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Is Your USB-C Cable Slowing Your MacBook's Charging? We Measured It

Published October 15, 2026. Last updated October 15, 2026.

Yes, a cable can cut your MacBook’s charging power by more than half. Charging power is negotiated between the charger, the cable, and the Mac, and the result is capped by the weakest of the three. In our tests, a single 30W charger delivered 30W, 15W, or 8W to the same MacBook depending only on how it was connected. Below are the measurements, what sets a cable’s limit, how to check yours in about a minute, and which cable to buy.

How much can a cable change charging speed?

We connected the same chargers to a 14-inch MacBook Pro (M1 Pro) in different ways and read the negotiated power from macOS:

Two lessons come out of this table. A USB-A connection caps you at 5 volts, because USB-A has no Power Delivery, so even a good USB-A cable tops out at 15W. That is below what Apple recommends for every current MacBook, including the 20W MacBook Neo. And a cable that is properly rated changes nothing: the second cable on the Apple charger delivered the same 94W as Apple’s own.

What sets a cable’s limit?

Four things, in the order you are likely to hit them:

• USB-A or USB-C at the charger end. Power Delivery, the protocol that lets a charger go above 5 volts, needs USB-C at both ends. A USB-A to USB-C cable stays at 5 volts, so 15W at best and often less.
• 3 amps or 5 amps. A standard USB-C cable carries up to 3 amps, which at 20 volts is 60W. Above 60W the cable needs a small chip, called an e-marker, that tells the charger it can carry 5 amps. These are sold as 100W cables. Without one, a 96W charger delivers about 60W.
• 240W cables for the biggest chargers. Charging above 100W uses the extended power range of USB Power Delivery, which needs a cable rated for 240W, or Apple’s USB-C to MagSafe 3 cable.
• Wear. A damaged wire or worn contacts can break the e-marker handshake or the connection itself, so the cable falls back to lower power or drops out.

Can macOS show that the cable is the problem?

Indirectly, yes. During our tests, macOS reported a different Family Code for each type of connection, both in pmset -g ac and in System Information under Power:

Apple does not document these values; this is what they lined up with on our test Mac. If you see 0xe000400a, your cable and charger are talking Power Delivery. If you see either of the others, the setup fell back to plain 5V USB, and the cable or the charger port is the first thing to change.

How do I test my cable?

1. Plug in and wait 30 seconds. Some chargers start low and climb: one of ours took 23 seconds to go from 15W to 30W.
2. Read the watts. In Terminal run pmset -g ac, or open System Information, select Power, and look under AC Charger Information.
3. Swap only the cable and read again.

If the watts jump with the second cable, retire the first one. If nothing changes, the cable is fine and the limit is the charger, a dock, or a port shared with another device; see Mac says “Slow Charger” for those.

Broken Charger shows the negotiated watts live in the menu bar, so this swap test takes a glance instead of a Terminal command, and it records every dropout, so an intermittent cable gets caught even when you are not watching.

What are the signs of a failing charging cable?

• Charging stops or starts when the cable is moved or held at an angle
• The charging chime repeats while nobody touches anything
• The watts are lower than the same setup gave last month
• The plug end is warm or discolored, or the jacket is split near the connector

The first two cause intermittent charging, covered in MacBook charging on and off. A cable that is warm at the plug end should be replaced, whatever the watts say.

Which cable should I buy for my MacBook?

Apple’s own 240W USB-C Charge Cable (2 m) covers every MacBook charger. For wattage by model, see how many watts a MacBook charger needs.

Frequently asked questions

Is it safe to charge a MacBook with a USB-A cable? It is safe and slow. A USB-A cable gives 15W at most, which is below what Apple recommends for every current MacBook, so the battery may not gain charge while you work.

Do all USB-C cables look the same? From the outside, often yes. The rating is printed on the packaging and sometimes on the connector. If you are unsure, the swap test above tells you in a minute.

Can a bad cable damage my Mac? A cable that fails usually delivers less power or none. The real risk is physical damage: a cable that is split, scorched, or hot at the plug should go in the bin.

Why does my 96W charger show only 60W? Almost always the cable. A 3A cable caps the negotiation at 60W. Use a 100W (5A) cable and the number should rise to about 94W.

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