sducato
New User
- Location
- Northern California
- Occupation
- Foreman/estimator
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?
EV Charger sizing
- Thread starter sducato
- Start date
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Welcome to the forum.
208 delta to 480/277 wye, greater than 30kva.
208 delta to 480/277 wye, greater than 30kva.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Greater than 30 kVA? Are you assuming that 30 kW is an output rating, and the charger's input rating will be slightly higher?
Cheers, Wayne
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
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.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
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
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
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
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.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I would think the power factor for battery charging would be really high.
JoeStillman
Senior Member
- Location
- West Chester, PA
I assumed it was the output rating.
Probably high enough to be negligible, compared to internal losses.