Transformers for motor loads

[augie47]

Is there a 'rule of thumb' for sizing transformers in relation to motor loads ?
Typical situation 1: Customer has 240 three phase service and is selecting a transformer for a piece of equipment with a 60HP soft-start motor load.
What size transformer would be needed ?

Typical situation 2:
Similar scenario: 40 HP across-the-line start ?

I understand there are a lot of variables relating to torque requirements, etc., but general conditions taking normal starting load into consideration.

 

[jim dungar]

From Acme question #34:
http://www.acmepowerdist.com/pdf/page_6-10.pdf

"Minimum Transformer kVA =Running Load Amperes x 1.73 x Motor Operating Voltage/1000
NOTE: If motor is to be started more than once per hour add
20% additional kVA"

"...When one transformer is used to operate one motor, the
running amperes of the motor should not exceed 65% of the transformer?s full load ampere rating."

"If several motors are being operated from one transformer, avoid having all motors start at the same time. If this is
impractical, then size the transformer so that the total running current does not exceed 65% of the transformer?s
full load ampere rating"

 

[augie47]

Thanks
(I actually went to ACME's site prior to posting here,,,,back to remedial reading class again )

 

[petersonra]

There are plenty of rules of thumb. I like the ones that come from some authoritative source like the manufacturer. I don't have to defend the reasoning then.