2nd Gen TR will draw ~250W. X399 boards have at least 1x 4-Pin and 1x 8-Pin ATX-12V connector, so at least 225W. The rest is supplied by the ATX 24 pin connection. Some X399 boards use 2x 8-Pin ATX-12V connectors, that works out to 300W, which should be more than enough for Threadripper 2.