For one, any PSU that has an integrated SDC (or a non-integrated one, for that matter) will not work properly. Basically, when you turn off the ignition, the power is cut to the PSU, and the PC would do a hard ****down, which isn't a good idea.
The proper way of wiring a SDC is to have a line running from the battery for constant 12v power and another running from an ignition switched source, as well as a ground line. The state of the ignition line triggers startup/shutdown.
Amperage is not so much an issue as long as your wiring is good.
You haven't given any details as to your setup, other than this tidbit of wiring information. It would help if you provided more details.