When calculating power draw for a PSU, which components pull from which rails?

[King_V]

This, obviously, comes up most often with CPU and GPU upgrades, and deals with the 12V rail(s).

However, I know that I personally tend to add up all the components and try to come up with a number, but that this may not be accurate.

So, what about the other parts? When it says this component pulls some number of watts, what rail are those components actually pulling from?

• 
  ■CPU - 12V
  ■GPU - 12V
  ■RAM - ?
  ■Motherboard - ? (I assume 12V, but is this correct?)
  ■SATA HDD, SDD, and Optical Drives - ?
  ■M.2 SDD - ?
  ■Fans - ?

Are there other items I'm missing?

 

[TJ Hooker]

SATA drives - 5V and 12V https://en.wikipedia.org/wiki/Serial_ATA#Power_connectors
Fans - 12V
M.2 - assuming they are powered the same as a x4 PCIe device, it'd be 3.3V and 12V
Motherboards - probably use all the rails to one extent or another
RAM - I would guess 12V, but not sure

Honestly, all these components tend to draw a relatively small amount of power, like a few watts each (e.g. the Z270 mobo chipset is rated for 6W TDP). Unless you have a ton of fans and drives, it's typical to just allot 50W to all that stuff. I wouldn't really worry about it when trying to see how many amps you need on a given rail. 12V rail amps are the only ones you typically need to worry about, and you should always give yourself enough headroom that the small amount of current these other devices draw shouldn't make a difference.