1) It seems to be highly unlikely that a modern system will ever overload either the +5V or +3.3V lines of a ATX12V 2.x compliant power supply. Neither of these lines ever drew more than 5A under any circumstance, and many power supplies rate them above 20A. The power draw on these lines tended to be quite stable and did not fluctuate much with load.
2) The +12V lines, on the other hand, are very heavily used, especially under load. For Intel-based systems with no external VGA card, this power comes almost exclusively from the +12V2 line. Adding a high powered VGA card may add some load to the +12V1 line, although not all cards use +12V. The systems with AMD CPUs tended to draw power more evenly across the two +12V lines, mainly because they do not consume as much power as Intel CPUs.
3) Looking at the total power draw alone, it would appear that all of our systems could easily be handled by a 300W power supply. Given that as much as 90% of that power comes from the +12V lines, it is likely that that the ratings for the +12V lines matters more than the total wattage. If these lines are inadequate, the power supply may not provide enough power even if its "wattage rating" exceeds the total power draw of the system. It would be wiser to qualify our statement thus: When it comes to adequate power delivery, all of our test systems could easily be handled by a 300W power supply that conforms to ATX12V v2.xx. Conversely, an older PSU rated honestly for 300W output may not be adequate for the most powerful system examined here because of the much lower 12V current capacity on models that comply with v1.3 and earlier versions of the ATX12V spec.
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