# What do OUTPUT charts of PC Power Supplies mean?

I'm looking at various power supplies and they all come with different output cables.

They also have different `OUTPUT` charts, for example:

What I'm wondering:

1. What are these output columns are indicating? Does it mean:

• There is 1 cable with `3.3v` output that can provide 17 amps,
• Together with another cable which can supply `5V` at 13 amps,
• Together with another cable at `12V` at 15 amps?

So totally 3 cable outputs with 17, 13 and 15 amp capacity each?

2. How do I know which cable supplies at what volts / amps?

3. What if cable number does not match column number?

4. What do negative ones indicate, such as `-5v` or `-12v`?

• You do understand what a negative voltage means right? It means in reference to ground ( 0 V ) the measured value is below it. – Ramhound Sep 4 '14 at 17:58
• Can you tell if you do indeed know why a PC would need it? – Phil Sep 4 '14 at 18:00
• Why would a PC need what exactly? A power supply with the ability to output a negative voltage on a rail then a postive voltage on another? A `Serial` connection has `+` and `-` voltages also its not unsual. To understand the specific reason would require you to study electrical engineering. If I had to sum it up in a single sentence. Some digital logic circuits will use a negative voltage to compare the input voltage to the postive voltage. There also is the ATX standard which require it and in addition for serial connections like RS-232 for example. – Ramhound Sep 4 '14 at 18:08

They are totals.

All the 3.3V lines combined allow for up to 17A of draw, all of the 5V lines combined allow for up to 13A draw, etc.

This is because each of those voltage columns listed in the chart represent a voltage "rail" in the PSU, and all output cables relating to that voltage are tied to the same rail. So (for example) there is a single 3.3v rail inside the PSU, and all the 3.3V lines coming out are all coming from that same 3.3V rail.

To answer #4, check out this question over on the Electrical Engineering StackExchange site: What is negative voltage?

In reference as to why they exist in a computer PSU, Wikipedia offers that information in their Computer Power Supply Unit entry:

The −12 V rail was used primarily to provide the negative supply voltage to the RS-232 serial ports. A -5 V rail was provided for peripherals on the ISA bus, but was not used by the motherboard.

The ratings reflect the total current that the power supply can supply on each voltage (Some power supplies have multiple 12V "rails" - effectively 2 separate 12V supplies to increase the available power). This is typically available on multiple cables. In PC power supplies the color code is:

• Red: 5V
• Yellow: 12V
• Orange: 3.3V
• Black: Ground (0V)
• Blue: -12V
• Purple: 5VSB (standby power, available even if rest of Power supply is switched off)
• White: -5V (not used on modern power supplies)
• Others are mainly used for signals, like green that is used to switch on the power supply

The current that a connector can safely supply will be determined by the load on all other cables from the same supply, the thickness of the wiring as well as the power supply's rated output for the rail.

The power supply lines are generally referred to as rails and there may be multiple physical wires attached to each rail. The documentation for the power supply should say what each pin on the connectors does. There may also be multiple rails at the same voltage. Many higher wattage power supplies have two, three, or even more 12V rails.

The amperage ratings are the maximum amps that can be pulled from the rail without damaging things or even causing a fire from overheating. One thing to pay attention to is the total wattage (200W here) versus the total calculated from the voltage and amperage ratings of each of the rails (W=AV, thus we have 236.1W just from the +3.3V and +12V rails). This means you can't use the full capacity of all rails at the same time, the power supply can't output that much power.