Could someone explain how combining amps across a 12v rail works

Hi,

For some time the ATX12V specification limited the maximum amount of AC power on any single output wire to 240 VA (volt amps). A short from a +12 volt supply (yellow) to the DC reference (black) must not draw more than 20 amperes before tripping the overcurrent protection (OCP) mechanism. Furthermore, the wires themselves may not rise more than 30 degrees above case ambient. Manufacturers that use narrower wires may have to limit power delivery below 240 VA.

Strict adherence to the ATX standard meant that delivering more than 240 volt amps on a single rail wasn't possible. To work around this, many manufacturers either ignored the standard or implemented multiple OCP devices each feeding a separate output.

PSUs that have multiple 12 volt rails often use an organization scheme like the one below

Rail 1: ATX12V 24 pin connector + ATX 4 pin connector / EPS 8 pin connector (dual rail)

Rail 2: EPS 8 pin connector (dual rail) + SATA/Molex +12v supplies

Rail 3: 6 pin PCIe power (75 watts) + 8 pin PCIe power (150 watts) = 225 watts

Rail 4: 6 pin PCIe power (75 watts) + 8 pin PCIe power (150 watts) = 225 watts

There is often a combined limit. Each rail may be able to deliver 18-20 amperes as that's the point where the OCP will kick in, but the combined delivery across all four +12v rails may not exceed, say, 40 amperes (480 volt amps). The data sheet for your FSP600-80EPN states that the limit across all four +12V rails is 504 watts, or 42 amperes. Furthermore, the 3.3V and 5V rails together cannot exceed 130 watts, and the entire PSU cannot exceed 600 watts.

Each 12 volt rail is capable of delivering up to 18 amperes because that's what the OCP was set at.

In ATX12V 2.3 the strict 240 VA limit was removed. This allowed manufacturers to deliver more than 20 amperes on a single rail without violating the specification. However, the wires must still remain within temperature spec. Single-rail designs have become more common since ATX12V 2.3 was introduced, but there are still plenty of very good multi-rail designs on the market. The difference between them is that in a multi-rail configuration one must be aware of the rail configuration to avoid drawing too much current across a single set of wires. Most PSUs are designed to prevent this from happening accidentally.

EDIT: wording

watts = volts * amps. in this case 12x48 or 12x(4x18) except the latter doesn't take into account other limits within the psu itself.

just about any complex product on the planet has a limiting factor (bottleneck) somewhere, almost nothing is perfectly balanced from beginning to end. like your cpu and gpu, at some point one will hit its limit well before the other does. it might be perfectly balanced for game A, but game C will stress the gpu more while game B will stress the cpu more.

i've no idea if the upgrade was necessary - they both seem to have similar power ratings, but there could be other things involved, like age or quality of components or isolated failures or just general glitchiness. and if a psu fails or does nasty things, it can fry more than itself, so maybe the thought was "just in case, replace it"

yeah, to an extent. more than anything, the capacitors are the ones that degrade the most (and are fun to watch explode). when some components blow, they just crack or puff smoke. capacitors tend to be more... extravagant. a little more flamboyant.