# Does wire gauge significantly impact the charging rate of smartphones?

This article claims that wire gauge in USB cables significantly impacts the charging rate of smartphones:

USB cables have a data wire and a charging wire within the cable itself. Most USB cables, probably over 99+% are 28/28. Buying after market micro USB cables will pretty much always result in getting cheap quality 28/28 cables and your device will barely break 500mah [sic] when it charges, maybe even less.

The misuse of units makes me skeptical of the claims made by the article. Furthermore, the length of cables should also impact the total resistance, and the article does not mention cable lengths at all.

Is the article correct in saying that increasing the wire gauge from 28 to 24 (which has ~40% the resistance) would allow the charging current to increase from 500mA to 2A?

The contention is absolutely true.

AWG 28, often used in USB cables, at 2m is about 0.5 Ω either way, and charging at 1A thru such a cable would require the phone to draw 1A at 4v, which won't work.

At 2A it would be even worse.

The current situation is unacceptable, and USB cables offered should never have a resistance greater than 0.1Ω either way, regardless of length.

To check in reality, buy a USB current/voltage meter insert device. Usually about USD4-5 on EBay.

It sounds very suspicious for multiple reasons.

1) Resistance of the wire is not the limiting factor for power transfer. Let's do a few calculations. Based on the resistance for copper wire provided by this chart, 1000 ft of 28 gauge wire is approximately 64 ohm. Say we have a 3 ft cable, the resistance of the cable is approximately 0.2 ohm. 0.2 ohms combined with the 5 V power from USB would be able to provide many amps of current. Ultimately the current is controlled by the charger itself. See the next point.

2) USB current is specified pretty well in the USB spec. See USB Power. By default, USB will provide between 150 mA to 500 mA based on the USB version. Past that, the USB controller and device must negotiate for a higher current. 2 A most certainly is only possible with USB negotiation and the USB controller will be the limiting factor, not the cable.

3) Based on the same wire gauge chart, 28 gauge can handle 1.4 A, 24 can handle 3.5 A. Copper wires don't fatigue over use. I'm more inclined to think that there is physical wear from bending/inserting/removing the cable multiple times.

• I notice a lot of people reporting this actually. I would love to do some tests to check this claim. Any ideas as to the things I should be looking for? – shaunakde Mar 27 '16 at 8:16

It is at least plausible that there is significant voltage drop in a 28 AWG cable used to carry 2A. See this calculator, for instance: http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=212.9&voltage=5&phase=dc&noofconductor=1&distance=3&distanceunit=feet&amperes=2&x=73&y=6

Using a short cable with large conductor area will help.

Wether a 0.7 volt drop affects charging rate would depend on the voltage drop of the charging circuit in the phone. The phone battery is probably lithium ion and requires about 4.2V to reach maximum charge. So if there is any voltage drop in the phone charging circiit, then thin cables could plausibly alter the charging rate.

To determine this one would have to make experiments :-). Unfortunately I don't know of any such experiments.