USB-C and Thunderbolt Explained: Why Not All USB-C Cables Are Equal

USB-C is the universal connector that replaced dozens of port types – or was supposed to. In practice, a single USB-C port can be five different things: a basic charging port, a USB 3.2 data port, a Thunderbolt 4 port, an HDMI output, or a DisplayPort output. Two identical-looking cables can deliver wildly different speeds. This confusion is not accidental – it reflects how multiple independent standards converged on the same physical connector. This explainer untangles it.

USB-C Is a Connector Shape, Not a Standard

This is the most important thing to understand. “USB-C” describes the physical shape of the connector – the reversible oval plug. It says nothing about what protocols run through the cable or what the port supports. A USB-C port on a $15 phone charger and a Thunderbolt 4 port on a MacBook Pro look identical from the outside but have dramatically different capabilities.

The protocols that can run through USB-C connectors include:

• USB 2.0 (480 Mbps) – bare minimum, found on budget chargers and some phone cables
• USB 3.2 Gen 1 (5 Gbps) – common on mid-range devices
• USB 3.2 Gen 2 (10 Gbps) – faster data transfers, common on recent laptops and SSDs
• USB 3.2 Gen 2×2 (20 Gbps) – uncommon, found on some high-end external SSDs
• USB4 (40 Gbps) – the current USB top standard, uses the Thunderbolt 3 specification
• Thunderbolt 3 (40 Gbps) – Intel’s protocol, superset of USB4, adds display and daisy-chaining
• Thunderbolt 4 (40 Gbps) – stricter minimum specifications than TB3, more consistent behavior
• Thunderbolt 5 (120 Gbps) – the current Thunderbolt maximum, for demanding pro workloads
• DisplayPort Alt Mode – carries DisplayPort video signal over USB-C
• HDMI Alt Mode – carries HDMI signal (less common)
• Power Delivery (PD) – defines charging wattage up to 240W

USB Versions: What the Speed Numbers Mean

USB standards have confusing naming history, but the relevant speeds for current hardware:

Marketing Name	Technical Name	Max Speed
USB 3.0 / USB 3.1 Gen 1	USB 3.2 Gen 1	5 Gbps
USB 3.1 Gen 2 / USB 3.2	USB 3.2 Gen 2	10 Gbps
USB 3.2 Gen 2×2	USB 3.2 Gen 2×2	20 Gbps
USB4 Gen 2×2	USB4 Gen 2×2	20 Gbps
USB4 Gen 3×2	USB4 Gen 3×2	40 Gbps

These speeds are theoretical maximums. Real-world transfer speeds on a 10 Gbps USB port with a fast external SSD reach around 900-950 MB/s (7.2-7.6 Gbps effective) – the protocol overhead and drive speed cap the theoretical maximum.

Thunderbolt: Intel’s Superset Protocol

Thunderbolt is a protocol developed by Intel (in collaboration with Apple) that combines PCIe (Peripheral Component Interconnect Express) and DisplayPort signals over a USB-C connector. Thunderbolt does everything USB does, plus:

• Carries DisplayPort video natively (not just Alt Mode – it is a first-class feature)
• Carries PCIe signals for external GPUs (eGPUs) and high-speed external NVMe enclosures
• Supports daisy-chaining – up to six Thunderbolt devices in a chain from a single port
• Enables higher-bandwidth docking stations with multiple simultaneous 4K displays

Thunderbolt 3 vs Thunderbolt 4

Both Thunderbolt 3 and Thunderbolt 4 operate at 40 Gbps. Thunderbolt 4 added stricter minimum requirements: it must support two external 4K displays (Thunderbolt 3 allowed single 4K support in lower configurations), it must support PCIe at 32 Gbps (not 16 Gbps as in some Thunderbolt 3 implementations), and it requires Intel VT-d-based DMA protection for security. In practice, Thunderbolt 4 means more consistent capabilities across devices – you can trust that any Thunderbolt 4 device will work with any Thunderbolt 4 dock.

Thunderbolt 5

Thunderbolt 5 uses USB4 v2 at 80 Gbps bidirectional, with asymmetric mode (Bandwidth Boost) pushing 120 Gbps in one direction for display-heavy workloads. This enables three external 4K displays, 8K display support, and the bandwidth for eGPUs to be more practical. Intel’s Lunar Lake (Core Ultra 200V) and Arrow Lake CPUs support Thunderbolt 5. Apple’s M-series Macs use Apple’s own high-speed protocols rather than Thunderbolt 5, though compatibility with Thunderbolt 4 accessories is maintained.

Power Delivery: Why Charging Speeds Vary

USB Power Delivery (USB PD) is the standard that defines how much power flows over USB-C for charging. The maximum wattage a port delivers depends on what both the charger and the device support:

• 5W – basic USB, slow charging for phones
• 18W – older fast charging standard, common in bundled Apple chargers
• 30W – charges MacBook Air at full speed, iPhone 15 Pro at ~27W effectively
• 65W – handles most Windows laptops at full speed
• 96-100W – required for MacBook Pro 14-inch full-speed charging
• 140W – MacBook Pro 16-inch, some high-performance Windows laptops
• 240W – the new USB PD 3.1 maximum, for desktop replacements

A charger provides up to its rated wattage; the device draws only what it needs. Plugging a 140W charger into an iPhone is safe – the iPhone draws 27W and ignores the rest.

How to Tell What Your USB-C Port Supports

The fastest way: look for the Thunderbolt symbol (a lightning bolt icon) next to the port – that confirms Thunderbolt support. USB4 ports may have a “40” number. A plain USB-C symbol with no additional marking is USB 3.x at best, possibly USB 2.0 at worst.

Check your device’s spec page on the manufacturer’s website for the exact protocol and wattage each port supports – not all ports on the same device are equal. Some MacBooks have a MagSafe charging port plus two Thunderbolt 4 ports and a headphone jack; some Windows laptops have one full-speed Thunderbolt 4 USB-C and one USB 3.2 Gen 1 USB-C that looks identical.

For choosing the right hub or dock for your specific laptop’s ports, see our best USB-C hubs and docking stations guide which specifies which docks require Thunderbolt ports versus working with standard USB-C.