Hi OP,

Not sure if you are trying to learn theory, or trying to know if a particular setup you have is safe. If you need to plug in a bunch of chargers, Belkin makes a 12 outlet 15A power strip. I looked at my 2.1A apple charger and it draws 0.5A from the outlet. The fact that it puts out 2.1A is irrelevant. The 0.5A is the "input" current to the charger and is what is important for determining your maximum system. Most chargers have this printed on them. So if I plugged 12 of my 2.1A apple chargers into the outlets on this strip, it would only consume 6A maximum from the outlet, which is well under the 15A limit. Sir Hippo is correct that the outlets in most houses are 15 or 20 amps (you can tell by the outlet style). In a house multiple outlets are often connected to the same 15A breaker. So the current from anything plugged into the connected outlets is added and cannot exceed the breaker rating without it tripping. Like if someone plugs a vacuum into an adjacent outlet.

You can theoretically connect many power strips together and be perfectly safe although violating code (much like doing 60MPH in a 55MPH zone). A power strip adds together all of the current drawn by all of its' outlets and pulls them through its plug. For example, if you had 2 15A power strips, and if power strip 2 is plugged into power strip 1, which is plugged into a wall -> If strip 2 had 15A of loading, that load is also flowing through power strip 1 and the wall. So if you add more stuff to power strip 1, power strip 1 and the wall are now overloaded. One of the breakers will give. If a breaker fails you could be in danger.

Incidentally I can overload a wall by plugging in two 15A heaters; one into the top and one into the bottom outlet of a receptical and it will blow the breaker without any power strips involved.

Bottom line, if you aren't sure what you are doing, don't risk it. You absolutely can risk a fire.Like I said Belkin makes a 12 outlet strip.

Good Luck - be safe