Why Do USB Cables Get Hot While Ethernet Cables Stay Cool?

Power Delivery Is the Key to USB Heating

When USB cables are used for data transfer and charging, the same conductor carries both signals and a large current. According to USB PD 3.1, fast‑charging can draw up to 5 A. By Joule’s law, the resistance of the wire converts this current into noticeable heat.

Additionally, the metal contacts have small contact resistance that increases with oxidation or dirt, and the compact design of USB‑C connectors limits heat dissipation, causing the cable and connector to feel warm.

Ethernet Cables Transmit Pure Signals with Minimal Current

Unlike USB, Ethernet does not provide power; devices use separate adapters. Ethernet uses differential signaling over twisted pairs, which inherently resists interference and requires only a few milliamps of current, resulting in negligible heat generation.

The low voltage swing (≈±1 V) and high sensitivity mean the driver current is tiny, typically a few to tens of milliamps.

Some Ethernet applications, such as Power‑over‑Ethernet (PoE), do carry power, but the current is limited to about 2 A and is distributed across eight conductors, so the cable temperature rise is modest compared with USB fast charging.