Guide · 6 min read
The USB-C cable guide.
Chargers get the blame; cables cause the problem. A USB-C cable carries a current rating, a data rating and a length penalty, and all three are independent — which is why a cable that transfers video perfectly can still cap your laptop at 60W.
Power and data are separate specifications
Every USB-C to USB-C cable defaults to 3A. At the PD ceiling of 20V that is 60W, and it needs no chip to advertise it. Going above 3A requires an e-marker declaring 5A, which lifts the cable to 100W at 20V. Going above 100W requires an EPR-rated cable declaring 50V/5A, for the 140W, 180W and 240W rails.
Data is a different axis entirely. A cable with only the USB 2.0 pairs wired — no SuperSpeed conductors at all, 480 Mbps maximum — can legally and safely carry 100W if it is e-marked for 5A, and most 240W charging cables are exactly that: thick power conductors, USB 2.0 data. Conversely a thin 40 Gbps Thunderbolt cable is not automatically a 240W cable. Do not infer either rating from the other; the numbers you want are the printed ones.
| Cable type | Max power | Max data | Practical length |
|---|---|---|---|
| USB 2.0 C-to-C, no e-marker | 60W (20V/3A) | 480 Mbps | Up to 4m |
| USB 2.0 C-to-C, 5A e-marked | 100W (20V/5A) | 480 Mbps | Up to 2m typical |
| EPR 240W certified | 240W (48V/5A) | 480 Mbps on most | 1–2m |
| USB 3.2 Gen 2 (10 Gbps) | 60W or 100W if e-marked | 10 Gbps | 1m passive |
| USB4 / Thunderbolt 4 passive | Usually 100W | 40 Gbps | 0.8m passive |
| C-to-Lightning | ~27W to the iPhone | 480 Mbps | 1–2m |
Length, gauge and the voltage-drop trap
Copper has resistance, and a cable's round-trip resistance drops voltage in proportion to current: V = I × R. Take a long, thin 28AWG cable with roughly 0.3Ω round trip. At 5V and 3A that is a 0.9V loss, so the device sees about 4.1V — below the threshold where many devices will keep charging at full rate, which is why a phone on a cheap 3m cable charges slowly and the cable gets warm.
Now run the same 15W at 9V instead of 5V. Current falls to 1.67A, the drop falls to 0.5V, and — because resistive loss is I²R — the wasted power falls by more than two-thirds. Raising voltage to cut current is precisely why USB-C PD climbs to 9V, 20V and 48V rather than pushing more amps. It is also why high-wattage cables are short and thick: 240W at 48V is still 5A, and 5A over 3 metres of thin wire is not a cable anyone certifies.
Why the cable is usually the culprit
Four common cases. A 3A cable on a 100W charger: the laptop gets 60W and charges while asleep but drains while working. A charging-only cable from a power bank or a car kit, with no data pairs and sometimes no e-marker. A worn cable where the conductors near the plug have fatigued — resistance rises long before it fails outright. A USB-A to USB-C cable, which cannot carry a PD contract at all and caps at roughly 12–18W no matter what it is plugged into.
Buy for the ceiling you might need, not today's device: a 100W 5A e-marked cable costs little more than a 3A one and removes the whole category of problem. Above 100W, the cable must say 240W. More symptoms and fixes in fixing slow charging and phone not charging; the high-power end is covered in 240W USB-C explained and on the 100W adapter page.
FAQ
Does a more expensive USB-C cable charge faster?
Only if it lifts a limit you were hitting — 5A instead of 3A, or EPR instead of 100W. Beyond that, price buys durability and data speed, not watts.
Can I use a Thunderbolt cable to charge a laptop?
Yes. Thunderbolt 4 cables are e-marked and typically rated to 100W, which covers most laptops. They are not necessarily rated for 140W or 240W.
Why does my cable get warm while charging?
Some warmth is normal resistive loss. Hot is not: it means current above what the conductors are sized for, usually a thin or damaged cable on a high-current contract.
How long can a USB-C charging cable be?
Up to 4m for a USB 2.0 cable at 3A, but 1–2m is the sensible range for high-wattage charging. Data speed, not power, sets the shorter limits for 10 Gbps and 40 Gbps cables.