Budgeting your power needs

First things first: you need to think of your power supply not as a single unit, but a box that contains multiple, independent power supplies. That is, if you look at the label on a power supply, you’ll see that the maximum current is listed independently for +3.3V, +5V, and +12V. Thought of another way, imagine the total wattage being the power of the pump at the water reservoir, and that there is a pump for +3.3V water, +5.5 water, and +12V water. Just be patient with me, it’ll all make sense by the end of this article.

Determining your Power Budget

Some people tell you to add up the number of watts that your components support. For example, an AMD Athlon XP 2000+ (Palomino) uses 87.5W. Your motherboard is 23.5W, and so your power needs are 111W for the moment. Add up all your components and you’re done. The problem is that because the power supply is not a unified source of energy, the 87.5W from the Athlon XP is meaningless unless you know how those 87.5 watts are distributed; are they on the +5V rail or +12V rail?

And therein lies the problem. You see, the Pentium III and all CPUs before it ran on the +5V rail. Since the CPU remains the largest consumer of electricity in an x86 machine, power supplies were engineered to provide significant amounts of current on the +5V rail. Modern CPUs such as the Athlon and Pentium 4 run on the +12V rail. The problem is that many power supplies are still based on older Pentium III-era designs and so even for many mid-range gaming systems, chances are that the +12V component of the power supply is not going to be adequate.

This will all make sense when you actually calculate your power consumption budget. In the following table, you’ll find estimates of power consumption for common devices that we obtained from AMD technical documents; they are a bit on the high-end (but you’re better safe than sorry)

Current Consumption (Estimates)
+3.3V	+5V	+12V	Device
3	2	0.3	Motherboard w/ onboard devices
0	0	0.25	High Performance fans (including power supply fans)
0	2	0	Memory (128MB DDR)
3	0	0	VGA
0.5	0.5	0	PCI Sound
0.4	0.4	0	PCI Network Card
0	0.8	2	High performance hard drive
0	1.2	0.8	CD-RW
0	1.2	1.1	DVD
0	0.8	0	Floppy
0	0.5	0	USB devices
0	0.25	0	Keyboard
0	0.25	0	Mouse
0	0.5	0	PCI Modem
0	1.6	0	FireWire

Athlon and Pentium 4 CPUs run entirely on the +12V rail and you can determine their power consumption using the following formula that takes into account voltage regular inefficiencies:

1.25/12*core voltage* current

The processor’s current can be determined by reading AMD and Intel PDF tech documents, but as a general rule of thumb, the higher the raw megahertz, the higher the current, and the smaller the die process, the lower the current. We’ve listed some common CPUs below:

Processor Consumption
+12V Current	CPU
5.742188	Thunderbird 1GHz
6.289063	Thunderbird 1.1GHz
6.466667	Thoroughbred 2100+
6.55	Thoroughbred 2200+
6.872396	Palomino 1800+
7.115625	Thoroughbred 2400+
7.115625	Thoroughbred 2600+
7.291667	Palomino 2000+
7.492188	Palomino 2100+
8.145	Northwood 2GHz
8.505521	Northwood 2.4GHz
8.53526	Northwood 2.2GHz
9.292188	Northwood 2.6
9.707813	Northwood 3.06GHz
10.1175	Northwood 2.8GHz

It’s worth noting that although Pentium 4’s typically run cooler and have lower voltages than AMD CPUs, their higher raw clockspeed means that they draw more power. We’re not sure why the 3GHz P4 draws less power than the 2.8GHz model, but that’s what Intel’s documents list…