Quick answer: A USB-A to USB-C cable is a passive cable with a 56 kohm pull-up on the C end, which tells the device it may draw up to 3 A at 5 V, so 15 W is the ceiling. It cannot negotiate USB Power Delivery, because PD messages travel on the configuration channel and a USB-A port has no configuration channel. It also cannot carry DisplayPort, because there are no high-speed lanes wired for it. Speed depends entirely on the cable: 480 Mbit/s for a charge-only lead, 5 or 10 Gbit/s only if the lead is wired for SuperSpeed and both ends support it.
The connector pair looks symmetric and is not. USB-A defines four or nine contacts with the host controlling power, while USB-C defines 24 contacts, a reversible plug and a negotiation channel. When the two meet, the cable has to impersonate a fixed 3 A source using a resistor, and everything clever that USB-C normally does is unavailable.
What is inside the cable
Wiring and the single resistor
The A end presents a standard USB 2.0 pair, power and ground, plus SuperSpeed pairs on a 9 pin version. The C end adds a 56 kohm resistor from CC to VBUS, which advertises a default current of 3 A. A cable with a 22 kohm resistor advertises 1.5 A and is meant for a C-to-C lead, so it must never be fitted to an A-to-C product, where it would under-report available current.
No e-marker, no negotiation
The e-marker is a small chip in the cable that reports current rating, data rate and whether the lead is optically isolated. A-to-C cables are not permitted to carry one, because the resistor already fixes the current. Without an e-marker there is no path to 20 V, so a laptop that needs 20 V and 3.25 A will charge slowly or not at all from a 15 W A port, and it will report a low-power charger warning on screen.
What it can and cannot deliver
| Cable | Data | Power | Video |
|---|
| A-to-C, 2.0 wiring | 480 Mbit/s | 5 V, 3 A, 15 W | None |
| A-to-C, 3.2 Gen 1 | 5 Gbit/s | 5 V, 3 A, 15 W | None |
| A-to-C, 3.2 Gen 2 | 10 Gbit/s | 5 V, 3 A, 15 W | None |
| C-to-C with e-marker | Up to 40 Gbit/s | 100 W, 20 V 5 A | DP Alt Mode |
| C-to-C, EPR rated | Up to 80 Gbit/s | 240 W, 48 V 5 A | DP Alt Mode |
Three failures worth recognising
A phone that charges slowly on a laptop A port is not faulty. The port may cap at 500 mA in suspend and 900 mA when awake, which is well below the 3 A the cable advertises, and the device will simply take what it is given. A monitor that never appears is also expected: DisplayPort Alt Mode needs lanes that an A port does not have, so no A-to-C cable can drive a screen. And an external SSD that mounts as a slow drive is usually a charge-only lead with no SuperSpeed pairs; the connector looks identical.
Cable identification is the practical headache. Two leads with identical moulded ends can differ by a factor of twenty in data rate, and nothing on the outside distinguishes them. A 9 pin USB 3 connector has extra contacts visible inside the A shell, and the C end of a SuperSpeed lead is thicker and less flexible; a charge-only lead uses four wires and is noticeably thinner. Marking the leads at installation costs a minute and saves an afternoon of debugging a drive that mounts at 480 Mbit/s.
Where the cable does earn its place is in a fixed installation. A USB-A extension cable plus an A-to-C lead is often the cheapest way to reach a wall-mounted display or a car dock, and a Type-C hub on the far end can add ports that the A host cannot offer directly. For laptops that must charge and expand from one connector, a USB hub with USB-C input and a C-to-C lead is the correct combination.
Practical scenarios
- Charging a phone from a wall charger A port: expect 15 W at best, and accept that a phone needing 25 W will take roughly twice as long as on a C-to-C PD charger.
- External SSD on a desktop front port: check the port’s SuperSpeed capability and use a lead rated 10 Gbit/s, otherwise the drive will run at 480 Mbit/s and feel like a hard disk.
- Car head unit with A port and C phone: a 1 m A-to-C lead is the right part, but choose one rated 3 A so the phone charges while navigation runs.
- Kiosk or digital sign with a C device: do not plan video over A-to-C; use a C host or an active converter with its own power.
FAQ
Can a USB-A to USB-C cable fast charge?
It can reach 15 W, which many phones call fast charging, but it cannot reach the 25 to 100 W that USB Power Delivery provides. PD requires messages on the configuration channel, and a USB-A port has none. If the device needs more than 15 W, the charger and the cable both have to be C-to-C.
Will an A-to-C cable carry video to a monitor?
No. DisplayPort Alt Mode uses the SuperSpeed pairs, which a USB-A port does not route for video, and there is no alternative mode negotiation without CC pins. Video from an A port requires an active adapter using USB graphics, which compresses the image and adds latency.
Why does my device only draw 500 mA?
The host port is limiting it. Unconfigured USB 2.0 ports supply 100 mA, configured ports 500 mA, and battery charging ports up to 1.5 A. The cable’s resistor tells the device what the cable can carry, not what the host will give, so a laptop port will still cap below the cable rating.
