Hi all,
A couple of the responses here could be misleading.
For the most common types of wire and insulating materials used in electrical cords in the US, an 18ga wire can handle up to a 10 amp or 1200 watt load, a 16ga wire a 12.5 amp load, a 14ga wire a 15 amp load, and a 12ga wire a 20 amp load. These are based on the insulation covering the conductors being able to handle a defined temperature rise. If the insulation can only safely handle less temperature rise, the wire would be rated to handle only smaller amp loads. But, lets use these figures for our purposes here.
Receptacles in the wall are sized for amp load as well and use wire sized accordingly. Plus, those circuits are protected by a circuit breaker which will not allow more than the rated amp load to pass for any protracted period of time. They will allow greater amp loads to pass for short periods of time, time short enough that the insulation on the wire will not melt.
As Bob points out correctly, the two plug-it cords use 18ga and 16ga wire. The 18ga wire is only sized to handle a maximum of a 10 amp load (1200 watts at 120 volts) while the 16ga wire can handle a maximum 12.5 amp load (1500 watts at 120 volts). The male plug on both will connect to a plug in the wall (receptacle) capable of handling a maximum of 15 amps (1800 watts at 120 volts) but the wire in each cord will only safely handle the amp load noted so that is all the current that can be safely passed through that cord. The "larger" 16ga cord can safely handle the current draw from any of the plug-it equipped tools (all of which have a less than 1500 watt draw). The smaller 18ga cord can only handle the current draw of the smaller tools (those with less than a 1200 watt draw) and not the larger amp draw tools like saws and large routers. The notch in the plug-it end of those cords makes it possible to plug the larger 16amp cord into any plug-it tool while the smaller 18ga cord will only plug into the smaller amp draw tools.
Bottom line, just because the cord is capable of being plugged into a 15amp receptacle at the wall does not mean the wire in the cord can safely carry a 15 amp load, hence the need to prevent the user from inadvertently plugging the wrong (smaller) wire cord into the larger amp draw tools.
On a related note, the dust collectors come with much larger 12ga wire in their cords so they can handle a total of 2400 watts (20 amps times 120 volts) combined between the wattage required by the DC itself plus whatever is plugged into it. Most household receptacles in the US are wired with 14ga wire capable of only handling 15 amps or 1800 watts total load. Those receptacles have two vertical flat slots for carrying power and one round ground slot. That is the plug which is on the end of the cord for the dust collectors. A second, less common household receptacle has one vertical and one horizontal slot for carrying power plus the round ground slot. Those are wired with 12ga wire and are capable of carrying a 20 amp or 2400 watt load. The auxillary cord that comes with the dust collectors has a male end with one of those 20 amp style plugs and a female end with the 15 amp style plug so you can connect the DC to either a 15 or a 20 amp receptacle (since the wire itself can handle either load) depending on the total power draw required for both the DC itself plus whatever is plugged into it.
Finally, for really large power draw tools like the 2200 router, the cord supplied would have to be 12ga wire and the plug shaped with the one horizontal and one vertical posts for power plus the one round ground post to be able to plug into a 20 amp receptacle sized to handle the full 2200 watts of continuous power the motor is able to handle safely. The wire on the version of the 2200 sold in the US is 14ga and uses the 15amp variety plug which has two vertical and one round posts so it can be plugged into a receptacle rated only to handle 1800 watts. Hence, it must could only draw more than 1800 watts for a short period of time before the circuit breaker tripped. It can draw the full 2200 watts the motor is capable of, but only for a short period of time. That is lots of power for an efficient tool like the 2200. The name plate lists it as a 15 amp maximum draw tool in US configuration.
All the large routers sold in the US which claim to be 3 1/4 to 3 1/2 HP are also sold with 14ga wire cords and 15 amp plugs. Since one HP electrical is defined as a 746 watt draw, then 3.25 hp would be a 2425 watt draw or slightly over 20 amps at 120 volts. Hummmmmmm. How can that be? Answer must be that those manufacturers rate HP only for the brief period of time the motors draw more than 15 amps before the circuit breaker trips or they are internally limited to 1800 watts continuous draw or 2.41 HP.
Hope this somewhat clears up what can be a confusing topic, but one that is very important for all of us who use power tools to understand.
Jerry