Maximum Amount of EV Chargers on a 75kVA Transformer

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mmcfly

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Hello, I have a 75kVA 3 phase transformer with a 480V primary and a 208Y/120 secondary. This transformer is feeding a panel with a 250A MCB. This panel is dedicated for Level 2 32A EV chargers.

I am trying to determine the maximum number of chargers that can be added to this transformer/panel. With a 250A MCB, I am assuming that the EV chargers are a continuous load and the EV charging load should not exceed over 200A (250A MCB x .8 = 200A load, or 200A load x 1.25 = 250A MCB).

The 32A EV chargers use 2-pole 40A breakers. When energized I am assuming there are 32 amps on both phases (AB, BC, or CA). I can balance three chargers to distribute the amps evenly across the three phases to get 64A for three chargers. Multiply this by 3 and I get 9 chargers with 192A on each phase. Do any of these assumptions or math raise any red flags? My one concern is just the eye test, 9 chargers at 208V and 32 amps = 9 * 208 * 32 = 59904 VA or 59.904 kVA on a 75kVA transformer, and I could potentially add charger(s).

https://forums.mikeholt.com/threads/ev-charger-loads-in-208y-120v-panel-schedule.2569877/

I have seen posts in the above mentioning we can alternatively use VA for these panel calculations. They have indicated splitting the load evenly between phases, i.e. 208 * 32 = 6656/2 = 3328 VA on each phase. When I get to 9 chargers balanced evenly, I have 19,968 VA on each phase (3328 VA * 6) and when added together I get 59,904 VA again. They have then mentioned to get our current on each phase we take 59,904 VA divide by 208V and sqrt(3) to get ~166A. If this is true, could I add an additional 32A charger to get me to 10 chargers on this transformer? Don’t think there is a possibility to get to 11. I am still learning the NEC and 2-pole panel calculations so I just want to make sure I add the correct amount of infrastructure.
 
mmcfly said:
Hello, I have a 75kVA 3 phase transformer with a 480V primary and a 208Y/120 secondary. This transformer is feeding a panel with a 250A MCB. This panel is dedicated for Level 2 32A EV chargers.

I am trying to determine the maximum number of chargers that can be added to this transformer/panel. With a 250A MCB, I am assuming that the EV chargers are a continuous load and the EV charging load should not exceed over 200A (250A MCB x .8 = 200A load, or 200A load x 1.25 = 250A MCB).

The 32A EV chargers use 2-pole 40A breakers. When energized I am assuming there are 32 amps on both phases (AB, BC, or CA). I can balance three chargers to distribute the amps evenly across the three phases to get 64A for three chargers. Multiply this by 3 and I get 9 chargers with 192A on each phase. Do any of these assumptions or math raise any red flags? My one concern is just the eye test, 9 chargers at 208V and 32 amps = 9 * 208 * 32 = 59904 VA or 59.904 kVA on a 75kVA transformer, and I could potentially add charger(s).

https://forums.mikeholt.com/threads/ev-charger-loads-in-208y-120v-panel-schedule.2569877/

I have seen posts in the above mentioning we can alternatively use VA for these panel calculations. They have indicated splitting the load evenly between phases, i.e. 208 * 32 = 6656/2 = 3328 VA on each phase. When I get to 9 chargers balanced evenly, I have 19,968 VA on each phase (3328 VA * 6) and when added together I get 59,904 VA again. They have then mentioned to get our current on each phase we take 59,904 VA divide by 208V and sqrt(3) to get ~166A. If this is true, could I add an additional 32A charger to get me to 10 chargers on this transformer? Don’t think there is a possibility to get to 11. I am still learning the NEC and 2-pole panel calculations so I just want to make sure I add the correct amount of infrastructure.
Click to expand...
Does this transformer have primary protection ….if it does what size ocd does it have on primary side?
 
Try your calculations only with charger quantities of multiples of three.

Single-phase loads don't share power among phases evenly as you add one at a time.

In other words, if you can't support 12 chargers, you're limited to 9.
 
mmcfly said:
The 32A EV chargers use 2-pole 40A breakers. When energized I am assuming there are 32 amps on both phases (AB, BC, or CA). I can balance three chargers to distribute the amps evenly across the three phases to get 64A for three chargers.
Click to expand...
That's not correct. The currents on A from AB and from AC are 60 degrees out of phase. So their sum turns out to be 32A * sqrt(3), not 32A * 2.

Cheers, Wayne
 
mmcfly said:
Hello, I have a 75kVA 3 phase transformer with a 480V primary and a 208Y/120 secondary. This transformer is feeding a panel with a 250A MCB. This panel is dedicated for Level 2 32A EV chargers.

I am trying to determine the maximum number of chargers that can be added to this transformer/panel. With a 250A MCB, I am assuming that the EV chargers are a continuous load and the EV charging load should not exceed over 200A (250A MCB x .8 = 200A load, or 200A load x 1.25 = 250A MCB).

The 32A EV chargers use 2-pole 40A breakers. When energized I am assuming there are 32 amps on both phases (AB, BC, or CA). I can balance three chargers to distribute the amps evenly across the three phases to get 64A for three chargers. Multiply this by 3 and I get 9 chargers with 192A on each phase. Do any of these assumptions or math raise any red flags? My one concern is just the eye test, 9 chargers at 208V and 32 amps = 9 * 208 * 32 = 59904 VA or 59.904 kVA on a 75kVA transformer, and I could potentially add charger(s).

https://forums.mikeholt.com/threads/ev-charger-loads-in-208y-120v-panel-schedule.2569877/

I have seen posts in the above mentioning we can alternatively use VA for these panel calculations. They have indicated splitting the load evenly between phases, i.e. 208 * 32 = 6656/2 = 3328 VA on each phase. When I get to 9 chargers balanced evenly, I have 19,968 VA on each phase (3328 VA * 6) and when added together I get 59,904 VA again. They have then mentioned to get our current on each phase we take 59,904 VA divide by 208V and sqrt(3) to get ~166A. If this is true, could I add an additional 32A charger to get me to 10 chargers on this transformer? Don’t think there is a possibility to get to 11. I am still learning the NEC and 2-pole panel calculations so I just want to make sure I add the correct amount of infrastructure.
Click to expand...
IMO the 250 MCB should be a 225 MCB.
 
Also, as the 75 kVA rating of the transformer is, to my understanding, a continuous rating, if you want to put 75 kVA continuous of EVSEs on the transformer (if it happened to divide out nicely), then per 625.41, you'd need to size the OCPD based on 75 kVA * 125% = 93.75 kVA.

In practice, you're limited to 10 EVSEs in the OP since the math doesn't divide out nicely. [You could do 9 @ 32A plus 3 @ 24A and use the transformer capacity more completely.] So the current on the legs with 7 EVSEs would be the sum of 32 * 3 * sqrt(3) = 166.3A plus another 32A at 30 degrees from that, so sqrt((166.3+32cos30)2+(32sin30)2) = 195A.

So a 250A OCPD suffices in this case, assuming it won't trip on transformer inrush. [If you did 9 @ 32A plus 3 @ 24A, then the minimum OCPD size per 625.41 would be 260A, so you'd need to go up to a 300A OCPD.]

Cheers, Wayne
 
wwhitney said:
So a 250A OCPD suffices in this case, assuming it won't trip on transformer inrush. [If you did 9 @ 32A plus 3 @ 24A, then the minimum OCPD size per 625.41 would be 260A, so you'd need to go up to a 300A OCPD.]
Click to expand...
Breakers on the secondary do not see transformer inrush, which occurs only on the primary side.
 
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