The only thing I would add to what Nascon listed is to total the burdens of what you are feeding with the PT (meters, relays, etc.) plus consider how long your wiring will be for voltage drop issues. Most PTs for meter, instrument, and protection use are from several hundred to several thousand VA rated, so pick one that gives you adequate capacity for your loads.
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Potential Transformer Size-Up
Re: potential transformer size-up
