EV Charging Station Load Calculation

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YUBI

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Hi everyone,

Just wanna seek your comments/advices if I got the calculations correctly. I am trying to solve this problem related to my examinations .

A commercial establishment has a service of 400A 480V three-phase solely dedicated to EV chargers and is proposing to install a 225 kVA step-down transformer (primary: 480V, secondary: 208/120V delta to wye). How many chargers can be installed if the proposed EV charger is a single-phase 208V equipment with a rated input current of 48A?

Below is my answer:

Since there is no nameplate rating given, we can assume the input power to be:

208 V x 48 A = 9.984 kW

*NEC 220.57 Electric Vehicle Supply Equipment (EVSE) Load

The EVSE load shall be calculated at either 7200 watts (volt-amperes) or the nameplate rating of the equipment, whichever is larger.

Since the EVSE are considered continuous load, by using the 225 kVA transformer (normally rated to sustain a continuous load at specified kVA);

225 / 9.984 = 22.5

Thus, we can connect a maximum of 22 EV chargers

Thank you!
 
The power only calculation approach works fine for balanced sets of EVSEs. But the 22nd EVSE is an unbalanced load on the transformer. So it is possible that even though the power requirement of 22 EVSEs is less than the transformer rating, the unbalanced loading would subject one of the coils of the transformer to a higher current than is allowed.

So your computation shows that 21 EVSEs is definitely OK, and a 22nd EVSE may be OK. Details on checking the 22nd EVSE to follow.

Cheers, Wayne
 
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Since they are single phase, you’ll need to distribute them evenly across each phase. 7 per limits you to 21.

Edit - Wayne posted as I was (slowly) typing. His post is more complete.
 
With 21 EVSEs at 48A in a balanced arrangement, each line conductor will see 7 (number of sets) * sqrt(3) (factor for how the currents will add) * 48A = 582A. If we add one more EVSE, its 48A current will be 30 degrees out of phase from the balanced sets, so the total current will become sqrt((582+cos(30)*48)^2 + (sin(30)*48)^2) = 624A.

While the transformer is rated for 225 kVA / 208V / sqrt(3) = 624.5A.

So the 22nd EVSE just squeaks in. You can see that if the power only computation came up with, say, 22.3 EVSEs, the above computation would show that the 22nd EVSE could overload two line conductors.

(Edit: speculation retracted).

Cheers, Wayne
 
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