I'm looking at (flex)ATX power supplies, and notice that some have multiple 12 volt rails. For example this 400w one (https://www.fspgroupusa.com/ecommerce/fsp400-60fggba.html) has a maximum capacity of 400 watts, but two rails of max 216 watts (12v x 18a) each.
Can I simply use up to 400 watts and will the rails autobalance? Ignoring the 3v and 5v for a moment. I've seen that the 24pin connector has 2x +12v pins at position 11 and 12, the EPS has 4x +12v pins, and a PCIE 6/8pin has 3x +12v pins. Are those hooked up to both rails and can I just draw 400 watts and have the PSU split it up evenly across both rails?
Can I find out what cables are connected to each rail? Is it standardized for example to use pin 11 for rail 1 and pin 12 for rail 2? Or is that vandor specific? (Also, curiosity - I've seen PSUs with 3 rails, how do those factor in since the 24pin only has 2 cables)? The linked PSU has a diagram showing the available cables but not what rails they are connected to - perhaps this is more detailed info I need to ask, or perhaps it's standardized and therefore omitted.
Can I find out what computer parts are connected to what pin? Does ATX has a standard way of distributing power? For example using pin 11 for the CPU and pin 12 for the chipset? Or is that dependent on each individual motherboard or CPU: e.g. MSI motherboard using pin 11 for CPU but gigabyte using pin 12?
Can I simply budget my watts like this? Rail 1 has motherboard+CPU, rail 2 has PCIE 6/8pin cable. CPU (ryzen 3600) takes up 80 watts, estimate the motherboard (chipset,wifi,usb,m.2, ram) uses about 50 watts. Combines to 130w on the first rail, with 86 remaining in the rail budget. The GPU can take its 66w through the PCIE slot, with 20w margin left. 66w + 216w = absolute maximum 284 watts for the GPU. In theory?
Context: building a small formfactor mini-itx PC and wanting to know what kind of a CPU/GPU I can put in. I wouldn't actually put in 284w; about 225w (80% of capacity) seems safer. But if the maximum I can use for the GPU is 1 rail, the safe limit seems something like 170 watts. And that's the difference between a 2060 nonsuper and 2070 super, or 5600xt or 5700xt.
