well given that my 350mm heatpad will drawer 27 amps.
As you'v got a 400mm x 400 plate. I'd say yes, not enough power.

Also nema 17 moptors draw around 1.7 amps. So 8x1.7 is only 13.6
(reads post again) is the heat pad attached directly to the mains or via the main psu ?

Have a look at led lighting power supplies.
I got a 600watt 50 amp one for £15 :-)

Same units but much cheaper if they think you're fitting led lighting rather than building a 3d printer