EV Chargers Service Calculation

[brycenesbitt]

What size service I will need for 4 48amp ev chargers. It’s for an apartment complex.

Existing construction?
The service size I recommend in some cases whatever the building has now.
---
The building gets a CT clamp on the main, and the EV chargers share whatever capacity is left over after
the residents stop cooking, running space heaters, grow lamps, vibrators or whatever.
All we care about is the cars are full in the morning. Capacity increase needed for that is 0 amps
---
8 hour dwell time, but going off peak so midnight to 6am, just six hours.... 240V 200A from the building....
48kw * 6 = 288 kwh. 288 kwh/4 miles per khw * 7 days = 504 miles of EV driving per week assuming everyone works and commutes 7 days a week.
Or start charging 9pm, so nine hours active -> 800 ish miles of EV driving per week for the complex.
But in reality more, as some people will park longer, work from home, or take a gas car for weekend drives.

....
Once that gets tight the owner can pull in new service.

 

[jimjones]

Existing construction?
The service size I recommend in some cases whatever the building has now.
---
The building gets a CT clamp on the main, and the EV chargers share whatever capacity is left over after
the residents stop cooking, running space heaters, grow lamps, vibrators or whatever.
All we care about is the cars are full in the morning. Capacity increase needed for that is 0 amps
---
8 hour dwell time, but going off peak so midnight to 6am, just six hours.... 240V 200A from the building....
48kw * 6 = 288 kwh. 288 kwh/4 miles per khw * 7 days = 504 miles of EV driving per week assuming everyone works and commutes 7 days a week.
Or start charging 9pm, so nine hours active -> 800 ish miles of EV driving per week for the complex.
But in reality more, as some people will park longer, work from home, or take a gas car for weekend drives.

....
Once that gets tight the owner can pull in new service.

@brycenesbitt it will be a 120/208 transformer. The service for the chargers will come from that transformer.

 

[jimjones]

Three 48A single phase 208V loads on a 3 phase panel will not be 48A per phase -- they add to each other by a factor of 1.73 or so. This is easiest to see when you use VA as a calculation: 48*208 = 10KW. Three 10KW loads is 30KW. 30KW on a 3 phase panel, evenly balanced, is 30000/208/1.732 = 84A. But since there are 4 chargers, two legs are going to have an additional 48A load. So the panel needs to support 84+48 = 132A * 125% = 165A (168A * 125% = 210A if you want to support 6 chargers).

Question: do the chargers still draw 48A if fed with a 208V

 

[jimjones]

@suemarkp yes, but if course the kw will change because of lower voltage. Right?

 

[acrwc10]

@suemarkp yes, but if course the kw will change because of lower voltage. Right?

The KW is a combination of Volts and Amps, when the voltage is lower the amperage is higher, 7200KW divided by 240 volts is 30 amps. 7200 divided by 208 volts is 34.6 amps.

 

[jimjones]

The KW is a combination of Volts and Amps, when the voltage is lower the amperage is higher, 7200KW divided by 240 volts is 30 amps. 7200 divided by 208 volts is 34.6 amps.

Correct…

 

[suemarkp]

But watts typically aren't forced, they are the result of the consumed amps at a given supply voltage. There are loads that act like fixed resistance (e.g. heating elements). Lower the voltage, they draw less amps, consume less watts, and make less heat. There are loads that tend to be more constant work like motors. Lower the voltage and the amps tend to increase, because the motor is trying to do the same amount of work if it is a constant RPM motor. A universal motor that changes RPM with voltage may do less work as the voltage lowers.

I don't know how to characterize a car charger. If they are designed to provide a constant current, then they will draw less power when the voltage is reduced and therefore charge the battery slower. I doubt they increase current to make up for a lower supply voltage. I thought most car charges also had jumpers to select the current limit. So if you set it to 48A, hopefully it will do that whether you supply it 120V, 208V, or 240V.

 

[retirede]

Car chargers are current-controlled. The maximum charge current does not change as the voltage is varied.

The EVSE tells the car the maximum current available. The charger in the car determines the actual charge current, not exceeding the maximum as signaled by the EVSE. 208 vs 240 doesn’t matter.

 

[tortuga]

I think the code (625.42) is a bit unclear if the continuous factor applies on the service & feeder conductors, its clearly saying include that 25% adder on the branch circuit but not the feeder / service entrance. I'd add the 25% for headroom anyways.

 

[jimjones]

Car chargers are current-controlled. The maximum charge current does not change as the voltage is varied.

The EVSE tells the car the maximum current available. The charger in the car determines the actual charge current, not exceeding the maximum as signaled by the EVSE. 208 vs 240 doesn’t matter.

That makes sense. The owner of the chargers said it can be controlled by the setting where it never goes over current.

 

[jimjones]

But watts typically aren't forced, they are the result of the consumed amps at a given supply voltage. There are loads that act like fixed resistance (e.g. heating elements). Lower the voltage, they draw less amps, consume less watts, and make less heat. There are loads that tend to be more constant work like motors. Lower the voltage and the amps tend to increase, because the motor is trying to do the same amount of work if it is a constant RPM motor. A universal motor that changes RPM with voltage may do less work as the voltage lowers.

I don't know how to characterize a car charger. If they are designed to provide a constant current, then they will draw less power when the voltage is reduced and therefore charge the battery slower. I doubt they increase current to make up for a lower supply voltage. I thought most car charges also had jumpers to select the current limit. So if you set it to 48A, hopefully it will do that whether you supply it 120V, 208V, or 240V.

 

[jimjones]

Thanks…!

 

[brycenesbitt]

That makes sense. The owner of the chargers said it can be controlled by the setting where it never goes over current.

Hopefully they get the right advice.
If you just have 4x chargers at 48amps each, your service will be WAY oversized to the actual demand.
---
Individual cars will not be plugged in, incapable of charging at that full rate, or have a battery that's partially full
and charging slower.

The way to maximize utilization of your main service is to have many more charging heads: perhaps 8x or 10x.
Then allocate power dynamically. Only then will the utilization factor go up, maximizing the value of the investment made.
If you've got 10 cars charging you may be able max out a 200A main. With 4 cars charging, it's unlikely.

 

[letgomywago]

I think the code (625.42) is a bit unclear if the continuous factor applies on the service & feeder conductors, its clearly saying include that 25% adder on the branch circuit but not the feeder / service entrance. I'd add the 25% for headroom anyways.

I agree on a dedicated service for EV itd seam reasonable to do this. I wouldn't want that added to the code for a mixed use service like a dwelling or an entire commercial building. I'd also point out that most metering equipment is already rated for that increase. My meters I use most often say 200a continuous. They use bolts and busbar for the most part and that gives them a lot of fudge room where you could upsize wire if needed to accommodate for the heat.

 

[rlqdot]

Three 48A single phase 208V loads on a 3 phase panel will not be 48A per phase -- they add to each other by a factor of 1.73 or so. This is easiest to see when you use VA as a calculation: 48*208 = 10KW. Three 10KW loads is 30KW. 30KW on a 3 phase panel, evenly balanced, is 30000/208/1.732 = 84A. But since there are 4 chargers, two legs are going to have an additional 48A load. So the panel needs to support 84+48 = 132A * 125% = 165A (168A * 125% = 210A if you want to support 6 chargers).

Question: do the chargers still draw 48A if fed with a 208V supply?

i cannot speak for every charger manufacturer, but my office does a considerable amount of Tesla building (delivery and collision repair centers) designs where Tesla Wall Connectors are installed. 99% of those buildings have 208Y120 supply for the Level 2 chargers. as far as i know (and have been told by Tesla), the Tesla chargers are listed for a maximum supply circuit of 60 amps but can be commissioned for circuits rated at lower ampacities. according to Tesla, when they commission these chargers, they adjust the output maximum to 80% of the breaker rating - so a 60 amp branch circuit to the charger results in it being commissioned for 48 amp output ... 48 amps x 208 volts = 9.98 KW (that 10 KW noted above). obviously, if fed from a 60 amp 240 volt source, the output would be greater than 10 KW (actual output = 11.5 KW) ... and if fed from a lower-rated branch circuit, the output would be less. if we have a problem with load at the service or sub-panel level, Tesla will commissioin the chargers for a lower output to control the loads.

 

[brycenesbitt]

i cannot speak for every charger manufacturer....according to Tesla, when they commission these chargers, they adjust the output maximum to 80% of the breaker rating - so a 60 amp branch circuit to the charger results in it being commissioned for 48 amp output...

Yes that's a pretty typical feature. The vehicle limits current draw to the lesser of it's own need, or the amount advertised by the EVSE. It reacts quite quickly, so load shedding devices use this one way communication method to shed power when needed.

 

[LarryFine]

The KW is a combination of Volts and Amps, when the voltage is lower the amperage is higher, 7200KW divided by 240 volts is 30 amps. 7200 divided by 208 volts is 34.6 amps.

If the load is altered to use the same amount of power at the new voltage, yes.

If the load is a constant-power load like most types of motors, yes.

For most other, constant-impedance loads, no.

 

[retirede]

If the load is altered to use the same amount of power at the new voltage, yes.

If the load is a constant-power load like most types of motors, yes.

For most other, constant-impedance loads, no.

Level 2 EV charging is current-controlled. The car sets the current, the resulting KW depends on the voltage.

 

[ggunn]

I treat EV charger conductors and OCPD the same as PV inverter output circuits.

 

[tortuga]

I treat EV charger conductors and OCPD the same as PV inverter output circuits.

How would you size the service entrance conductors for the OP's proposed four 48A chargers?