GPUs are GTX Titan (the older 6 GiB cards, not the Titan X), and currently I only use 3 of those with an EVGA SuperNOVA 1300 G2. Full config:

Z10PE-D8 WS mobo, 2 x Xeon E5-2620 v4 (32 threads), 128 GiB DDR4 ECC RAM (8*16 GiB), 256 GiB SM961 NVMe, 12 TiB SATA (4 x 3 TiB HGST server drives), 3 x GTX Titan 6GB, Soldam black knight XR1 casing, 2 x DeepCool Gammax 400 CPU coolers, EVGA SuperNOVA 1300 G2 PSU, 2 x Noctua NF-A9 PWM, 2 x Noctua NF-A14 3000 PWM

I have a similar setup to grandparent (Z10PE-D16 WS), running four last-gen TitanX GPUs with an EVGA SuperNOVA P2 1600W PSU.

I was a little worried because this is the most power-hungry machine I've ever built, but so far I haven't had any issues. But I'm not going out of my way to start an electrical fire, either.

The motherboard in question is pretty deluxe, the only thing I'd complain about is the boot time.