Current Requirements for RAMPS 1.4

[Jeff Zacher]

On the RepRap wiki it says the RAMPS 1.4 has a 5Amp block for the extruder and motors and an 11 Amp block for the heatbed. RAMP Layout I have an ATX PSU that can deliver 16 amps. Is this enough to drive my heat bed. I'm using an MK3 bed and it is reported to use 6 AMPS. I tested the resistance on mine and calculated about 7 AMPs.  

Is there a reason that in this implementation a 20 AMP PSU is recommended (Is it just that 20 is the round number above 16)? 

Can I use the supplied PSU on the 5 AMP block and the ATX on the 11 AMP heatbed block safely (are they isolated circuits? Or am I risking smoke?). 

I'm trying to layout my fuses and connection blocks for the PSU side so I can hook this  PSU up to not only my printer, but also to use the 5V to power my Pi so I need to understand if I have enough current to work with and how I need to fuse everything. Thanks in advance.

[Bob Wood]

If you have a 20 amp it would be ok to use for both connections. The extra current rating is to make sure you do not run the power supply too hard. for the heated bed, extruder and stepper motors I would use the 20 amp ( 12 x 20= 240 watt).

The 5 amp side will not draw more current than it needs.