[Nexus 6P Accessories] vido.ardes:
I'm not an engineer, but I have read both the USB Battery Charging Spec and the USB 3.1 Spec which includes the specification around Type-C plugs at great length and have a decent working knowlege of how consumer electronics work.

The battery charging spec (which is different from Power Delivery) discusses several scenarios, one of which is called DCP or dedicated charging port. This is the part of the spec that people seem to ignore and keep misquoting the power delivery figures that are only relevant when connected to a data source. DCP is applicable when talking about wall and car chargers, because it specifically designed for the scenario where no data transfer will ever take place. It allows for up to 5A to be delivered, regardless of the port type on either end of the cable. This can be seen on page 54 of the first link. This is the part of the spec that people have been using to build chargers for phones and tablets for the last 3 years.

The issue that has been discussed by the Google engineer doesn't mean that a type-A port can't handle 3A, to the contrary it actually proves it can. The issue described by the engineer is what the USB spec tries to cover and prevent. If you read the 3.1 spec, specifically the section on Type-C to legacy connector, it describes a scenario that means the cable should incorporate a resistor that limits the current draw to 1.5A. This is to prevent damage to legacy chargers and doesn't reflect on the capability of the cable/port.

Consider the scenario before the 6P and 5x; no phone or tablet drew more than 2.4A maximum. The way a wall charger works is it simply provides power from the socket, the device on the other end of the cable decides how much current / voltage it can draw. Considering no device could draw more than 2.4A, the chargers didn't have to cope with that scenario. The issue that USB 3.1 provides a scenario that means it can draw more than 2.4A and if your charger is incorrectly built and your cable doesn't identify itself as legacy, it could potentially draw more current that the charger is rated for. There are 3 outcomes to using an out of spec cable on a legacy charger:

The charger is well built, resists the extra current and supplies a lower current, probably 2.0A or 2.4A depending on the charger

The charger isn't smart, but has high quality parts that can safely provide the extra 0.6A and operates at full capacity

The charger fails to limit the current and has cheap components

The third result is bad, very bad indeed, but it is the minority scenario. Best case in this scenario, the charger dies and stops working. The worst case in this scenario is that the charger could actually burst into flames. This is why the USB IF have decided that for legacy implementations, the old ports (Type A and B) should be limited, even if the phone and the cable is capable of providing the current.

What we have been seeing is that people have been ignoring the spec and building cables that pull the full 3A. In the majority of cases, most chargers react by fulfilling item 1 on my list; they correctly provide the max current they are rated for, and you get slightly slower charging at 2.4A. The reason they have done this is so that when they build chargers with Type-A ports that can provide 3A/5V, their cables will work fine at the full speed, and the vast majority of chargers will behave correctly when one of these out of spec cables is plugged in and lower the current draw. Several manufactures have stated they are working on 3A/5V chargers with Type-A ports.

The USB IF have aired on the side of caution, and said that to comply with the spec type-A to type-C shouldn't provide high current despite being able to handle it, because of legacy issues. This is also done to drive the adoption of type-C forward, which is correct behavior for a standards body. This doesn't mean everyone has to build to the spec, they are guidelines, and people have been ignoring them for years.

Are these cables / chargers out of spec? Yes, they are, but 90% of the chargers that come with your phone in the last 3 years are. The specs have never really kept up with the battery charging needs of our devices and the USB IF are very slow to react. With the new PD spec allowing for up to 100W, everyone should be able to have spec compliant chargers and needs met for the foreseeable future. In the next 3 - 5 years however while Type-C gains traction, we will still be using out of spec chargers while everyone plays catch-up.

Hope this helps clear up any confusion.