What is EPR, and why does it need a 240 W cable?
Extended Power Range adds 28, 36 and 48 volt rungs to USB-C, taking the ceiling from 100 W to 240 W.
Three new rungs above 20 V took USB-C from 100 W to 240 W.
Three new rungs
USB Power Delivery 3.1 introduced Extended Power Range: three new voltage rungs above 20 V, at 28, 36 and 48 volts. At 5 amps those give 140, 180 and 240 watts, which is what finally made USB-C viable for gaming laptops and workstation machines.
The cable decides
EPR is strict about the cable. A cable must declare itself EPR-capable in its e-marker chip, and only then will the charger offer the higher voltages. An ordinary 100 W cable with a perfectly good e-marker will still hold the link at 20 V, so a 240 W charger and a 240 W laptop connected by a 100 W cable charge at 100 W and nothing warns you.
What the box does not say
Almost every charger above 140 W on the market advertises EPR, but the cable in the box is not always rated for the full figure. Check the cable's own rating, not the charger's.
240 W needs 48 V at 5 A. If either the charger or the cable stops short, the whole link stops short.
EPR cables carry a USB-IF logo with 240 W on the connector shell or the sleeve near it.