Guide · 5 min read
E-marked cables, explained.
An e-marker is a tiny chip in the cable's plug that answers the charger's questions before any high-power contract is offered. No chip, no answer, no more than 3A — which is the reason a 100W charger sometimes delivers 60W.
What the chip is and how it is interrogated
USB-C sources talk to devices using packets addressed SOP. To talk to the cable itself, they use a separate address, SOP' (SOP-prime), which only the e-marker IC answers. The chip is powered from VCONN, a pin repurposed for exactly this, and it responds with a set of Vendor Defined Messages describing what it is:
Maximum current (3A or 5A) · maximum voltage (20V, or 50V for EPR cables) · maximum data rate (USB 2.0, 5/10/20 Gbps, or 40 Gbps) · whether the cable is passive or active · latency and cable type · vendor and product ID · firmware version. The charger reads that, then decides which entries in its own capability list it is allowed to advertise. If the cable says 3A, the 20V/5A entry is withheld — the phone or laptop never even sees it as an option.
This is a genuine safety mechanism rather than a licensing gate. 5A through conductors sized for 3A is how cables cook, and the source has no other way to know what it is plugged into.
When an e-marker is mandatory
| Situation | E-marker required? | What you get without one |
|---|---|---|
| Up to 60W (20V/3A) | No | Full speed — no chip needed |
| Above 3A / above 60W | Yes, 5A rated | Capped at 3A, so 60W maximum |
| Above 100W (EPR, 28/36/48V) | Yes, 240W certified | Capped at 100W; EPR mode never engages |
| USB 3.2 Gen 1 and faster | Yes, by spec | Cable would not be compliant |
| USB4 / Thunderbolt | Yes, always | Not applicable — all are marked |
Note the middle rows carefully: an e-marked 5A cable is not automatically a 240W cable. The 100W and 240W tiers use different e-marker declarations, and a source running PD 3.1 EPR will refuse to leave 100W without a cable that advertises 50V. Everything in-between — 61W to 100W, the range that covers most USB-C laptops — needs the ordinary 5A marker.
Identifying one you already own
In descending order of reliability: read the printed rating on the connector shell or the moulded sleeve (5A, 100W, 240W, or the USB-IF "Certified USB 240W" / "Certified USB 40Gbps" logos on the packaging); plug it into an inline USB-C power meter, which reports the negotiated contract and therefore reveals the cap; or run an empirical test — connect a 100W-capable adapter to a laptop that can draw more than 60W and check what it actually pulls. On a Mac, hold Option, click the Apple menu, open System Information and select Power: the AC Charger Information block shows the wattage of the contract in force. A 60W reading on a 96W charger is a 3A cable, every time.
What to buy
One 100W 5A e-marked cable per charger covers phones, tablets and every laptop below the EPR tier, and costs a few dollars more than a 3A cable. Add a certified 240W cable only if you own a source and a sink that both do EPR — currently the Apple 140W brick with a 16-inch MacBook Pro, or a 240W desk charger. Keep the short 3A cables for AirPods and watches, where they are perfectly adequate and easier to coil.
Related reading: the full USB-C cable guide for length and voltage-drop effects, USB standards explained for how data and power ratings diverged, and laptop USB-C charging for the wattages your machine will actually request.
FAQ
How do I know if my cable is e-marked without buying a tester?
Check the printing on the plug for 5A, 100W or 240W, then verify empirically: if a 96W or 100W charger only ever delivers 60W to a laptop, the cable is a 3A cable.
Do e-marked cables charge phones faster?
Almost never. Phones stay under 3A on their fast-charge profiles, so the marker changes nothing. The exception is Samsung's 45W mode, which runs above 3A.
Is every USB-C cable that came with a laptop e-marked?
Usually, if the laptop charges above 60W. Cables bundled with phones and tablets typically are not, because they never need to be.
Can an e-marker fail?
Yes, and the symptom is a sudden drop to 60W or a cable that stops charging entirely while still passing data. It is not repairable; replace the cable.