Thanks very much for the reply and the link to the cable chart. The cables are phobya ones i bought quite cheaply and take a 16awg 6pin cable and split it into 2x 20awg 6pin cables.
Do you know if the cards draw most of there power through the riser or its dedicated cable socket?
It depends on the card. Some cards pull more from the PCI-E x16 slot than others, theoretically up to 75W which is the spec limit. From running a dual PSU setup, where all the risers are powered by the same PSU as the motherboard and the secondary PSU only powers the VGA power on the top of the cards, my RX 570/580's pull between 40-50 W from the risers and up to 90-140 W from the VGA power inputs each when dual mining. I would imagine that GTX 1060's should pull less from the riser, since they use much less power.