you have an zotac and zotac take more power from the wall so at 70% you probably have 250w (like founders edition which have 250w at 100% tdp) from the wall or something close, if you let card at 100% tdp probably you will have close to 300w from the wall so check your power consumption, personally i have an gtx1070 amp extreme! and at 100% tdp it has 220w from the wall, with 80% tdp i have something between 170-180w, 1070 is rating 160W normally (founders edition and specs), on 80% power i get 2100-2113 rock stable at gpu speed on mining, gaming etc, at 100+ tdp i don't get any more speed, this is frequency limit, but under 80% tdp i got lower frequency.