Dr. Pyro, this is NOT to argue with you but to simplify it for electrically challenge readers who are already confused enough.
Everyone -- the safe current carrying capacity of a circuit is dependent on only 3 things. First clean tight connections with good breakers and plugins. The second is wire size. The third is the plugin rating. The size of a breaker is irrelevant to how much current your circuit can safely carry. A given wire size of a given length can only safely carry a given amount of current regardless of the breaker rating!
ALL CURRENT RATINGS THAT ARE DISCUSSED HERE WILL BE FOR COPPER WIRE UNLESS OTHERWISE NOTED! If you have aluminum wiring forget about it and get the heck out of there. It is very unsafe eventually. The odds are very great against finding but there may still be a little of it around. Never work on aluminum wiring because it is very dangerous even when used by a licensed electrician.
For anything less that 75' long circuits the following are the current ratings for the most common wire sizes encountered in residential wiring:
14 gauge COPPER = 15 amps
12 gauge COPPER = 20 amps
10 gauge Copper = 30 amps.
Most three prong plug with parallel blades are rated at 15 amps. Most 20 amp plugs have at least one blade cross ways and 30 amp have both blades cross ways. They are made this way to prevent your plugging a 30 amp appliance into a 15 amp circuit.
It makes no difference what size the circuit breaker is, the highest safe amperage for that circuit is determined by the lower of the rating for the wire or the plug.
If I have a 14 gauge wire and put a 30 amp plug on the end and a 20 amp breaker in the box I still have a 15 amp circuit because the wire will over heat at either 20 or 30 amps.
Likewise if I have 10 gauge wire and a30 amp breaker but a 15 amp plug, for that plug at least I have a 15 amp circuit. However, on a circuit you can have multiple plugs of what ever rating but the total safe load is still determined by wire size.
What a breaker or fuse does is to limit the amount of current you can draw through that circuit. They are designed to fail (blow for a fuse, trip/break for a breaker) BEFORE the wiring gets hot enough to start a fire, but just changing the fuse/breaker to a larger one does NOT increase the current the wire will handle.
Changing a 2 wire circuit to a 3 prong plug, regardless of whether it is a normal plug or a GFCI plug does NOT make it the standard safer grounded circuit! The purpose of the third wire is to create a path between the hot wire and the earth that is of less resistance than through your body so that the circuit is more likely to flow through the ground circuit than through you if a short occurs.
A GFCI is a Ground Fault Circuit Interrupt device which senses the difference between the current flowing through the hot wire and the neutral wire and shuts down if it senses that part of the current is going to ground instead of staying in the power circuit. In order to sense this is MUST have a good ground wire for a reference. The advantage of a GFCI is that if will shut down the circuit in micro seconds, usually long before you even feel a tingle. GFCIs are NOT intended to be used as circuit breakers -- they are designed to prevent shock/electrocution NOT to prevent fires.
The best advice I can give anyone who isn't sure about a circuit's capacity is to have someone who KNOWS how to tell wire size check the circuit at the fuse box/circuit breaker panel and then cross check to be sure that the breaker rating matches the rating of the wiring. It should cost less than $100US, but that is a whole lot cheaper that a fire or an electrocution!
If you don't KNOW exactly what the safe rating of the wiring in the wall is then you are playing Russian Roulette any time you are plugging in anything, even a trouble light.
Hopefully this gives the uninformed a safe place to start.
Good smoking everyone!
