# How do I do the math?

#### mouse24

##### Splendid
So for power supplies, I am getting conflicting information, how do you do the math when you have say 25a on one 12v rail and 12v on another. Do you just add it up then multiply? (25+25x12).

I don't think thats very accurate though seeing as the specs on my EA-500D are 12v1 22a 12v2 22a now normally that would come up to about 528 but they are saying that it only has 444w

Now I know there is a bit of resistance in the wires but... that seems a bit steep...

So I'm just thinking maybe my math is wrong? an obviously you would go by review sites and not manufacturer numbers anywho.

#### Pinhedd

##### Champion
If you have to worry about what kind of current your rails are providing you are most definitely looking at the wrong power supply. Out of all the components in your computer, the power supply is the one that takes the most beating.

Do not ever skimp out and buy a cheap power supply. With that said, lets get started

There's no regulated way of representing a power supply's capabilities. Most manufacturers sum up the current capacity on each rail multiplied by the voltage of that rail. These specifications should be plastered on the side of the power supply. The marketed wattage of the power supply is the sum of the maximum power on each rail. However this does not mean that the power supply is capable of maxing out all the rails at once and as such it may not perform up to its marketed rating.

PSUs have at least 3 rails. 12v, 5v and 3.3v. Good gaming PSUs will have most of their wattage rated on the 12v rails while poor ones will put unnecessary gobs of it on the 5v rail in a vain attempt to boost the numbers.

There are also a variety of other performance implications. How much does the DC voltage swing with respect to AC swing and line noise? I've seen some absolutely horrific power supplies that are very unstable.

How close to the rail voltage are the actual voltages? Is your 12v rail at 12v or is it at 12.5, 11.5? This should not vary

How does the rail voltage change with respect to load changes that are within spec? This is crucial for gaming computers in which graphics cards can go from 0 to fast in a microsecond. If the powersupply cannot handle the transient that results from a change in load then the constant voltage supplied to the rail won't stay constant.

Unlike most PC components, PSUs are very easy to counterfeit and there are a lot of really poor ones out there. They are the Rolex of the PC world. Don't pay attention to the marketing, look at the specifications and professional reviews here on Toms. Don't skimp out and buy a cheap PSU, you will regret it.

#### DJDeCiBeL

##### Splendid
As I understand it, the rating on each rail is just the max that that particular rail can supply, and should NOT be added together with other rails to get get total wattage output. I'll admit though, it's definitely confusing. I've always stuck to single rail PSU's, so I haven't really put a lot of thought into that. As far as I've seen, the only reason for multiple rails is for stability, but a GOOD single rail should be plenty anyway, in my own admittedly slightly biased opinion.

#### mouse24

##### Splendid

(Saying this right now, I don't know technical electrical jargon)

You'll find more power supplies are just single rails split into two, with each being able to draw close to the maximum wattage of the entire 12v rail (or atleast 4/6ths of it before OCP or similar come into play) or at least this is how it was explained to me a while back.

And pinhedd, thanks for the info. Already knew most of it but thanks for putting in the effort and helping those who might not have known

IIRC power supplies implement a load sharing system of some sort when power is split between rails on the same voltage line.

(IE its not split down the middle, so if one is powering some low power device using say 100w, then the other can pull the remaining as long as it doesnt trip OCP or whatever else is in place)

#### Brett928S2

##### Champion
BANNED
Hi

Easy maths...

1 graphics card = 500 watts
2 graphics card = 1000 watts

All the best Brett

#### HEXiT

##### Champion
thats poor bret... very poor...
if you have 2 12v rails you dont add em together. you can connect both to a device but you only ever calculate the amps separately.

#### ko888

##### Titan
The Antec EarthWatts Green Series 500W (EA-500D Green) has a combined +12 Volt continuous current rating of 37 Amps.

+12V1, 12V2 Maximum combined output: 444W

444 Watts ÷ 12 Volts = 37 Amps

#### mouse24

##### Splendid

... no... I take it you don't do much work working on actual systems in the pc repair shops you own...

if you have 2 12v rails you dont add em together. you can connect both to a device but you only ever calculate the amps separately.

Is there a way I can tell which plugs are hooked into which voltage line? I mean without disassembling the unit.

The Antec EarthWatts Green Series 500W (EA-500D Green) has a combined +12 Volt continuous current rating of 37 Amps.

+12V1, 12V2 Maximum combined output: 444W

444 Watts ÷ 12 Volts = 37 Amps

Thanks but lets say for whatever reason the manufacturer doesn't state the combined 12v rating. How am I supposed to get the 444w in the first place?

I know it 99% of the time won't be useful to me to know this as its always stated or atleast on the website but I'd still like to know

G

#### Guest

##### Guest

aside from testing it yourself . . .
i suggest you do not purchase that PSU. if the manufacturer is not forth coming with their specs, its a undesirable PSU.

#### DJDeCiBeL

##### Splendid
Is there a way I can tell which plugs are hooked into which voltage line? I mean without disassembling the unit.

I would think on a 2 rail unit, one rail is for the system and CPU (24 pin and 4/8 pin connectors) and the other rail is for peripherals. With more than 2 rails, I'm not sure though.

#### ko888

##### Titan

A power supply manufacturer that doesn't state the PSU's combined +12V output on its label is trying to hide something (i.e. the combined +12V output is weaker than its competitors and they don't want the uninformed consumer to to be able to make an obvious comparison). Those are the power supplies that should be avoided.

If the power supply has any 80 PLUS certification you should be able to look up its combined +12V output on:

G

#### Guest

##### Guest

i BELIEVE that 12 volt wires are yellow and the yellow wires with the black line on them are meant for the CPU/MoBo 12 volts. (first rail on a multi 12volt rails system)

i read that and then looked in my box to confirm :lol:

#### ko888

##### Titan

Doesn't work for fully modular power supplies that don't have colored stripes on the yellow wires because modular cables usually don't have any color coded wiring.

#### delluser1

##### Splendid
In some cases there's more to it than just reading the label.
Dual rail is typically 1 rail to cpu ( MB not included ) and 1 rail to everything else, this can vary by manufacturer.
Rail ratings can also be deceiving.
OCP is sometime's set higher than the labeled rating in order to provide some overhead.
Best thing to do is look for a comprehensive review of the unit in question.
http://www.peakpower.org/antecea500dpg1.htm

G

#### Guest

##### Guest

you kids with your modular stuff, i like spaghetti that i stuff in an empty 5.5" bay.

j/k

good point.

#### ko888

##### Titan
If there's no multi-rail over-current protection then the PSU only has a single rail despite what the label may show.

Just look at the Seasonic S12-II 520W model as an example. Its label shows +12V1@20A and +12V2@20A but internally it's all wired to a single +12V rail.

#### mouse24

##### Splendid

IIRC isn't that how most of these units work? I mean sure there are a few true multi rail systems but in effect most of them have OCP set to almost the total rating of both rails.

atleast thats how my EA-500D is (its explained on there website somewhere)

#### ko888

##### Titan

Each of your +12V rails latches off at 27-28 Amps during the OCP (over current protection) test on both 12V rails according to the test review link that @delluser1 provided.

That's 5 to 6 Amps above what the label specifies for each +12V rail.