# EV Charging Transformer Sizing Calculations

#### jselectricservs

##### Member
When sizing a transformer for some new EVSE station for an apartment community we do simply all all the loads together and size equipment or can we size using the optional method. Example the plan is to install 186 level chargers. 186 x 32 amp x 125% = 7440 amps . If there are no demand factors and my calculation are correct this would require a 2700 KVA Transformer. Or can we use table 220.84, for over 62 units at 23%. This would bring the amp to 1711 amps.

Thank you.

#### retirede

##### Senior Member
IMO, 220.84 could be only used if each EVSE was fed from an individual apartment’s feeder.

If you are installing a new service/transformer/feeder solely to power the EVSEs, that wouldn’t fly. I feel the NEC needs to better address vehicle charging loads for these such installations. Common sense says that all 132 units will never be supplying full output at the same time, but what is the right demand factor???

#### augie47

##### Moderator
Staff member
I agree with all the points in Post #2 but might add that, for load calculation, I don't see the need to add the 25%.

#### garbo

##### Senior Member
IMO, 220.84 could be only used if each EVSE was fed from an individual apartment’s feeder.

If you are installing a new service/transformer/feeder solely to power the EVSEs, that wouldn’t fly. I feel the NEC needs to better address vehicle charging loads for these such installations. Common sense says that all 132 units will never be supplying full output at the same time, but what is the right demand factor???
With majority of people working Monday to Friday during the day can only imagine that between the hours of 6 to 10 PM would be the heaviest time for EV charging. That along with EV chargers considered a continuous load if only halve of the charges are in use the load would be above 2,900 amps . Have to wonder if many of the utility companies will have generation plants, transmission lines & hard to get transformers installed within the next 15 years.

#### retirede

##### Senior Member
Also - check your math. 32 x 208 x 1.25 = 8.32 KW. 8.32 x 186 = 1548 KW.

It looks like you assumed the 32A chargers are 3 phase. They’re not.

#### augie47

##### Moderator
Staff member
Curious as to why you would be supplying a transformer. I would think with that much added load it would require a new service and you could get the desired voltage from POCO.

#### jselectricservs

##### Member
IMO, 220.84 could be only used if each EVSE was fed from an individual apartment’s feeder.

If you are installing a new service/transformer/feeder solely to power the EVSEs, that wouldn’t fly. I feel the NEC needs to better address vehicle charging loads for these such installations. Common sense says that all 132 units will never be supplying full output at the same time, but what is the right demand factor???
This apartment complex has an existing 750KVA transformer with minimal loads, are you saying if we this we can use the the table in 220.84 or i am misinterpreting the above reference.

Thank you.

#### jselectricservs

##### Member
Also - check your math. 32 x 208 x 1.25 = 8.32 KW. 8.32 x 186 = 1548 KW.

It looks like you assumed the 32A chargers are 3 phase. They’re not.
You are correct, I don't know why I used the 3 phase calculation, but i am glad you pointed it out. I would have sent it in as it was.
Thank you for catching that!

#### wwhitney

##### Senior Member
As augie mentioned in post #3, the 125% factor for continuous loads is for OCPD sizing, and hence for conductor sizing when those conductors are protected by the upsized OCPD. Transformer ratings are continuous, to my understanding, so there is no need to use a 125% factor for continuous loads when considering transformer size or capacity.

Cheers, Wayne

#### retirede

##### Senior Member
This apartment complex has an existing 750KVA transformer with minimal loads, are you saying if we this we can use the the table in 220.84 or i am misinterpreting the above reference.

Thank you.

IMO, you cannot use that table. It only applies to the loads of the individual dwelling units.

A 750 KVA transformer would be able to supply a maximum of 112 32A EVSEs without applying a diversity factor.

#### augie47

##### Moderator
Staff member
I might have overlooked an important point. Ist the transformer going to be a single phase or 3 phase

#### dvcraven0522!

##### Member
There are load management systems that can help reduce the size of your service. It basically works first in first service. If more chargers are pugged in it will split the loads evenly depending upon the number of chargers.

#### GeorgeB

##### ElectroHydraulics engineer (retired)
I might have overlooked an important point. Ist the transformer going to be a single phase or 3 phase
Almost certainly the transformer would be 3 phase. Almost certainly the EVSEs would be single phase. To use COTS EVSEs, I'd probably specify 240/415 secondary. Distances and layout could have an impact.

This does not address your initial question and more knowledge is needed.

#### retirede

##### Senior Member
I might have overlooked an important point. Ist the transformer going to be a single phase or 3 phase

Since his calculations seemed to be based on 208V (not 240) I presumed 3 phase also.

#### retirede

##### Senior Member
Almost certainly the transformer would be 3 phase. Almost certainly the EVSEs would be single phase. To use COTS EVSEs, I'd probably specify 240/415 secondary. Distances and layout could have an impact.

This does not address your initial question and more knowledge is needed.

Since AC charging is current-controlled, running them on 240V instead of 208V will increase the required transformer size even more.

#### ruxton.stanislaw

##### Senior Member
Absolutely, going with 480/277 V would make sense for this. However, put it on the lowest tap in the direction of 415/240 like GeorgeB mentioned, as there are some certain Tesla models that won't charge past 280v.

Some other things to consider are that if the cars charge faster, fewer cars will be charging at once on average. However, the evening rush hour may still see the same peak for a shorter duration of time. This can actually be better for transformer life. Also, the upper limit of the charge rate of these vehicles is based on power (KW). Though with almost all models, it would be more than 32A.

#### raberding

##### Senior Member
There are load management systems that can help reduce the size of your service. It basically works first in first service. If more chargers are pugged in it will split the loads evenly depending upon the number of chargers.
yeah...I just learned of 220.70 and 750.30 in the 2023. EVSE is a great application

#### winnie

##### Senior Member
IMO, 220.84 could be only used if each EVSE was fed from an individual apartment’s feeder.

If you are installing a new service/transformer/feeder solely to power the EVSEs, that wouldn’t fly. I feel the NEC needs to better address vehicle charging loads for these such installations. Common sense says that all 132 units will never be supplying full output at the same time, but what is the right demand factor???

Even more than this, a well designed installation will have load management and time of use management, specifically to minimize peak loading and shift the bulk of the power consumption to optimal times.

Evening rush charging should be intentionally minimized; most users will want to get home, plug their car in, and have it ready to go by the time they leave in the morning; if the system immediately jumps to full power as soon as each user plugs in then it will be bad for the electrical service, bad for the car's batteries, and bad for the grid as a whole.

-Jon

#### retirede

##### Senior Member
Absolutely, going with 480/277 V would make sense for this. However, put it on the lowest tap in the direction of 415/240 like GeorgeB mentioned, as there are some certain Tesla models that won't charge past 280v.

Some other things to consider are that if the cars charge faster, fewer cars will be charging at once on average. However, the evening rush hour may still see the same peak for a shorter duration of time. This can actually be better for transformer life. Also, the upper limit of the charge rate of these vehicles is based on power (KW). Though with almost all models, it would be more than 32A.

View attachment 2567585

Tesla is (was) the only EV that will accept anything over a nominal 240VAC. I believe the current generation of wall connectors are no longer rated for 277V.

Edit - this comment only applies to EVs in the US market.

#### retirede

##### Senior Member
Even more than this, a well designed installation will have load management and time of use management, specifically to minimize peak loading and shift the bulk of the power consumption to optimal times.

Evening rush charging should be intentionally minimized; most users will want to get home, plug their car in, and have it ready to go by the time they leave in the morning; if the system immediately jumps to full power as soon as each user plugs in then it will be bad for the electrical service, bad for the car's batteries, and bad for the grid as a whole.

-Jon

This is best accomplished when the drivers are paying for the power under a TOU rate plan. The vehicles have the capability to schedule the charging so the car can be plugged in when parked, but the charging doesn’t start until scheduled.

Battery health in modern EVs is generally not measurably affected by L2 AC charging. We’re mostly talking about L2 rates of 11 KW (the units in this post are 7 KW) or less into a battery designed to be fast charged at 150 KW or more.