The Book title is: Over 50 years of working with power electronics electronics with the past 30+ as Sr. Engineer designing industrial laser systems... 
As far as specifics on the PCIe connectors, their max rating first of all depends on the quality/condition of the pins used and that sets max current per-pin. Anywho, @12vdc it works out to 288w max - with best quality pins for both the socket/plug, eg gold or silver-flashed and not plugged/unplugged more than 10 times. Even then then connector will get warm(ish). Best Practice for long life is keep connectors around 50% of max rating when feasible. ref the section on PCIe at: https://www.moddiy.com/pages/Power-Supply-Connectors-%26-Pinouts.html
Um, that advice for max sustained load per connector definitely applies to your wall sockets and cords as well....
That would put us at 144w/connector = 432w per board using 3 connectors. The 721's with their 4x connectors per-board and less power draw - perfect design choice. The 741 with more power draw and 3 connectors.... Why? To save maybe $0.50 tops at the cost of the connectors aging faster??? Throw in many folks using non-optimum PSU's that may have only 18ga wires (higher voltage drop on top of iffy regulation) and their choice is is -- not the way I would go...
As far as specifics on the PCIe connectors, their max rating first of all depends on the quality/condition of the pins used and that sets max current per-pin. Anywho, @12vdc it works out to 288w max - with best quality pins for both the socket/plug, eg gold or silver-flashed and not plugged/unplugged more than 10 times. Even then then connector will get warm(ish). Best Practice for long life is keep connectors around 50% of max rating when feasible. ref the section on PCIe at: https://www.moddiy.com/pages/Power-Supply-Connectors-%26-Pinouts.html
Um, that advice for max sustained load per connector definitely applies to your wall sockets and cords as well....
That would put us at 144w/connector = 432w per board using 3 connectors. The 721's with their 4x connectors per-board and less power draw - perfect design choice. The 741 with more power draw and 3 connectors.... Why? To save maybe $0.50 tops at the cost of the connectors aging faster??? Throw in many folks using non-optimum PSU's that may have only 18ga wires (higher voltage drop on top of iffy regulation) and their choice is is -- not the way I would go...
KNC Neptune use one PCIE 6pin connector for one cube with consumption aprox. 400W... With good quality AWG16 cables I had minimum problems with that (but not overclocked to 500MHz). But of course with shit/AWG18 cables yes...
