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How much amperage do i have on a dual 12 v rail . Advice required .

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March 1, 2013 6:42:17 AM

My psu has 2 12v rail . One has 18A (max) and another 16A (max) and unite power is less than or equal to 270 w as mentioned on the psu . I was going to buy asus radeon 7750 1 gb gddr5 edition without the external power supply . So will i be able to run it . What is the total current that i have .
March 1, 2013 7:22:20 AM

Actually what u think , is not that straight . A hd 7750 consumes power from a pcie x16 slot. It doesn't require any external power. So, if ur psu's wattage is around 400w , by assuming that it has an efficiency of 70% , it shall deliver u 280w of full power in full load , but keep in my mind that temperature affects efficiency of psu . So any medium quality 400w psu should able to handle your total cpu along with ur graphics card. So amperage in psu railing affects little when it comes to gf cards which consumes it power from motherboard pcie slot.
March 1, 2013 8:19:20 AM

Thanx for replying . But how to calculate it in case i bought other card .
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March 1, 2013 11:24:01 AM

Well the easiest way to do it use a psu calculator . The one is provided in cooler master site . Use there psu calculator ( provide them the system specs of ur pc ) they will give the amount of power (in wattage ) ur pc will consume . When u get the resultant power multiply it with 1.4 , that should be the rating of psu you must buy . Also don't forget the amperage rating , a good 35-45 amps in ur 12v rail should do the job. The higher it is , the better the psu is.
March 1, 2013 11:45:22 AM

Forgot to add , multiply 1.4 in case of 70% efficiency in all cases (20,50,100) or multiply 1.25 in case of 80-85% efficiency in all cases.
a b U Graphics card
March 1, 2013 12:03:15 PM

Diabowx said:
So, if ur psu's wattage is around 400w , by assuming that it has an efficiency of 70% , it shall deliver u 280w of full power in full load , but keep in my mind that temperature affects efficiency of psu .


You have a very flawed understanding of how PSUs work. efficiency has nothing to do with power delivered from the PSU. It has to do with the amount of power you need to draw from the wall. For your example a PC pulling 400W off a 70% efficient PSU means the PSU itself is pulling about 570W from the wall.

Ratings don't affect load either, just the maximum rated load.
a b U Graphics card
March 1, 2013 12:04:06 PM

I don't see a psu calculator on cooler master's site, but there's one here:
http://www.extreme.outervision.com/psucalculatorlite.js...

The wattage rating of a psu is supposed to be output wattage, not input wattage (divide by efficiency rating to get how much wattage it draws from the wall)

If that 270W is the total wattage available on the +12V rails, that would be 22.5 amps total, but with multiple +12V rails, it can be hard to predict which components draw from which rails. If that 270W is the overall wattage for the whole psu, the total amps for the +12V rails would be less. But two +12V rails is very strange either way - among good quality psus, normally only models with over 600W use multiple +12V rails, and not always even then.

OP, can you provide exact make and model of your psu?
a b U Graphics card
March 1, 2013 12:09:37 PM

Lahsiv said:
My psu has 2 12v rail . One has 18A (max) and another 16A (max) and unite power is less than or equal to 270 w as mentioned on the psu . I was going to buy asus radeon 7750 1 gb gddr5 edition without the external power supply . So will i be able to run it . What is the total current that i have .


We need to know the actual PSU model. And the COMBINED 12V rating. It should be in a separate area. You can't add the rails because they aren't independent, they are just current limiters on the same power source to protect the wires. For example (16A + 18A) * 12V = 408W. Obviously not correct.
If 270W is the combined wattage for those rails only then COMBINED they can pull 22.5A

Realistically that should run a 7750 without issue, depending in the rest of the system. As long as the PSU maker isn't lying. Need to know full system specs
a b U Graphics card
March 1, 2013 12:25:25 PM

MauveCloud said:
But two +12V rails is very strange either way - among good quality psus, normally only models with over 600W use multiple +12V rails, and not always even then.


This actually happens because of an outdated ATX spec(I believe it was intels idea). Basically it required currant limiters on a chain of connectors so you couldn't burn the wires by pulling too much currant through any one group. The original limit was 18 I think so on a 22A supply you get stuff like this to meet the spec, but not make hooking up stuff a pain. The main PSU limiter takes care of the actual total limit(if it has one).

Never was truely adopted, some have "rails" but no actual limiters, and it was pulled from the spec years ago. Some still used/use it after for marketing (look we have multiple rails! Its a "feature")Modern ones use a single rail and only have so many connectors per wired group with rated max currant pull for devices on those connectors (like pcie)
a b U Graphics card
March 1, 2013 12:35:59 PM

Keep it simple.
12volts x 18 amps = 216watts
12volts x 16 amps = 192watts

The 7750 is only a 55 watt part.
Im going to guess the cpu is a core 2 at 60 watts, or an i5 at 77 watts.

In any one of those scenarious the cpu and gpu will not draw more than 135 watts or 135watts / 12volts = 12 amps.

No matter how you plug things in or load the seperate rails it should be fine. Install the card and go go go.
March 1, 2013 3:06:52 PM

@unksol Thanks for enlighting me . I was little confused about Transformer and psu . In our college it has been taught to us that if u choose to setup a transformer , then u must not give it full load so it is better to set up a new transformer that has more wattage than a required one . Also giving full load to psu may affect it's life after end of warranty period . Also it is better to buy more wattage than needed one in case we upgrade the system . We have been taught that never give full load to a transformer/psu (even if it is able to deliver the it's rated power) , as it may affect it's longevity.
March 1, 2013 4:09:03 PM

My rig is core 2 duo e7500 2.93 ghz . 6 gb ddr3 ram . 500 gb internal harddisk . 20 " monitor . I am running hd 5450 currently and was planning to buy a new gf card . The psu came with the cabinet . It says 450 watt . 170w for the other rails like the 5v 3v etc dont know the exact values and 270 w for the two 12 v rails one 18A and another 16A Both max . I was using my old gf card which requires 20 amp as mention on many websites without any problems so can i run the new one ie hd 7750 . The psu is from pc tech version 450w-p4 . I've tried searching that in internet but found nothing . So can i run 7750 . Any other gf cards do u recommend , priced around the 7750 ?
a c 99 U Graphics card
March 1, 2013 4:23:10 PM

Well, I sure am glad unksol was here to straighten all this out.

The 7750 is one of the more power efficient cards on the market that can at least handle some gaming(mid level in most cases).

Intel specs the card at 65 watts. so your chances are quite good.

Just an observation.....

You power supply is a much older design and it shows because it is rated at 450 watts and has a much smaller portion of its power dedicated to the 12 volt rail. I would keep an eye out for a good power supply in the future.

Most newer power supplies will give almost all the power to the 12 volt rail(the current limiters have already been explained for multi-rail setups) and the 5 and 3.3 will also get power from this rail(dc to dc converters). The means that if you load up the lower rails the 12 volt rail will loose some of its power(it is generally never a problem). Modern systems take most(but not all) power from the 12 volt rail.

For the time being, you should be fine.

I actually have a 300 watt power supply with 22 amps on the 12 volt rail and run a more power hungry cpu(i7 750) and video card(650 ti) without issues. My system is also SFF so all the parts take less power too(itx board/notebook hard drive[replaced the full 3.5 inch one]/notebook dvd/ssd).

As you can see, 22 amps @ 12 volts(264 watts), but also 125watts @ 3.3 and 5 combined. You can not max out the lower rails without loosing power from the 12 volt rail(s). 264 + 125 = over 300(389) :) 


I would call you in the safe range, but it is advisable to look for a new power supply in the future.
March 1, 2013 4:51:19 PM

Thank you all . So i can buy the 7750 . Do u suggest any other card suitable for me . I will change my psu may be in a few months but first the graphic card . And i play games mostly at 1600x900 and heavy games at 1280x720
a c 99 U Graphics card
March 1, 2013 4:58:44 PM

the 7750 performs similar to a 5770/6770 card(in most games. some favor on or the other. CGN shines on some games). I played quite a few games at 1920 x 1200 on that card. But as always newer games need details reduced(shadows/AA/SAOO should be tuned first for most games).
March 2, 2013 4:45:00 AM

I dont mind reducing the details . So 7750 it is . Thanx everyone :-)
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