This is a question I answered on the now defunct makers stackexchange QandA site:

On the below RAMPS 1.4 wiring layout, I see 11A and 5A, what does it mean?

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You need an ATX power supply, these are what power most desktop computers. ATX power supplies are cheap, readily available online and at local computer repair stores.

The markings on the diagram 11A and 5A, refer to the number of amps or current each set of components require. The 11A terminal is used to power a heated build platform, and the 5A terminal is used to power the RAMPS board, stepper motors and extruder. If you don’t have a heated build platform, you won’t need to wire anything into the 11A terminal. You can just power your printer from the 5A terminal.

Both the 11A and 5A terminals run at 12 volts, you could run the heated build platform (11A terminal) with say 5 volts but you will find it will take much, much longer to heat up. So it is best to run both with 12 volts. This means you need a power supply capable of outputting 12 volts at 16 amps - not every ATX power supply does this! So you need to be careful when purchasing your power supply. It is OK to have more than 16 amps, but not less. If you only had say 8 amps of 12 volt power, your power supply will become overloaded and a fuse in the power supply will pop.

The cheapest power supply I found that matched this spec was the Gigabyte Superb 360, purchased locally for around $20. When shopping for power supplies look at the specification pages and look for one that has one 12 volt output of 16 amps or greater, or two 12 volt outputs. The Gigabyte Superb has +12v1 that runs at 8A (this gets wired into the 5A terminal on your ramps) and a +12v2 that runs at 14A (this gets wired into the 11A terminal on your ramps). The sticker on the side of the power supply will tell you what coloured wires are what.