ATX PSU Electronics question

[Rastalovich]

A Q. for those in the know:

If I were to take a ~275W ATX psu for a single purpose of powering HardDrives and a gang of fans, how feasible and/or safe is it to power a group of drives off the usual Molex supplied connectors - 1 for 1 - but in addition, use the mobo 20pin connector`s 12/gnd/5v supplies to power more drives and the fans ?

Additionally, and possibly more importantly, if I`m trying to stay as legit as possible (outside of the fact it`s an obvious mod), can more than 2 drives per 12/gnd/5v ~advisably~ be used ? Trying to assume that the mobo connector is geared towards powering a mobo (obviously) which supposedly draws more than the usual molex connectors 2 x ~25w. -- or would it figure that the 12v rails of the mobo connector are considered the same as the molex - so stick to 2 drives ?

(I`ve got the pin outs etc)

 

[Rastalovich]

Going by the stickers that emblazen the sides of the PSUs, it appears that some are designed differently than others.

I`ve seen some complicated looked schematics that show a rail for each connection and their expected current supplies, and I see a 400w psu (that I took out of a cheap case) that shows all 12v @ 20A - implying that the whole of the 12v supply is bound to 20A, not individual 12v rails. (it`s an el-cheapo COLORSit badge).

What that says to me, off the top of my head, is 240w supplied on the 12v - and it`d make no difference which way the drives would be connected - other than all the current going through long thin wires, instead of lots of spread over lots of shorter ones.

5v gives 30A, which implies 150w - giving a total 390w over possible molex connections.

With this under consideration, a psu designed like this would massively effect power given to a mobo if u normally setup a pc with it - and used a good grouping of drives etc.

There must be suffixes or descriptions given to psus that don`t supply power in this fashion.

EDIT: Hmmm it`s all pretty much explained here http://www.playtool.com/pages/psumultirail/multirails.html Seems I just gotta check what wattage requirements the drives and fans require.