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EV Charger sizing

Merry Christmas
E

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
sducato said:
I have a customer who wants to put a 30KW 480 volt Bus Charger on a 208 volt circuit. How would I size the step-up transformer?
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The beautiful thing about KW is it is the power being used by the charger. The KW is the same for 480V as it is for 208V. What changes is the current. 30KW with 480V L-L is 62.5A and 208V L-L is 144.2A.

The Transformer is rated typically in KVA which can be easily sized based on KW and power factor. If you are not sure of the power factor (PF) then a worst case can be assumed of 0.8, It is rare to have one lower than 0.8.

You then take the KW and divide by the PF -- 30kW/0.8 = 37.5 KVA transformer.

Another reason why I chose 0.8 is because car, bus or truck chargers are considered continuous loads. When sizing wire and the overcurrent protection for the charger you will want to multiply the load by 125% (30kw*1.25 = 37.5 KW). If your load has a good power factor (most loads do), then 37.5KVA is sufficient.

I personally would go with a transformer between 37.5KVA and 50kVA depending on availability and pricing.
 
E

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
wwhitney said:
Isn't a transformer's rating a continuous rating? That would mean there's no need to use a 125% factor when sizing the transformer.

Cheers, Wayne
Click to expand...

Yes. I believe they are. I have always been under the impression that most can be slightly overloaded even continuously and be fine.

I think it has more to do with insulation temperature? Or maybe the core temperature? I haven't looked to far into it.

I was wrong to apply that to the sizing of the transformer.
 
JoeStillman

JoeStillman

Senior Member
Location
West Chester, PA
Only a 30kW charger with zero losses and unity power factor can draw 30 kVA. A real-world charger will draw more. You'll need the next size larger transformer - 45 kVA in most catalogs.
 
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