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Will they still be able to negotiate the correct charging amount of 60W and not overload the cable?

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    How did you determine that your cable is rated to 60W? – Allan Apr 19 '18 at 10:24
  • As above, it's unlikely the cable is rated to 60W. In general, you can charge 60W and 85W devices from a 85W brick and only 85W bricks can charge a 85W device. – Turkeyphant Apr 19 '18 at 16:29
  • @Allan The item is listed as 'charging up to 60W' apple.com/au/shop/product/HKK12ZM/A/… – Harry Allen Apr 20 '18 at 6:32
  • It would just charge 65W since that's all the cable can handle. The extra 20W just doesn't happen. – IIllIIll Apr 20 '18 at 13:44
  • I don't know of any cable that has a rating for work (a watt is a unit of work) so that looks like an error. Since TB3 supports USB 3.1 which includes the PD specification of upto 20V at 5A or 100W, that cable should support it. – Allan Apr 20 '18 at 16:23
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What happens if I use a 60W rated USB-C cable to charge my MacBook from the 85W power brick?

Nothing. It will work fine.

A watt is a unit of work (power) and is calculated with the formula:

Watts = Volts * Amps 

A cable is rated for how much current (amps) it can handle - volts mean nothing. For example, using the same cable and a 60W load on 22 AWG wire (22 gauge) with two different scenarios

  • 3V at 10A would fry the cable
  • 12V at 5A would be fine

I could put 50,000 volts so long as I didn't exceed the amperage, I'm ok.

This is why a wattage rating on a cable is incorrect.

Given that the Thunderbolt cable conforms to USB 3.1 and the power delivery spec for calls for up to 20V at 5A or 100W, your cable will handle it.

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