Scotty "O"
Member
I have a customer that needs (25) 30A 240V outlets. I will need to install a transformer fed by a 460V primary. How many amps(460V) would the primary need to be sized for? What about the secondary(240V).
Transformer sizing
- Thread starter Scotty "O"
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
There is some information missing from your description. The fact that an outlet is rated for 30 amps does not mean that the load it will carry is 30 amps. For example, marinas and RV parks have demand factors that allow us to provide less than 100% of the receptacle’s rated current. What type of facility is this? Also, is the power source 3 phase, and do you intend to use a 3 phase transformer?
Scotty "O"
Member
response to Charlie B.
response to Charlie B.
My bad. It is an auto distribution center. The outlets will be used for charging electric cars. I asked about the actual load and the customer said to figure on the full 30amps. But to answer the other question, the power source is 3ph and i would be using a 3ph transformer.
response to Charlie B.
My bad. It is an auto distribution center. The outlets will be used for charging electric cars. I asked about the actual load and the customer said to figure on the full 30amps. But to answer the other question, the power source is 3ph and i would be using a 3ph transformer.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Were you to ignore Charlie B's advice and base everything on just the outlet maximum amp size and assume single phase, you would need 750A at 240V. That's a 200kVA transformer, so you would need a 480V circuit capable of 416A minimum, which needs a 600A circuit breaker. That''s a LOT of single phase power!
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
OK. The first step is to calculate the load, and then you pick the transformer rating, and finally you determine the conductor sizes (based on primary and secondary currents).
? 30 amps times 240 volts times 25 units is a total of 180 KVA.
? I don?t know about 460V to 240V transformers, but from the ones I work with the next higher standard size is 225 KVA.
? Although you could size the conductors on the basis of the calculated load, I tend to prefer to size them on the basis of the transformer?s rating.
? 225 KVA divided by 460 volts, then divided again by the square root of 3, gives you the rated primary current of 282 amps. If you prefer to base all design choices on the load instead, then you get 180 KVA / (460 * 1.732), or 226 amps.
? On the secondary side, 225 KVA / (240*1.732) = 541 amps. Or if you prefer, 180 KVA / (240 * 1.732) = 433 amps.
That is the basic process. Where do you need to go from there?
? 30 amps times 240 volts times 25 units is a total of 180 KVA.
? I don?t know about 460V to 240V transformers, but from the ones I work with the next higher standard size is 225 KVA.
? Although you could size the conductors on the basis of the calculated load, I tend to prefer to size them on the basis of the transformer?s rating.
? 225 KVA divided by 460 volts, then divided again by the square root of 3, gives you the rated primary current of 282 amps. If you prefer to base all design choices on the load instead, then you get 180 KVA / (460 * 1.732), or 226 amps.
? On the secondary side, 225 KVA / (240*1.732) = 541 amps. Or if you prefer, 180 KVA / (240 * 1.732) = 433 amps.
That is the basic process. Where do you need to go from there?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
You were not able to see the OP's statement that this was a 3-phase system before you posted, since the OP is a new member, and the second post had not yet been approved by a moderator.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
It sat on my browser for a while before I could get back to it to hit Submit so I assumed it crossed, but that explains it better, thanks.
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