuous load capability on a 200A panel (the 80% rule applies to your main breaker too).

I suppose it would make sense to have interconnected chargers that can share a circuit, maybe 80 amps total giving each car 32 amps to charge the car with if both are charging at a time. It would also allow a single car to draw up to 64 amps such as my X which maxes out at 72 amps. I plugged into a destination charger once that gave the full 72 amps. It was only 15 kW because it was 208V. Still, much better than 7 kW. There were fewer Superchargers back then. Now I don't worry with destination chargers unless I'm in a hotel.

 

[jdr93]

Did you need to upgrade the transformer? I'm wondering if they make people pay for that. The wire from the transformer is owner responsibility... at least if it's above ground. I expect they take responsibility for the transformer and everything upstream. That doesn't mean they don't make you pay for an upgrade though.

no, for some reason it was an oversize one. perhaps because of the grid-tie solar.

 

[pb2000]

That doesn't mean they don't make you pay for an upgrade though.

A friend of mine had to pay to upgrade his transformer when he rebuilt a much larger house on his property. He had pre-emptively had 400A service installed years before starting the rebuild project, but either one of his neighbors upgraded while he was disconnected during construction or the power company sold him the upgrade never thinking he would actually use it.

I suppose it would make sense to have interconnected chargers that can share a circuit, maybe 80 amps total giving each car 32 amps to charge the car with if both are charging at a time.

That's what the wall chargers do, although last I heard the gen3 chargers didn't have the feature working (might be fixed now).

 

[kayak1]

Where I am Central Maine Power by default uses a 10kva transformer for a 200 amp service (which would be 38.4kva at a 80% load). My house is heating with heat pumps, an electric double oven, heat pump water heater, and two electric cars. At times I have seen loads of ~24kva, as part of my solar installation I had to pay to upgrade the transformer that feeds my house to 25kva.

When I replace the Leaf with the Cybertruck, I expect that I will see times of ~30kva, which would be acceptable according to the power company as they rather we see a voltage drop than install an oversized transformer and have more transformer losses. They wouldn't let me opt up for the 35kva transformer during the solar install in case we added additional panels in the future.

 

[UncleCreepy]

Where I am Central Maine Power by default uses a 10kva transformer for a 200 amp service (which would be 38.4kva at a 80% load).

Not sure I'm following... 240 V x 200 A is 48 kW. How do you provide a 200 Amp service with a 10 kVA transformer?

 

[kayak1]

Not sure I'm following... 240 V x 200 A is 48 kW. How do you provide a 200 Amp service with a 10 kVA transformer?

They bank on the service not being at 48KW for hours and hours. The transformer is surrounded by oil to help dissipate the heat, they will undersize the transformer as a standard. The more recent installations will have a cutout with a fuse. The fuses for the 10kva transformers are normally 3amps or 21.6kva (7200 volts).

If your house is connected to a non-shared transformer take a look at the markings. Many of them will have a sticker to designate the size.

 

[Big Earl]

Not sure I'm following... 240 V x 200 A is 48 kW. How do you provide a 200 Amp service with a 10 kVA transformer?

The same way my power company puts 7 houses on a 50 kVA transformer. The models they use to allocate power and plan their transmission and distribution network don’t include electric vehicles. They work on the assumption that houses on a transformer won’t have simultaneous high loads. That will obviously need to change as EVs become more prevalent.