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Watts = Volts * Amps
This is the equation that will do it all. If you are in the United States, V or Volts will be about 120 or less. Basically the standard feed to a house in the united states is two 120v legs or a total of 240v. Some large building will have a higher feed, but they know that the outlets will need to provide a typical house voltage to stay within the standard. So outlet circuits in any place you hold a party should be 120v. Makes things simple ... kinda. Remember a few sentances back when I said `about 120 or less'? Well, electricity loses it's voltage over distance. And after a long run to your house and that long run through your 100' extension cord ... it might not be a full 120v. Plan on 110v being what's actually at the fusebox.
Speaking of the fusebox ... breakers will normally be rated by Amps. If you know the Voltage (we do), and the Amperage of the circuit (we do if we look in the fusebox), then we know how many watts we can put on the circuit. A typical 15Amp, 110v circuit can support 1650 watts. Woo hoo!
In any building or house, a circuit refers to the wires providing power from the fusebox to any appliance, light, or outlet. Each circuit has one breaker in the fusebox. Once you get to commercial or industrial buildings, circuits tend to allow more amperage and cover less outlets per circuit (which is a good thing).
Power enters the building via the fusebox, where it is distributed among the many circuits. There is a breaker that isolates the fusebox from the main power feed, and there are breakers that isolate each circuit from the main power feed. The purpose of the main breaker is to keep the total power usage of the entire building from exceeding a certain amperage. Each of the smaller circuit breakers prevent each circuit from exceeding a smaller amperage. This places two limitations on your power needs. You can't exceed the main breakers amperage or the individual circuit breakers amperage. If you stay under the smaller circuit breakers amperage limit, you should never have to worry about popping that mighty big main breaker. If the main breaker pops due to heavy draw, the entire building will lose power, so it's worth the simple calculation if you can make sure you'll stay under it's limit.
At this point you might ask `Why do we have fuse boxes that limit our power draw?' It is very good that you ask. If you draw 20Amps of power over a 10Amp extension cord, there is a good chance the cord will melt, spark, or even catch on fire. Imagine those conditions inside the walls of your house. The breakers prevent you from abusing or damaging the wiring inside your house, and the wiring to the power company overall. On this note I will warn you not to alter or change the wiring or fuses in your house or building unless you are a trained electrician and know what you are doing.
Standard CPU 300w 17" Monitor 139w (60% of 231w) Speakers ---------------------------- Total 439w (or 3.9Amps)
1) Plug the radio/light into the extension cord.
2) Draw a diagram of the outlets in the building.
3) Draw a diagram of the fusebox.
4) Plug the extension cord into an outlet.
5) Start flipping breakers off, then on.
6) When the light/radio goes off, mark the outlet and the breaker with a matching symbol.
7) Repeat until you know what outlets are on what breakers.
If there are large appliances in the area, you may want to note which of them switch off when you flip breakers. Having a heavy duty freezer on your server circuit can cause problems when it's compressor kicks in and steals your power. Try to find out what EVERY breaker does. It's a little extra work but it can help if you know there's an extra outlet somewhere in case you need a back-up circuit during your party. Look at each fuse and there should be an amperage rating on each one. Now you can tell where the power is, and how much there is at each location. A 20Amp circuit will provide you with 2200 watts of power.