Check what the cable is rated at by checking the cable it's self, your PSU box, or the PSU booklet or website. I'm sure it varies with PSUs. This is why it's highly suggested you never mix modular cables from different PSUs.
The PINOUT on modular cables from different PSUs is often not the same - even the "same Manufacturer" often has different cables on different models, as those different models are often not actually MADE by the same manufacturer.
THAT is why you don't mix-and-match without doing careful research first.
Draw from the PCI-E bus varies widely depending on the specific card.
There have been tests that showed peak draw on some cards EXCEEDING the PCI-E spec by 5-10 watts - yes, that's with all of the PCI-E POWER connectors attached to the card at the time.
The AMD RX 470/480 cards were NOTORIOUS this way, 'till AMD released a new driver version that apparently adjusted the "power split" the cards used to pull less from the bus.
Some of the affected models were 150 watt or so TDP but had an *8 pin* PCI-E power connector.
