Future proof power supply

[tigersty1e]

So 8 pin graphics card connectors are coming up. These don't seem to be necessary, right?


The most important factor seems to be the number of amps on the 12v.

How many amps on the 12v should I be aiming for for the future proof PSU. Maybe 1-2 years future proofed.

 

[mpilchfamily]

Its really hard to guage that. We hope that the power requierments don't go too much higher then they already are. As a guess i'de say shoot for a PSU with over 40A on the combined +12v rails. Perferably a PSU that has a single +12v rail not one with multiple +12v rails. As you said the amperage is a little more important then having the 8 pin connector but since your getting a new PSU might as well get one with the connector(s).

 

[tigersty1e]

I never really understood the multiple 12v rails.

What is the advantage to having 1 12v rail that ouputs 60amps compared to 3 12v rails that output a total of 60 amps?

 

[mpilchfamily]

Its all in the distrabution of power. Lets look at an example shall we. For this example i'd like to use this PSU. Its got 4 rails but more imprtantly it shows on the picture of the Label what each rail powers.

http://www.newegg.com/Product/...x?Item=N82E16817233002

Now you can clearly see the breakdown as such:

Rail 1= CPU
Rail 2= PCI-e
Rail 3= Motherboard
Rail 4= ALl other connectors and the 2nd PCI-e

Now rail 2 also mentions CPU2. Not sure if the 8 pin EPS connector is devided between these 2 rails or not. For the sake of aurgument we will say it uses only the 1st rail.

Now each of the 4 rails are rated at 18A with a total combined amperage of 41A. Lets look at how the power is divided up and see what we are left with. First we need a theoretical system that this would power. So lets go with some modest power requierments.

E6600 6A to 10A draw
High end motherboard. about 2A
2 raptors in Raid 0 1.5A per drive
dual 8800 GTSs in SLI 9A to 11A per card
Single DVD-RW drive 1.5A
About 4 x 120mm LED fans. about 2A total

That would be a normal system right? All the power values are aproximate. They are not completly acurate and may be over stated but it gives you a general idea.

So lets look at how it all loads up the rails and see what we have left. Now remeber that the 18A rating is the max that rail can handle but not what that rail will actually supply. The PSU is only able to provide a max of 41A and that gets divided among the 4 rails.

So here is the max power draw on each rail if everything was running 100% basied on the numbers above.

Rail 1= 10A
Rail 2= 11A
Rail 3= 2A
Rail 4= 17.5A

So we have a total of about 39.5A under 100% load. Now this is a quality PSU. it balences out the power pretty well across the rails. There are others that have the same ratings as this one but offer more amperage on the combined +12v rails. As you can see in that situation you would easily be able to overpower a singel rail thinking you had plenty of amperage to spair. adding water cooling to the system would over power that 4th rail causing problems. If your talking about a single railed PSU then there is no balencing going on. You don't have to worry about over stressing a rail all the components have acess to the full power of the +12v rail. Most PSUs now have things pretty well balenced out so you don't have to worry about over stressing a single rial. But you still need to be careful. I find that many people here love to recomend the Earthwatts units to people with decent systems. Problme there is you quickly over power the 2nd rail on that unit. the 500W unit has a rating of 17A on each rail with a max output of 34A on the combined +12v rails. The motherboard and CPU are on one rail with everything else on the other. So if you have the same system above with only a single video card then you are over stressing the rail. At 27.5A you are still within the CPU's combined +12v amperage but you are tripping the PSU's over current protction on one of the rails.

Have i confued you any further?

 

[tigersty1e]

http://www.newegg.com/Product/...x?Item=N82E16817154015

This is my PSU. It only has 1 12v rail, but there are 4 different rails I can connect my video card to.



I don't understand your example. Let's say I had a PCIE card that needed 32 amps or higher. I've seen people powering up big cards with PSU units like the one you linked. According to your example, I wouldn't be able to power it up.

 

[mpilchfamily]

First of all don't call the cables coming out of the PSU rails. They are just strings of connectors or lines. Your PSU only has 1 x +12V rail like you said with only 20A on it. This is not enough to power most systems. The fact that you have it running the system in your sig is amazing. It won't be long before that PSU dies on you though.

Secondly the 32A recomendation for a video card is total system and not the cards requierments. No card as of yet needs more then 15A.

 

[Modelworks]

The rails thing started because of safety, its in the intel specs.
Most supplies that list 3 rails 12V@20A all go back to a single supply in the power supply of 60A@12V.
They put overload protection on 3 different connections to that 60A and so you have 3 "rails" at 20A each.
There isn't circuitry in there that does 3 independently generated supplies.

A short circuit of 12V @ 20A can easily start a fire. Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.

Manufacturers are starting to consider that the chance for a short is rare so the spec is getting relaxed about how much current can be on a single "wire". Also the size of the wire has to increase to carry higher current loads. Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.

 

[JEDIYoda]

Originally posted by: Modelworks
The rails thing started because of safety, its in the intel specs.
Most supplies that list 3 rails 12V@20A all go back to a single supply in the power supply of 60A@12V.
They put overload protection on 3 different connections to that 60A and so you have 3 "rails" at 20A each.
There isn't circuitry in there that does 3 independently generated supplies.

A short circuit of 12V @ 20A can easily start a fire. Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.

Manufacturers are starting to consider that the chance for a short is rare so the spec is getting relaxed about how much current can be on a single "wire". Also the size of the wire has to increase to carry higher current loads. Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.

all conjecture and fear mongering----Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.
Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.
-- huh....

 

[JEDIYoda]

Originally posted by: tigersty1e
http://www.newegg.com/Product/...x?Item=N82E16817154015

This is my PSU. It only has 1 12v rail, but there are 4 different rails I can connect my video card to.



I don't understand your example. Let's say I had a PCIE card that needed 32 amps or higher. I've seen people powering up big cards with PSU units like the one you linked. According to your example, I wouldn't be able to power it up.

Why cheap out on a no name PSU?

 

[BoomerD]

It amazes me how many people (including myself) will drop several hundred $$$ into their video cards, then cheap out on the PSU...the one component that can make or break a computer...I'm ordering a new one today to replace my "Perspire"...

 

[Modelworks]

Originally posted by: JEDIYoda

all conjecture and fear mongering----Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.
Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.
-- huh....

Not conjecture at all.

From the intel spec:
"The 12 V rail on the 2x2 power connector should be a separate current limited output to
meet the requirements of UL and EN 60950."

The above output was split rather than running one 12v rail to the connector, for safety reasons.

Wire sizes in supplies are very rarely the correct size for the current they carry.
Example:
A 12V@20A supply wire should be 12awg or better at a length of 2 feet.
Instead they use smaller gauges and allow for the loss due to resistance of the smaller wire.

Measure almost any supply at the supply and then again at the end on the connector.
You will find that under load the voltage at the connector is less than the voltage at the supply.

 

[PurdueRy]

Originally posted by: Modelworks

Originally posted by: JEDIYoda

all conjecture and fear mongering----Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.
Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.
-- huh....

Not conjecture at all.

From the intel spec:
"The 12 V rail on the 2x2 power connector should be a separate current limited output to
meet the requirements of UL and EN 60950."

The above output was split rather than running one 12v rail to the connector, for safety reasons.

Wire sizes in supplies are very rarely the correct size for the current they carry.
Example:
A 12V@20A supply wire should be 12awg or better at a length of 2 feet.
Instead they use smaller gauges and allow for the loss due to resistance of the smaller wire.

Measure almost any supply at the supply and then again at the end on the connector.
You will find that under load the voltage at the connector is less than the voltage at the supply.

Your stats are wrong however. You are figuring that 20A travels down 1 wire. This is not the case. For instance a 6 pin PCI express connector has three 12V wires. If you have a PSU with an 8 pin PCI express connector, it has 4 12V wires.

Once you divide the current up you realize that a graphics card pulls around 13A max. This would be the typical peak of a 8800GTX. The 8800GTX requires 2 6 pin PCI express connections. I am not sure if all graphics cards use all 3 12V wires so I will assume they use only two of the three. You then have 13/4 = 3A approximately per wire.

If you choose to split one wire to power both connectors you will still only be at 6A per wire which is still within limits for 18AWG wire which is approximately 16A.

Power supply manufacturers would be in serious trouble if they did not meet the specs for the connectors they had available.

Measure almost any supply at the supply and then again at the end on the connector.
You will find that under load the voltage at the connector is less than the voltage at the supply.

Of COURSE this will be the case. Take 00 AWG wire and you will still have SOME loss over the length. No matter how large the cross sectional area of the wire, you will always have some voltage drop across it. This drop is typically minimal due to the reasons desribed above.

 

[Modelworks]

thats fine for manufacturers that split a single rail between multiple wires, but there are many that do not and do put a single 20A wire out of the supply, often putting several connectors on the same wire.

The problem is that many supplies will connect those 4 wires on the 8 pin pci express connector back to circuits that are protected for 15-20A per wire, not to the same 20A connection.


As for the difference in voltage at the supply and connector of smaller gauge wires, I'm not referring to small losses, but losses in the .5 volt range, which is more than 10% on the 5volt lines, over the limit for the spec.

 

[PurdueRy]

Originally posted by: Modelworks
thats fine for manufacturers that split a single rail between multiple wires, but there are many that do not and do put a single 20A wire out of the supply, often putting several connectors on the same wire.

The problem is that many supplies will connect those 4 wires on the 8 pin pci express connector back to circuits that are protected for 15-20A per wire, not to the same 20A connection.


As for the difference in voltage at the supply and connector of smaller gauge wires, I'm not referring to small losses, but losses in the .5 volt range, which is more than 10% on the 5volt lines, over the limit for the spec.

You're providing a lot of opinions and not many substantiated facts. Show me how you are going to pull 20A on one wire(not one connector...one wire) out of the power supply.

Also, do some resistance calculations/measurements and show me how you would get a .5V drop.

 

[Modelworks]

There are supplies that have a single set of wires for the 5v and 12v for the drive connections.
That wire may have up to 6 drive connections on it.
Granted you would have to use all 6 drives to probably reach the current limits, but the potential is there.

CM = (25 x I x L) / V

 

[PurdueRy]

Originally posted by: Modelworks
There are supplies that have a single set of wires for the 5v and 12v for the drive connections.
That wire may have up to 6 drive connections on it.
Granted you would have to use all 6 drives to probably reach the current limits, but the potential is there.

CM = (25 x I x L) / V

The most power hungry hard drives will peak at about 2A upon spin up only. Even with six of these on a chain all spinning up at the same time you are only at 12A. Still below the safe limits of 18AWG wire.

Typically, for instance the 7200.10's, take just a little over 1A on seek. This means you chain of 6 drives only needs approx. 7A. Again, way below the safe limits.

 

[thecoolnessrune]

Originally posted by: BoomerD
It amazes me how many people (including myself) will drop several hundred $$$ into their video cards, then cheap out on the PSU...the one component that can make or break a computer...I'm ordering a new one today to replace my "Perspire"...

I prefer "Assfire"

 

[BoomerD]

Originally posted by: thecoolnessrune

Originally posted by: BoomerD
It amazes me how many people (including myself) will drop several hundred $$$ into their video cards, then cheap out on the PSU...the one component that can make or break a computer...I'm ordering a new one today to replace my "Perspire"...

I prefer "Assfire"

Yeah, there are lots of names to describe them...but none really do them justice

 

[JEDIYoda]

Originally posted by: PurdueRy

Originally posted by: Modelworks
thats fine for manufacturers that split a single rail between multiple wires, but there are many that do not and do put a single 20A wire out of the supply, often putting several connectors on the same wire.

The problem is that many supplies will connect those 4 wires on the 8 pin pci express connector back to circuits that are protected for 15-20A per wire, not to the same 20A connection.


As for the difference in voltage at the supply and connector of smaller gauge wires, I'm not referring to small losses, but losses in the .5 volt range, which is more than 10% on the 5volt lines, over the limit for the spec.

You're providing a lot of opinions and not many substantiated facts. Show me how you are going to pull 20A on one wire(not one connector...one wire) out of the power supply.

Also, do some resistance calculations/measurements and show me how you would get a .5V drop.

I stand by my statement--
all conjecture and fear mongering----Now imagine that same at 60A on a single output shorted.
It could easily melt the wiring and anything connected to it.
Wire sizes in supplies are often not the correct gauge to carry the amount of current they spec.
-- huh....

 

[Modelworks]

huh ?