You are getting mistaken. the PCIe slot on a mobo is 75w the 6Pin PCIe Cable can handle 150w give or take a few (possibly more if someone wants to confirm).
A lot depends on cable gauge say
20ga = 75-100 watts.
18ga = 125-150 watts.
16ga = 175-200 watts.
note not official numbers.
mostly based on 7 years of mining using various cables.