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This is in danger of becoming a computer question, but I've always wondered why do computer power supplies have so many 12V "rails". I have a 800W dead supply with four 12V 16A "rails", but internally, they are all connected to the same transformer, use the same diodes and the same capacitors - in fact, they are all electrically connected to each other. So why call them separate rails (given they are in fact the same rail), is it just marketing or is there some proper reason?

Thomas O
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  • I imagine it has far more to due with separate cables / board connections. You need a seriously fat cable to pass 100A at 12V. – Mark Nov 14 '10 at 00:35
  • @Mark: Sure you do, but each rail splits into about 5 or 6 wires to go to Molex, SATA, PCI-e etc., it's not like each wire for each rail is carrying 16A. – Thomas O Nov 14 '10 at 00:47
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    you'd be surprised, Nvidia GX480 graphics card requires a minimum of 38A @ 12V. – Mark Nov 14 '10 at 00:52
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    @Mark: well, doesn't PCI-e's power expansion have 4x 12V conductors, allowing for ~64A according to my calculations, if not more? – Thomas O Nov 14 '10 at 00:56

2 Answers2

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Allow me to quote Wikipedia: Power supply unit (computer):

In computer power supplies that have more than one +12V power rail...

  • Multiple 12V power supply rails are separately current limited as a safety feature; they are not generated separately.
  • ... the IEC 60950 standard, which requires that no more than 240 volt-amps be present between any two accessible points. Thus, each {12 V} wire must be current-limited to no more than 20 A; ... Unlike a fuse or circuit breaker, these limits reset as soon as the overload is removed.
  • Because of the above standards, almost all high-power supplies claim to implement separate rails, however this claim is often false; many omit the necessary current-limit circuitry,{5} both for cost reasons and because it is an irritation to customers.{1} (The lack is sometimes advertised as a feature under names like "rail fusion" or "current sharing".)

...

So it's apparently for exactly the same reason that electricians claim that different outlets in your house are on "different circuits", even when they are all electrically connected to each other. Each circuit is allegedly current-limited with a fuse or circuit breaker. And yet it is an irritation to customers when a circuit breaker blows because I have too much stuff plugged into it, when that same stuff works just fine if they are plugged into outlets in different rooms, or if someone replaces that fuse with a penny :-/.

Ricardo
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davidcary
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  • There should (must by code?) be separate wires from each CB/fuse to whatever gang of outlets/switches/lights. True, the average house can draw 100-200 A without much problems, but you can't jam more than 15 A through 14 AWG, or 20 A through 12 AWG, safely in confined spaces. – Nick T Nov 15 '10 at 18:31
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    Let me be clear: Nick is right, you can't safely replace a fuse with a penny. – davidcary Nov 18 '10 at 01:36
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rails refers to the number of cables coming in carrying power. you put them in parallel to reduce effective resistance, this reduces power consumption of the cable, stopping potential thermal runaway. This also reduces the voltage sags on the wire.

Kortuk
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