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EnglishHow WhatCable Determines a USB-C Cable's Speed and Charging Rating
Updated
WhatCable does not test a cable's physical wires. It reads the identity and capability data that the cable's e-marker chip reports, reads the USB-C and USB Power Delivery details macOS already exposes for the port, and then compares the two against what the Mac actually negotiated. The result is a plain-English explanation of the cable's reported speed and charging rating, with the original readings available on demand. This applies to WhatCable on Apple Silicon Macs running macOS 14 or later.
Where the data comes from
WhatCable combines two sources:
- The cable's reported identity. USB-C cables that carry an e-marker chip declare their speed and charging rating electronically. WhatCable reads that declaration rather than guessing from the cable's appearance.
- The connection as macOS sees it. WhatCable reads the port, cable, charger, adapter and connected device together, because, as the site puts it, "a cable never works alone."
The app then checks the reported identity against USB Power Delivery rules and compares it with what the Mac actually negotiated. That comparison is the core of the method: a claim is one thing, observed behaviour is another.
What "speed" and "charging rating" mean in the app
| What you see | What it's based on |
|---|---|
| Reported speed | The speed the cable's identity declares (for example, USB4 Gen 4 at 80 Gbps, or USB 2.0) |
| Charging rating | The wattage the cable reports it supports (for example, rated for up to 240W, or 100W) |
| Connection type | The kind of connection in use, read from the port and device side |
| Technical details | The original readings, shown when you open them |
The site's own examples show this format: a CalDigit TB5 cable reported as USB4 80 Gbps, 240W, passive; an Anker 333 nylon cable as USB 2.0, 100W, passive.
How it explains slow charging
For a slow-charging question, WhatCable shows what the charger offers, what the cable supports, and whether the Mac is drawing the power it needs, then explains the result. The site gives a concrete example of the output: "Charger and cable are fine. The Mac will draw up to 94W when it needs to." That tells you the limit is not the cable or charger, so swapping components at random is unnecessary.
What it cannot tell you
WhatCable reads what a cable reports and what the Mac negotiates. It cannot see inside the cable jacket and, per the site, "never pretends it can." So a cable that misreports its identity will be shown as reporting that identity; the app surfaces the discrepancy between claim and negotiated behaviour rather than physically verifying the wires. Treat the readings as evidence about identity and negotiation, not as a lab measurement of the cable.
Checking a cable against the public record
WhatCable also maintains a public database of cable observations contributed by users, reported through the app. At the time of the site's description it held 145 cable observations, searchable by vendor, speed and identity. Nothing is uploaded without your action. If you want to see whether your cable has appeared before, browse the database by vendor, speed or identity before drawing conclusions from a single reading.
Free vs Pro, as described
The free version covers understanding the current connection. Pro is for inspecting the evidence, watching measurements and investigating changes over time. The app is open source with Pro optional, signed and notarised, and available in 19 languages. The site does not state Pro pricing on the pages summarised here, so check the Pro page for current terms.