What does it mean when a USB-C™ cable is rated at 3A?

Benson Leung
4 min readNov 20, 2018


Feb 26, 2016

What does it mean when a cable is rated at 3A?

tl;dr : The USB Type-C™ spec has a quantifiable benchmark that cables must meet or exceed for wiring and connector quality.

I’ve been running into a different kind of bad cable lately, and I wanted to educate about the requirements and what that means practically.

According to the USB Type-C specification, all Type-C cables of any type must at a minimum be rated at 3A. This includes legacy A-to-C cables, uB-to-C adapters, C-to-C cables, and C-to-uB cables and all variants. Some types of cables may be rated at even higher, or 5A levels in order to hit 100W power delivery levels.

(IMPORTANT NOTE : Just because the cables themselves must be rated at 3A, does not mean that it’s correct to use the 10kΩ Rp pullup resistor to advertise 3A capable. The Rp pullup is meant to advertise the power supply’s capabilities, not the cable. All legacy A-to-C cables and uB-to-C adapters must use a 56kΩ Rp pullup)

But just what does it mean when a cable is rated at 3A or 5A? The answer is in Section 4.4.1 “IR Drop” reproduced in the picture below.

USB Type-C Specification Section 4.4.1 and Figure 4–1

When you have a wire of any kind in a circuit like the one created by the USB cable when charging a device, applying a load on the sink side will cause the voltage measured at the sink end of the cable to be lower than at the source. This is because the wire itself in the cable and the connectors at both ends have some built-in resistance, and some energy is lost crossing the cable.

Figure 4.1 models the different resistances as explicit resistors. The wire itself provides Rcable, and the resistance contributed by the connectors is labeled Rcontact.

As you can see from Figure 4.1, the USB Type-C spec requires that for Vbus, the sum of both Rcontacts plus Rcable must be no more than 500mV. For Gnd, the sum of both Rcontacts plus Rcable must be no more than 250mV.

What does this mean for resistance levels? Remember that the Type-C spec requires that cables are rated at 3.0A. Ohm’s law states : V = I * R. We can rearrange that as : R = V / I

We can divide 500mV by 3.0 to get our maximum Vbus wire resistance limit at 3A : 0.166 ΩWe can do the same for 250mV by 3.0 to get our maxmium Gnd wire resistance limit : 0.0833Ω

In order for a USB Type-C cable to be spec compliant, it must have Vbus end-to-end resistance of no more than 0.166Ω, and Gnd end-to-end resistance of no more than 0.0833Ω.

What does it mean for you if you if you have a cable that does not meet these requirements? The short answer is that your phone, tablet, or laptop may charge MUCH slower or not at all. A device being charged is very sensitive to the voltage on Vbus at the connector. As it tries to apply a larger load on the cable and power supply by charging at higher current levels, the charge circuitry on the phone, tablet, or laptop is checking that the voltage levels do not dip too low.

Especially on phones, batteries may not charge at all if the voltage is too low because the input voltage level must be sufficiently higher than the voltage at the battery in order to make progress.

As a result, when the charge circuitry sees a big dip in Vbus as it is applying more load, it may back off. In my real world practical testing on Chromebook Pixel 2015, I have seen many cables that when the Pixel tries to ramp up toward 2.4A (the capability of the 12W power source I plug it into), caps out at only 1A or 1.5A because the voltage has dropped too much.

If a cable correctly meets the IR drop standard as outlined here and is actually rated at 3A, it should have no problem getting to 2.4A.

In all of my latest reviews, I am actually directly measuring Vbus and Gnd resistance using a high precision LCR Meter. Expect a lot more details and expect me to call out poor performing cables in addition to badly designed cables!

EDIT: What can a cable manufacturer do to make sure their cables meet this spec? There’s not one simple answer, and the Type-C spec doesn’t specify explicitly the parameters of how to design a cable to meet this spec. However, an important key is that the longer the cable wire, the larger the drop. However, larger gauge wire (ie thicker) has less resistance. When a cable manufacturer makes a cable longer ( for example 2M or 3M), they must also at the same time design that cable with much thicker wire than a 1M cable.

Many manufacturers that I’ve reviewed do this correctly. Cambond, for example, ships a 1ft, a 6.6ft, and a 10ft cable. They do not use the same thickness wire for each! The 1ft cable has much thinner wire, and the 10ft cable uses VERY thick wire. You can tell immediately by feeling the different cables.

What is VERY wrong is what some manufacturers do: They make a 1M cable that is spec compliant, but also a 2M cable that uses the same thickness of wire. Because it’s twice as long, it will have almost 2x the resistance, and that puts them out of spec.

For more information about cable resistance (which doesn’t just apply to USB-C cables, but also Micro-B and all kinds of USB cables) read here: http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/

Originally published at plus.google.com.