CPU Power Requirements

Who Needs the Power

These charts show the ‘Typical’ and ‘Maximum’ power dissipation at spec for all current Pentium class processors. Most of these numbers were taken directly from the data sheets, however in a few cases the values had to be calculated. Note that each manufacturer calculates maximum power dissipation in a slightly different manner. For Cyrix, the maximum is calculated using the maximum ‘safe’ voltage multiplied by the current draw when the processor is 100% utilized. For other manufacturers, the maximum power is calculated by using the highest recommended voltage (which may or may not be less than the maximum ‘safe’ voltage) time the current draw when the processor is 100% utilized. All manufacturers calculate the ‘Typical’ power by using the recommended processor voltage multiplied by the current draw under a ‘normal’ load (somewhat less than 100% CPU utilization).

For overclockers, this chart can also be used to estimate the power dissipation for various MHz and voltages. This is done by calculating the current draw for Typical and Maximum power draw (at the Standard Voltage or at Max Voltage, depending if a Cyrix processor or not), then dividing that by the rated MHz. Next, multiply the amps per MHz by the overclocked speed and then multiply by the voltage. This will be your estimated Typical and Estimated power dissipation.

For example, a Cyrix M II 300 running at 300MHz would be calculated as follows: Typical current = 15W / 2.9V = 5.17A and Max current = 24W / 3.0V = 8A. Now we calculate the amps per MHz as follows: 5.17A / 225MHz = .023A abd 8A / 225MHz = .035A (approx). Now we calculate the current at 350MHz like this: .023A * 300MHz = 6.9A and .035A * 300MHz = 10.5A. Finally, we calculate the power dissipation as: 6.9A * 2.9V = 20.01W (Typical) and 10.5A * 3.0V = 30.45W (Max). If the voltage were increased above 3.0V to get the chip working stably, that would need to be figured in as well.

Another example is the Intel Celeron 300A at 450MHz. In this case only the Max Power was provided in the data sheets, so this is what we will use. 19.05W / 2.0V = 9.53A. 9.53A / 300MHz = .032A/MHz. 450MHz * .032A = 14.4A. 14.4A * 2.0V = 28.8W. If the voltage is increased to 2.3V for ‘stability’, the power dissipation becomes 33.13W. One thing to note here is that motherboards begin to have problems when the current exceeds 10A, so while overclocking this much may not be a problem for the processor (in the case of the Celeron 300A), the motherboard may not be able to handle the current draw.

Note that these numbers are only estimates, because the current draw typically becomes slightly less as the MHz increases. Using these calculations will, however, allow you to make sure that your motherboard can handle the maximum load you are trying to throw at it.