Sizing a transformer

[mike1061]

The first picture is an A/C compressor the customer has. The compressor is 480 and the service is 240. I said you need a step up transformer. The second is the label of a transformer he found. I determined he needed the 30 Kva transformer. A lower kva did not seem to be available. This is from memory. I said it needs to be sized to the “20 amp max fuse” the compressor says. Someone told him it needs to be sized to the LRA that he has circled. We are in Chicago, and both units will be mounted outside.
Can I have some advice please. Thanks
Mike

 

[augie47]

You have to take LRA into account and it wouldn't be bad idea top run the numbers by the transformer manufacturer but, based on field installs I've seen, your 30kva should be sufficient

 

[Eddie702]

I agree the 30kva looks ok. But the unit needs to be protected by a 20A fused disconnect or circuit breaker

 

[winnie]

I agree with 'you have to take LRA into account'.

The transformer doesn't need to be sized to continuously supply LRA; remember that in normal use you see locked rotor current for a moment when the motor is starting. If the motor is drawing LRA for an extended period of time something is broken.

So 'take LRA into account' means that the transformer needs to have low enough impedance to maintain sufficient voltage during the momentary locked rotor at compressor starting.

Once running the load is less than 9kVA. But if you can't start the load or it fails because of low voltage during starting then you won't have a happy customer.

Calling the manufacturer is a good step at this point. They can also tell you what accessories you will need to mount the transformer outside.

-Jonathan

 

[mike1061]

Thank you, guys. I appreciate it.
Mike

 

[tortuga]

For accurate transformer sizing or other load calculations that use the 'full load' with nothing extra added you have to do a little math and total up the loads so your unit would total something like:

Compressor 1​	6.1​
Compressor 2​	6.1​
Blower​	2.9​
Fan​	0.85​
Combustion blower​	0.5​
Total Amps​	16.45​
VA 480V/3PH​	13675.87​
kVA​	13.7​

The total amps on your 240 side would be ~33 Amps.

LRA I am not an expert on, most transformers will have a 1/2 hour rating of around 150-200% for starting motors, and that unit probably as a VFD or soft start, so I'd propose to use a 15kVA if there are no other 480V loads planned.

 

[Joethemechanic]

The only thing I know of that takes LRA into account when sizing conductors and equipment is a fire pump

 

[Eddie702]

For accurate transformer sizing or other load calculations that use the 'full load' with nothing extra added you have to do a little math and total up the loads so your unit would total something like:

Compressor 1​	6.1​
Compressor 2​	6.1​
Blower​	2.9​
Fan​	0.85​
Combustion blower​	0.5​
Total Amps​	16.45​
VA 480V/3PH​	13675.87​
kVA​	13.7​

The total amps on your 240 side would be ~33 Amps.

LRA I am not an expert on, most transformers will have a 1/2 hour rating of around 150-200% for starting motors, and that unit probably as a VFD or soft start, so I'd propose to use a 15kVA if there are no other 480V loads planned.

Miniscule point but the combustion blower would not be on with the compressors. Combustion blower is 208/230 and fan is 480v single phase