Neptunes are 400W per PCI-E at stock, not 300W. And even then the failure isn't guaranteed.
I didn't know that but it would explain the MANY instances I have seen mentioned of Nepture power connectors MELTING. I also find it VERY HARD to believe they got away with that on a STOCK Neptune at all. Air cooling on the connector as mentioned by someone else might help some, though.
No, 300 watts per PCI-E isn't insane - but running a connector outside of spec increases the chance of failure, and it's NOT a linear function. More like exponential, kicks up VERY fast once you exceed the rated capasity of a connection. I'm amazed that ANY Neptune ever worked for more than a minute, if they were stressing the connections at 400 watts by default.
For the record, I'm not fond of the idea of pushing 288 watts through a PCI-E 6-pin connector - that's too close to the edge, leaves NO margin for folks that want to overclock or folks like me that don't have A/C and do have hot summers.
Yes, you can get away with it in a cool environment - part of the rating is an allowance for ambient temperature - but that does NOT make it a good idea.
They didn't insist on anything, it was arbitrarily inherited from the rev 3+ S3 board which had 2 PCI-E connectors per 160W board. And its not a rare thing, look back in this thread, plenty of people (including me) running S5s on single PCI-Es.
You say that like the board design was unchanged between the two.
S5 is a stringed power design, S3 had voltage conversion, which implies AT LEAST a major redesign if not a "from scratch" design of the S5 hash boards specifically including the power circuitry. There was no "inherit" based on looking at the COMPLETE REDESIGN of the power setup for the S5.
On the other hand, the bolded is actually reverse to what you say. If the ambient is warmer / hotter you'll probably get more HW errors than run the risk of pushing the power draw on the connector(s), but have a cooler ambient and you'll get fewer HW errors and more power draw as the chips do not get reset.
Silicon semiconductors draw MORE power as they get hotter - this is the cause of "thermal runaway". Plus the higher temperature makes the wiring run hotter even at the SAME temperature, pushing the connector closer to it's meltdown point. The "HW error reset" only affects the chips for a very short period IF at all.