The thickness of USB cable conductors is normally specified in American Wire Gauge (AWG). In this system, larger numbers indicate thinner wires. As the USB specification makes reference to AWG numbers of 20, 22, 24, 26 and 28, calculations were performed for these AWG figures.
The data used in calculations is summarized above. The conductor resistance is taken from Wikipedia and assumes a copper conductor. The USB contact resistance of 30mohm is a “middle of the road” figure – many connectors specify 10mohm when new, and 30mohm through their lifetime, although 50mohm is acceptable for micro connectors.
The voltage drop is derived from ohm’s law – Voltage = Current * Resistance.
The resistance itself is calculated by the route length which is twice the length of the cable (as current travels from the charger to the device and back). For the one with contact resistance, four times the contact resistance is added to compensate for the charger positive, device positive, device negative and charger negative contacts.
This figure is in mohms, and is divided by 1000 to get to ohms, and then finally multiplied by current in amps to get the voltage drop.
The results are colour coded as to their voltage loss. For a 5v output, the USB specification demands that the voltage remains within 5% (i.e. an acceptable voltage drop of 0.25v). All voltage drops less than 0.25v are colour coded green. From there, voltage drops of between 0.25v up to 0.5v are colour coded yellow. This is because, while the USB specifications are stringent, most devices only require 4.2v – 4.35v at the battery to fully charge. Most chargers are of the linear or buck (step-down) type, and thus the supply voltage must be greater than the battery voltage for charging to happen. Accounting for losses in the charging circuit, it is determined that approximately 4.5v is required to ensure a full charge. Any losses greater than 0.5v are coloured red, as they are likely to cause problems.
With Contact Resistance
With contact resistance taken into account, it can be seen that it is difficult to meet requirements at high currents of 2A and 2.4A. As a result, cables only up to about 50cm can be used with 24AWG, or maybe even 1m with boosted source voltage. This is most significant for tablets.
For smartphones, with a requirement closer to 1A, 24AWG wires up to 2m could be sufficient, or 1m at 26AWG, and 50cm at 28AWG.
However, at 500mA (the original spec), it can be seen that it is possible to meet the stringent USB voltage requirement at every length with 20AWG wire, and 2m with 24AWG (probably by design).
The disadvantage of pushing more current at low voltages is very clear – the resistance causes power loss very quickly!
A corollary of this is that if you have a 3m cable, of 24AWG, then it’s likely your charge current is not going to exceed 1A (it’s probably 500mA-1A) purely because of the voltage drop that is caused by the cable.
Without Contact Resistance
If we assume that connector resistance isn’t part of the equation, and look at the wires itself, the situation is a little less stringent, at high currents, but still illustrates the difficulty in keeping voltage drop under control.
One caveat of this, is that some vendors have realized the issue and decided to push the output voltage up to 5.1v or 5.2v, which is still within USB specification but allows for an extra 0.1-0.2v voltage drop. This is a potentially nice feature as it means the requirements on the cables are slightly relaxed (i.e. maybe even 0.7v voltage drop is tolerable).
It is also possible to see higher charge currents at the expense of voltage drop when the cell is fully discharged at 3v, as a ~3.2v input would be enough to start charging. Likewise, as the cell reaches full charge, the current tapers off, thus reducing the voltage drop. This might be happening already with some cables “pushing” what is possible. You know when it’s pushed too far when it doesn’t consistently charge fully (i.e. stuck at 94%, when eliminating all other issues).