Do I need a special cable for 240 W charging?
AnswerYes
Yes. Anything over 100 W needs a USB-C cable rated 240 W (an "EPR" cable). Any USB-C cable carries 60 W, and 5 A "100 W" cables carry 100 W. 140, 180 and 240 W use 28, 36 and 48 V, and a charger only offers those voltages when the cable's e-marker chip says it is EPR-rated. With a 100 W cable a 240 W laptop still charges, but only at 100 W.
The power arithmetic
| Volts × amps | Watts | Cable needed |
|---|---|---|
| 20 V × 3 A | 60 W | Any USB-C to USB-C cable |
| 20 V × 5 A | 100 W | 5 A e-marked ("100W") cable |
| 28 V × 5 A | 140 W | 240 W EPR cable |
| 36 V × 5 A | 180 W | 240 W EPR cable |
| 48 V × 5 A | 240 W | 240 W EPR cable |
Why the cable matters
Power is volts × amps. USB-C caps current at 5 A, so the only way past 100 W was to raise the voltage. USB Power Delivery 3.1 added fixed 28, 36 and 48 V levels, the Extended Power Range. Cables built before that, including every Thunderbolt 4 cable, are rated for 20 V. An EPR cable's e-marker declares 50 V and 5 A. If the charger can't read that, it never offers more than 20 V. That's a safety design, and it's why the wrong cable means slow charging rather than a fire.
How to tell
Look for "240W" on the cable or its packaging. The USB-IF's own label is a cable logo with "240W". Some EPR cables are USB 2.0 for data, which is fine for a charger but not for a monitor or SSD.