USB-C Cable Wattage Checker — Why Your 100W Charger Is Slow

1What cable do you have?

This cable tops out at

—

Charges at full speed

Held back on this cable

2Symptom → cause → fix

What you see	Most likely cause	Fix
A 100W charger delivers only about 60W	You're using a 3A cable. It physically caps at 20V × 3A = 60W.	Get a 5A E-Marker cable
Laptop says "Slow charging" or won't charge under load	Cable can't sustain 20V at 5A, so the laptop drops to a lower profile.	Swap to 5A E-Marker
MacBook Pro 16" stops at 100W with a 140W charger	140W needs 28V × 5A. A standard 100W E-Marker cable can't carry it.	Need an EPR 240W cable
Phone fast charging suddenly stopped	Cable doesn't support PPS, or the connector is worn.	Use a PPS-rated cable
Charging cuts in and out	Damaged conductor or loose connector, not a wattage issue.	Replace the cable
A very long cable (3m+) underperforms	Voltage drop over length. Passive cables lose too much past ~2m.	Use an active cable

3Which cable do I need?

Pick your most power-hungry device

How to tell what cable you own

Check the printing on the connector. Many cables have 3A, 5A, 60W or 240W moulded into the plastic housing.
Read the original packaging. If it says 100W or 240W, it carries an E-Marker chip.
E-Marker is invisible. It's a tiny chip inside the connector — you cannot see it, and any 5A-rated cable must have one by spec.
Cables that came with a phone are usually 3A / 60W. They will bottleneck any laptop-class charger.
When in doubt, buy a cable rated above what you need. A 240W cable works fine on a 20W phone — the device only draws what it asks for.

Frequently asked

Do all USB-C cables support the same power?

No. Physically identical connectors can differ hugely. A basic USB-C cable carries 3A (60W at 20V), while a 5A E-Marker cable carries 100W and an EPR cable carries 240W. This is the single most common reason a powerful charger underperforms.

What is an E-Marker chip?

A small identification chip inside the cable connector that tells the charger what current and voltage the cable is rated for. USB-C spec requires it for anything above 3A, so the charger can safely negotiate 5A instead of guessing.

Will a 240W cable damage my phone?

No. Power is pulled by the device, not pushed by the charger. A phone draws the ~20W it asks for regardless of whether the cable is rated 60W or 240W. Higher-rated cables are strictly safer.

Why does my MacBook Pro 16" only reach 100W?

Full 140W requires 28V at 5A, which is above the 20V limit of a standard 100W E-Marker cable. You need a cable explicitly rated for EPR / 240W, plus a charger that supports PD 3.1 or PD 3.2 EPR.

Does cable length matter?

Yes. Resistance builds up with length, so long passive cables lose voltage under load. Above roughly 2 metres at 5A, look for an active cable with signal conditioning built in.

Is a more expensive cable worth it?

Only in one direction: buy the rating your device needs, not the price tag. A correctly rated mid-priced 5A cable beats an overpriced 3A cable every time for laptops.