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Last Updated: , Created: Saturday, October 20th, 2001

How do you measure power coming into your house?

click to enlarge

You can get an idea of the total power possible to bring into the house by looking at the main fuses or breakers -- not the set of ordinary ones that you replace or reset often. The main breakers are in the top of, or above, or between the regular electrical box and the electrical meter. There will be two large fuses or breakers, probably rated 60 amps, 100 amps or 200 amps. That is the limit for the whole house and is what is used to say you have a "100 amp entrance". But that does not measure how much power is actually being used in the house, only the maximum flow of electricity you can get.

Measuring voltage with a simple volt meter does not tell us how much power we are using either. The math goes as follows:

W = V x A or Watts (power) = Volts (pressure) x Amps (flow)

A water hose makes a good analogy to electrical flow. Volts is the same as the pressure in the water line. A line with low pressure simply won't squirt water very far. More pressure will squirt the same stream of water much further. Amps is the same as the amount of water going by a given point at one instant in time. So given the same pressure, a little garden hose will have a certain diameter of water stream, while a fireman's hose will have a much larger diameter of water stream. Watts, or the power, is the combination of the two, and you could think of this as the quantity of water it takes to fill up the pool. A fireman may fill up the pool faster than the garden hose, but you both end up delivering the same amount of power.

So to measure instantanious power (not power consumption, that is how much power we use over a period of time) we need to measure the voltage and the amperage, and then multiply the two to find out how many watts we are using. In the old days an amp meter was a difficult to use instrument that required cutting into the wire, installing a calibrated "shunt" that would let most of the electricity flow normally, while a tiny but well calculated portion of that electricity would flow through the meter itself. Today we have "inductive amp meters" as you see in the photo that are much easier to use. You must, however, isolate a single wire (placing the meter over a bundle will give no results) and then place the claw type of gadget over the wire and centre the wire in the middle of the loop. Then you can directly read the amperage going through that wire at any given instant.

The electrical company multiplies watts by time to charge you for kilo (thousand) watt hours, how much power you actually consumed. This tells you not just how fast or powerful was the electricity you used, but the total consumption of electricity (or shall we say, how much water you actually put into the swimming pool). The electrical meter, the one slowly turning around and around, changes its speed as you use more or less electricity, instant by instant, and keeps track of time as well.

So the real answer to the question "How do you measure power coming into your house?": you don't measure it directly, you calculate it by measuring other things -- or you read the electrical meter which does all of that for you. In experimental houses, we often install a large number of electrical meters to allow us to isolate the electrical consumption of each element of the house.

Keywords: Electrical, Wire, Wiring, Measuring

Article 1375