Control transformer sizing

[newee]

Everything I know, I learned in school and from the internet. I am at somewhat of a disadvantage in that my first job (been here less than a year) is at a company with no EEs, electricians, or anyone with electrical experience for that matter.

Trying to size control transformers for some panels we want to build. This is really simple stuff, but I need someone to confirm that it really is as easy as it sounds.

For example, let's say I have a 5HP motor, 460/3/60 supply. Any control product catalog will tell you what to use for a contactor, overload, and overcurrent protection. The catalog will also give the inrush VA of the contactor.

The particular contactor I'm using has an inrush VA of 140 and a steady-state VA of 12. I don't have data on the lights or one DPDT relay that I'm using. Is it really as simple adding 140 to an overestimate inrush VA for the other devices, then adding the 12 to another overestimate? Based on that I can use a transformer mfr. table to get the appropriate transformer.

Does a 100VA CCT sound big enough for a motor control circuit? 1 motor, a couple lights, a couple switches, and a relay.

I am only slightly concerned because I have NO ONE from which I can get assistance on this simple matter. I'd hate to "engineer" a control panel and have an undersized CCT in there.

 

[jim dungar]

Yes, it is almost as simple as adding VA's.

Go to any control power transformer manufacturer and they will have instructions on selection (i.e. Square D Digest #175 page 14-17).

You need to know:
Inrush VA of each device.
Sealed VA of each device
Total amount of inrush at one time (how much turns on at once)
Total amount of sealed VA (much is on at steady state)
Largest of amount of sealed VA + largest amount of inrush (how much turns on when something else is already on)
Amount of acceptable voltage regulation (how much voltage drop is acceptable)