Quick answer: USB Power Delivery lets a charger and a device agree a voltage and current over the configuration channel, from 5 V up to 20 V at 5 A, and up to 48 V with extended power range. The cable matters: without an e-marker chip declaring 5 A, the link is limited to 3 A, so a 100 W charger gives 60 W at 20 V. A cable can also limit data rate, and the two ratings are independent.
Power Delivery is a negotiation, not a fixed output. The charger advertises a list, the device picks one, and the cable’s declaration caps the maximum. That is why the same laptop charges at 45 W, 60 W or 90 W from three chargers that all claim 100 W.
How the negotiation works
The configuration channel
One wire in each direction carries the protocol. The source advertises capabilities, the sink requests a profile, and both confirm before VBUS changes. If the CC wire is damaged, the port stays at 5 V and the device charges slowly or not at all.
The e-marker
A chip in the cable plug declares current rating, data capability and connector type. Without it, a compliant charger assumes 3 A. Captive cables and hard-wired docks also use this mechanism, which is why a dock’s rating is fixed by its own lead.
Power tiers and cable needs
| Profile | Voltage | Current | Cable |
|---|
| Default | 5 V | 3 A | Any rated lead |
| Standard PD | 9 to 20 V | Up to 5 A | E-marked 5 A for 100 W |
| PPS | 3.3 to 21 V steps | Up to 5 A | E-marked 5 A |
| EPR | 28 to 48 V | Up to 5 A | EPR-rated, e-marked |
Why charging is slower than expected
Three causes. The cable has no e-marker, so the current is capped at 3 A. The device requested a lower profile because its battery is warm or nearly full. Or the port is a hub output that shares power with other devices and advertises less than the charger can deliver.
Check the reported wattage in the operating system before replacing hardware. A laptop showing 60 W from a 100 W charger is usually reporting a cable limitation, not a charger fault.
Choosing cables
Buy a cable whose listing states both the current rating and the data rate, because the two are independent. A 100 W cable may still be USB 2.0 for data, and a 40 Gbit/s cable may be limited to 3 A. Keep the PD lead short, and put the length on the mains side where loss does not affect negotiation.
A Type-C hub with power pass-through re-negotiates the link, so its own rating caps the result; the USB-C extension cable page explains why extensions reduce the ceiling, and the USB-A extension cable route cannot exceed 15 W at any length.
Practical scenarios
- Laptop charging at 60 W from a 100 W brick: replace the cable with a 5 A e-marked lead, then check the reported wattage again.
- Phone on a laptop port: expect 15 W or less, because data ports rarely advertise high-voltage profiles.
- Powered speaker with a C input: confirm the device wants PD rather than a fixed 12 V supply before connecting.
- Wall socket behind furniture: use a short PD lead and a longer mains cable, not a long PD extension.
FAQ
Is any USB-C cable safe at 100 W?
Only if it is rated for 5 A with an e-marker. A 3 A cable will not overheat dangerously because the charger respects the declaration, but it will limit the power. Cables without any rating may not be compliant.
Does PD work in both directions?
Yes. A phone can charge a laptop at low power, and a laptop can charge a phone. The roles are negotiated, and the device with the higher capability usually becomes the source.
Why does the charger get warm but the laptop stays cool?
The laptop may be refusing the offered profile because of temperature or battery level, so the charger is running at a low current while its own electronics dissipate heat. Check the reported input wattage before assuming a fault.
