EV Charging Transformer Sizing Calculations

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
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.

I'd love to know what the plan is for truck stops. Current production chargers are one megawatt. Trucks will need to sit and charge for like an hour, compare that to 10 minutes to fuel a diesel. And the trucks will have to be charged more times to complete the same trip.

We could be talking 40-50 megawatts. Not to mention needing a lot more land
 

ruxton.stanislaw

Senior Member
Location
Arkansas
Occupation
Laboratory Engineer
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.

If going with the J1772 spec, a max of 265 V is permitted. To make that work, you can choose the transformers or negotiate with the utility for an overall -10% tap or use 415/240 transformers.
 

retirede

Senior Member
Location
Illinois
If going with the J1772 spec, a max of 265 V is permitted. To make that work, you can choose the transformers or negotiate with the utility for an overall -10% tap or use 415/240 transformers.

Ford’s EVs up into 2021 would show an error and stop charging if the input voltage was 255 or higher. I know this was changed, but I’m not sure if 265 will work.
 

retirede

Senior Member
Location
Illinois
I'd love to know what the plan is for truck stops. Current production chargers are one megawatt. Trucks will need to sit and charge for like an hour, compare that to 10 minutes to fuel a diesel. And the trucks will have to be charged more times to complete the same trip.

We could be talking 40-50 megawatts. Not to mention needing a lot more land

That has nothing to do with what’s being discussed on this thread, which is level 2 AC charging for an apartment.
 

ruxton.stanislaw

Senior Member
Location
Arkansas
Occupation
Laboratory Engineer
Ford’s EVs up into 2021 would show an error and stop charging if the input voltage was 255 or higher. I know this was changed, but I’m not sure if 265 will work.
This is true; there will always be some unknowns and supplying ~240 V or less nominal could avoid that. I have one other idea for this, though it depends on how the utility supplies equipment. E.g. ours is responsible for the transformer at their expense and optionally provides a 5% discount on medium voltage metering for customer supplied equipment in the case that something custom is required.

The idea is to use the utility's off the shelf 480/277 service with a small buck boost at each EV charger. That could be more economic than sourcing a more custom 415/240 transformer.
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
This whole thing was poorly thought out when developing charging standards by the auto manufacturers. The actual chargers are in the EVs and they only support 240v single phase
 

retirede

Senior Member
Location
Illinois
This whole thing was poorly thought out when developing charging standards by the auto manufacturers. The actual chargers are in the EVs and they only support 240v single phase

That’s not correct. They will work at any voltage between 120 and 240 plus some tolerance at each end that varies by manufacturer.
 

rlqdot

Member
Location
St. Louis, MO - USA
Occupation
Professional Engineer (multiple states) - building design
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.
while it is true that the chargers are single phase, by the time they hit the utility (or even the customer-owned) transformer, they can be arranged on a panel so that the load appears to be a 3 phase load. sizing the transformer based on 3 phase configuration is the right answer in my opinion.
 

rlqdot

Member
Location
St. Louis, MO - USA
Occupation
Professional Engineer (multiple states) - building design
my company actually prepares designs for Tesla SSD (Sales, Service and Delivery - essentially dealerships) and Collision repair facilities and we routinely hit their Wall Connectors with 60 amp 208 volt single phase branch circuits. Tesla commissions those chargers for a 48 amp 208 volt maximimum supply and we design the panels and transformers that support the whole charging system for 10 KW per charger, balancing the load as best we can to achieve nearly-equal loading on the 3 phase supply panels / transformers.
 

retirede

Senior Member
Location
Illinois
while it is true that the chargers are single phase, by the time they hit the utility (or even the customer-owned) transformer, they can be arranged on a panel so that the load appears to be a 3 phase load. sizing the transformer based on 3 phase configuration is the right answer in my opinion.

I don’t understand what you mean by this.
These are single phase loads, and yes, they should be equally distributed across the 3 phases, but that doesn’t change how you calculate the load.
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
That’s not correct. They will work at any voltage between 120 and 240 plus some tolerance at each end that varies by manufacturer.
I should have said 240 max. And 120 is worse

I've got some of these miller welders that automatically adjust to all low voltages, and both single and three phase, They cost one heck of a bit less than an EV. If they had some foresight when designing the EVs they would have known there would be large facilities with numorous charging locations. Limiting them to 240 single phase was not very smart.


Miller® power sources equipped with Auto-Line power management deliver consistent, powerful arcs using single-phase or three-phase electrical service from 110V to 575V* — even “dirty” power with dips and spikes.
 

rlqdot

Member
Location
St. Louis, MO - USA
Occupation
Professional Engineer (multiple states) - building design
I don’t understand what you mean by this.
These are single phase loads, and yes, they should be equally distributed across the 3 phases, but that doesn’t change how you calculate the load.
i'm sorry - i mis-read your post #5 ... you are completely correct, the load on the branch circuit serving each charger would be 8.32 KW. i interpreted your statement about assuming the chargers are 3 phase as being associated with the transformer selection and you were only pointing out that the load on each branch circuit was incorrect. i agree with that and with the statement that the 125% is not required beyond the branch circuit OC device and branch circuit wiring.

in our dealership designs, we supply these chargers with 60 amp 2-pole 208 volt circuits, commissioned to draw a maximum of 48 amps (10 KW). and, transformer and 480 volt feeder selection is based on a fairly well-balanced 3 phase load.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
i agree with that and with the statement that the 125% is not required beyond the branch circuit OC device and branch circuit wiring.
That's not correct, 625.41 requires that branch circuit and feeder OCPD be sized with a 125% factor. And I would say an EVSE is a continuous load, so the usual 125% factors required by 210.19/210.20, 215.2/215.3, and 230.42 would also apply.

Cheers, Wayne
 

ruxton.stanislaw

Senior Member
Location
Arkansas
Occupation
Laboratory Engineer
Ideally, these cars would all accept a wider range of DC input voltage as a standard, with rectification behind handled by the charging station. It is also practical to interconnect with solar energy storage this way.
 

rlqdot

Member
Location
St. Louis, MO - USA
Occupation
Professional Engineer (multiple states) - building design
That's not correct, 625.41 requires that branch circuit and feeder OCPD be sized with a 125% factor. And I would say an EVSE is a continuous load, so the usual 125% factors required by 210.19/210.20, 215.2/215.3, and 230.42 would also apply.

Cheers, Wayne
no - read 625.41 carefully - the title of that section is OVERCURRENT PROTECTION ... i agree that the feeder wire size and OC protective device has to be sized for 125% of the total EV charging load, but the transformer that supplies that feeder is sized based on the EV load only without the 125% factor as far as i can tell.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
no - read 625.41 carefully - the title of that section is OVERCURRENT PROTECTION ... i agree that the feeder wire size and OC protective device has to be sized for 125% of the total EV charging load, but the transformer that supplies that feeder is sized based on the EV load only without the 125% factor as far as i can tell.
Yes, agreed on the transformer, I made that comment earlier in the thread (post #9).

But you stated "the 125% is not required beyond the branch circuit OC device and branch circuit wiring." That implies that the feeder OCPD and feeder conductors could be sized at 100%, which is not correct.

Cheers, Wayne
 

retirede

Senior Member
Location
Illinois
Ideally, these cars would all accept a wider range of DC input voltage as a standard, with rectification behind handled by the charging station. It is also practical to interconnect with solar energy storage this way.

That’s exactly how level 3 DC charging works.

Which, again, has no bearing on the issue raised in this post.
 

Joethemechanic

Senior Member
Location
Hazleton Pa
Occupation
Electro-Mechanical Technician. Industrial machinery
I don't know much about the EV onboard chargers, But with this much single phase rectification going on, aren't the 3rd and 5th harmonics going to be crazy? Is this going to be an IEEE 519 problem?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In a related story...

I have a client who wants me to design an array of (4) charging stations which need to be fed by a 480V to 208V transformer. The chargers will each be fed by two phases of the 208/120V. When all the chargers are running at capacity simultaneously, the phase currents will obviously not be the same. I am assuming that for sizing the transformer I will need to size it as if there were six chargers. Is correct?
 
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