Power Distribution in Three PCs (2012)

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POWER DISTRIBUTION

Total power is one factor to consider when choosing a power supply, but equally important is the distribution along the three main rails, +12V, +5V, and +3.3V. In years past the available power was divided almost evenly (in terms of current) between the rails, though most components (e.g. CPUs, GPUs, hard drives) now draw the majority of their power from the +12V line. If half of a power supply's rated output is on rails that are mainly unused, its effective capacity can decrease significantly.

Power Distribution by Percentage: CPU Load
System
i7-965 XE + HD 5870
i5-2500K + HD 6870
X4 630 + HD 5550
Pent. D 820 + GMA 950
Athlon 64 3500+ + 6800GT
+12V
(8-pin CPU)
54%
53%
66%
85%
38%
+12V
(6-pin GPU)
2%
3%
N/A
N/A
N/A
+12V Total
77%
81%
77%
88%
75%
+5V
5%
12%
15%
11%
15%
+3.3V
17%
7%
8%
1%
10%
Based on power (watts) drawn from each line.

Despite the passage of time, the power drawn from each line is more or less similar to the high-end systems from six years past. The +5V and +3.3V lines play only small supporting roles when it comes to power distribution. The +12V rail pulls the bulk of the power, though the proportion coming through the 4-pin/8-pin AUX12V/EPS12V cable has changed. Intel CPUs now pull a greater percentage of power from the main 24-pin ATX connector, while AMD CPUs seem to be doing the opposite.

Power Distribution by Percentage: CPU Load + GPU Load
System
i7-965 XE + 2 x HD 4870
i7-965 XE + HD 5870
i5-2500K + HD 6870
X4 630 + HD 5550
+12V
(8-pin CPU)
22%
29%
25%
52%
+12V
(6-pin GPU)
38%
38%
38%
N/A
+12V Total
89%
88%
91%
81%
+5V
2%
3%
5%
13%
+3.3V
8%
9%
4%
6%
Based on power (watts) drawn from each line.

GPU power consumption is another shift from the time of our previous tests with modern graphics cards often packing more transistors than CPUs. On full CPU + GPU load, 38% of the total power output was delivered through the 6-pin PCI Express cables feeding the graphics cards paired with our Nehalem and Sandy Bridge configurations.

Note that these aren't particularly high-end graphics cards either, as the HD 6870's current street price is only ~US$160 and is considered a modest GPU in gaming circles. Using a low-end video card or integrated graphics allows one to cut their power requirements by more than half.

CONCLUSIONS

It is important to keep in mind that the measurements presented here are continuous loads. Our test equipment does not have the resolution to measure peaks, which may last for 10 ms or less and may be much higher than the continuous load. Most power supplies are rated for a continuous load with allowances for higher peaks, but the internal protection circuits may still be tripped by an exceptionally high peak. It is wise to leave perhaps 30% headroom for peaks when sizing a power supply.

With these caveats, some broad, predictable conclusions can be drawn:

1) The +5V and +3.3V lines remain relatively unimportant for desktop PCs. In our systems the +5V line never drew more than 4A under any circumstances. The +3.3V line was underutilized as well, drawing less than 3A except in the case of the Core i7-965 which belongs to the all but obsolete LGA1366 platform. Even if LGA1366 was still mainstream, the peak draw was only 12A; Most power supplies, even low wattage models, offer 20A or more.

2) The +12V line is more important than ever thanks to the growing power requirements of graphics cards. On CPU load alone, our systems were drawing about 80% of their power from the +12V rail, more or less the same as the high-end equivalents from six years ago. On combined CPU and GPU load, the portion was close to 90% and total power consumption effectively doubled. Even a sub-US$200 card can raise the power requirements of a system dramatically.

3) Excluding high-end graphics cards and/or multiple GPUs, it appears a 400W power supply is more than sufficient for most systems. Unless you're choosing components like the Core i7-965 intentionally for inefficiency, even a fairly powerful gaming system will not require a 500W+ power supply. Our Core i5-2500K and HD 6870 combination at full load used less than 240W DC while the +12V rail topped out at just 18A.

NOTE: None of the above conclusions are meant to suggest that power delivery alone are the only criterion by which a PSU should be chosen. We are only considering adequate power delivery. We have not touched on noise, efficiency, cooling, voltage regulation — in short, all of the other relevant criteria we examine in our PSU reviews.

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