EV Charging Transformer Sizing Calculations

[wwhitney]

The 127A on the lines with 3 chargers comes from 48*sqrt(3) A in phase plus another 48A 30 degrees out of phase. The latter out of phase load can be resolved as 48 * cos30 = 41.57A in phase and 48 * cos30 = 24A at 90 degrees out of phase, for a total of 124.7A in phase and 24A out of phase.

The allowable total line current is 50 kVA / 120V / 3 = 138.9A. If we add in phase current, the out of phase component remains fixed, so the allowable in phase component is sqrt(138.9^2 - 24^2) = 136.8. Given the 124.7A in phase, the headroom is the difference of 12.1A.

So sharpening was not useful in this case. : - )

Cheers, Wayne

 

[ggunn]

Is a 50 kVA 480:208/120V transformer a stock item with your supplier? Standard sizes are 45 and 75 kVA.

-Jon

Hammond's configurator tool shows 15, 30, 45, 50, 75, 112.5, 225, and larger kVA transformers.

 

[ggunn]

The 127A on the lines with 3 chargers comes from 48*sqrt(3) A in phase plus another 48A 30 degrees out of phase. The latter out of phase load can be resolved as 48 * cos30 = 41.57A in phase and 48 * cos30 = 24A at 90 degrees out of phase, for a total of 124.7A in phase and 24A out of phase.

The allowable total line current is 50 kVA / 120V / 3 = 138.9A. If we add in phase current, the out of phase component remains fixed, so the allowable in phase component is sqrt(138.9^2 - 24^2) = 136.8. Given the 124.7A in phase, the headroom is the difference of 12.1A.

So sharpening was not useful in this case. : - )

Cheers, Wayne

I am not familiar with the math you are using; I got the 127A from Ib = sqrt((Ibc^2 + Iab^2 + (Ibc)(Iab)) = 127A. By the same method I got Ic = 127A and Ia = 83.1A. As for a load analysis on the existing 1P-20A breakers, I don't know what they feed. It seems to me that the 75kVA transformer has enough headroom where I don't care about the loading on those breakers, while the 50kVA transformer is close to the edge, hence my conservative approach. Do you concur?

 

[wwhitney]

I am not familiar with the math you are using

Just resolving the current vectors into two perpendicular components (like Cartesian coordinates x and y), one in phase with the voltage, one 90 degrees out of phase with the voltage. And you can use the Pythagorean theorem as usual for the length of the vector sum.

Ib = sqrt((Ibc^2 + Iab^2 + (Ibc)(Iab))

That works with Iab and Ibc are 60 degrees out of phase, but the more general form for the length of the sum of two planar vectors A and B at angle theta to each other is

| A+B | = sqrt ( |A|^2 + |B}^2 + 2 |A| |B| cos theta)

Then when theta = 0, the sqrt becomes |A| + |B| as cos 0 = 1; when theta = 60, we get your formula, as cos 60 = 1/2; and when theta = 90, we get the Pythagorean theorem, as cos 90 = 0.

It seems to me that the 75kVA transformer has enough headroom where I don't care about the loading on those breakers, while the 50kVA transformer is close to the edge, hence my conservative approach. Do you concur?

75 kVA has plenty of headroom, the allowable current is 208A, so you've got over 80A on each line.

The 50 kVA does not work if the loads on the 4 circuits are all 20A. But I imagine it's not likely they are all fully loaded.

If the least loaded circuit is 12A or less, and the other three add up to 56A or less, then you can use a 50 kVA by putting the other three on the line with just 2 EVSEs connected, and the least loaded circuit on either of the other two lines. So if the client prefers 50 kVA and is OK with no future headroom and wants to pay for a load calc, it very likely will work out.

Cheers ,Wayne

 

[winnie]

Hammond's configurator tool shows 15, 30, 45, 50, 75, 112.5, 225, and larger kVA transformers.

Double check that 50 isn't for single phase transformers, and 45 for 3 phase transformers.

Quickly skimming their catalog, I only see 45 kVA 3 phase transformers, however I do see 50kVA single phase units:

https://documents.hammondpowersolutions.com/documents/Literature/Catalog_and_HPS_Info/HPS-Standard-Product-Catalog-(HTP-16).pdf

 

[ggunn]

Just resolving the current vectors into two perpendicular components (like Cartesian coordinates x and y), one in phase with the voltage, one 90 degrees out of phase with the voltage. And you can use the Pythagorean theorem as usual for the length of the vector sum.


That works with Iab and Ibc are 60 degrees out of phase, but the more general form for the length of the sum of two planar vectors A and B at angle theta to each other is

| A+B | = sqrt ( |A|^2 + |B}^2 + 2 |A| |B| cos theta)

Then when theta = 0, the sqrt becomes |A| + |B| as cos 0 = 1; when theta = 60, we get your formula, as cos 60 = 1/2; and when theta = 90, we get the Pythagorean theorem, as cos 90 = 0.


75 kVA has plenty of headroom, the allowable current is 208A, so you've got over 80A on each line.

The 50 kVA does not work if the loads on the 4 circuits are all 20A. But I imagine it's not likely they are all fully loaded.

If the least loaded circuit is 12A or less, and the other three add up to 56A or less, then you can use a 50 kVA by putting the other three on the line with just 2 EVSEs connected, and the least loaded circuit on either of the other two lines. So if the client prefers 50 kVA and is OK with no future headroom and wants to pay for a load calc, it very likely will work out.

Cheers ,Wayne

Thanks for your help; they are going to go with the 75kVA transformer. The added expense and larger footprint are in the noise; it's an adjunct to a fairly large commercial PV system project.

 

[ggunn]

Double check that 50 isn't for single phase transformers, and 45 for 3 phase transformers.

Quickly skimming their catalog, I only see 45 kVA 3 phase transformers, however I do see 50kVA single phase units:

https://documents.hammondpowersolutions.com/documents/Literature/Catalog_and_HPS_Info/HPS-Standard-Product-Catalog-(HTP-16).pdf

hps-toolbox

Hammond Power Solutions Inc. is the largest manufacturer of dry-type transformers in North America supporting clients in oil and gas, mining, steel, waste and water treatment, and renewal energy.

americas.hammondpowersolutions.com

Go to HPS E-Quotes Lite
Distribution - General Purpose transformers under Products line
General Duty and Three Phase under Transformer Rating
Standard kVA pulldown menu

There is a 75kVA listed.

 

[winnie]

hps-toolbox

Hammond Power Solutions Inc. is the largest manufacturer of dry-type transformers in North America supporting clients in oil and gas, mining, steel, waste and water treatment, and renewal energy.

americas.hammondpowersolutions.com

Go to HPS E-Quotes Lite
Distribution - General Purpose transformers under Products line
General Duty and Three Phase under Transformer Rating
Standard kVA pulldown menu

There is a 75kVA listed.

And also a 50kVA listing. Surprising, and thanks!

For what its worth, if you search for 'standard 3 phase transformer sizes' you get 45 and 75 but not 50, but the configurator certainly does show a 50.

Also if you check 'lead time' in the configurator, for the 50 it says 'typically build to order', but for the 45 it says 'typically stock'.

Probably not relevant since it looks like you are leaning toward using the 75 rather than the 50.

-Jon