Here's a better figure of ampacity for stranded cabling in DC setting:
http://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-AmpacityMore importantly however, the connectors used for PCIe (Molex mini-fit JR:
http://www.molex.com/pdm_docs/ps/PS-5556-001.pdf) specify a maximum amperage of 8-9A per 16awg circuit in a 3-powered-circuit connector, depending on the type of pins used. Or, 288-324W maximum per connector.
They are also rated to a maximum of 105*C (including a maximum 30*C temperature rise over ambient for the connector).
Edit: To answer your question, Bitmain "requires" that all 3 PCIe sockets are used per hashing board. Each hashing board on an S7 draws somewhere around 360-370W DC, so with proper 16awg cabling (Scott's is good quality 16awg), if you were to use a splitter for 2x of the PCIe sockets to feel better about it (and be 100% sure not to violate warranty terms), you would be well within the safe range for amperage of the wiring and connectors/pins.