Measuring Electricity

WATER IS MEASURED in gallons, wheat in bushels, meat in pounds. Electricity cannot be poured into a measure or weighed on scales, but rather is something that must be considered as always in motion.

UNITS OF MEASUREMENT

We need to measure how much electricity flows past a given point at a given moment or in total over a period of time. To arrive at a measurement of electricity, the rate at which a quantity of it flows (coulombs per second, or amperes) and the pressure it’s under when it flows (volts) are combined to arrive at wattage.

Amperes : The absolute measure of a quantity of electricity is the “coulomb.” We can speak of electricity in terms of “coulombs per second” in the way we might speak of water in motion in terms of gallons per second. Few people may recall ever hearing the word “coulomb” because instead of “coulombs per second” we use the simpler term “ampere” (abbreviated “amp”). One ampere of electricity is defined as current flowing at the rate of one coulomb per second. (Note that we don’t say “amperes per second’ but just “amperes’)

An electric current is the flow of electrons past a given point. One ampere is the movement of 6.28 billion billion (6,280,000,000,000,000,000) electrons per second.

Volts Water and air and other substances can be put under pressure which we commonly speak of in terms of pounds per square inch. Electric power is under pressure which is measured in volts (abbreviated “V”). Any ordinary dry cell or flashlight cell, when new, develops a pressure of about 1½ volts. One cell of a car battery develops 2 volts; six cells together develop 12 volts. Most house and farm wiring is at 120 volts for lighting and 240 volts for permanently installed appliances and for motors that run machinery. The voltage at which power is transmitted over high-voltage lines varies from 2,400 volts for short distances to 500,000 or more volts for long distances.

Watts and kilowatts : The total amount of power in a circuit at a given moment is expressed in “watts.” Amperes alone or volts alone don’t tell us the actual amount of power in a circuit. Both must be considered. Amperes and volts together tell us how much power is in a circuit at a given moment : volts x amperes = watts.

This formula is always correct with direct current, but it’s correct only part of the time with alternating current. It’s correct with lamps, ranges, toasters, and similar heating appliances. It’s not correct with motors, or loads with transformers (radio, TV) or with ballasts (fluorescent lamps)—in all these the watts are somewhat less than volts x amperes. For ac load calculations, the NEC uses volts-amperes (VA) instead of watts.

Any wattage may consist of either a low voltage and high amperage, or a higher voltage and lower amperage. A lamp drawing 5 amps from a 12-volt battery consumes 60 watts (5 x 12 = 60); another lamp drawing ‘/2 amp from a 120-volt line also consumes 60 watts (½ x 120 = 60 watts). The voltage and amperage differ widely, but the actual watts or power consumed by the two lamps is the same.

Watts measure power just as horsepower does. As a matter of fact, 746 watts is equal to 1 hp. A motor that delivers 1 hp delivers 746 watts and could just as well be called a 746-watt motor. (It uses more than 746 watts because some power is wasted as heat and it also takes some power to run the motor even when it’s not delivering power). A lamp that uses 746 watts could just as well be called a 1-hp lamp.

A watt is a very small amount of power. When speaking of large amounts of power, it’s simpler to speak of kilowatts (the Greek word “kilo” means thousand). One kilowatt (abbreviated “kW”) is 1,000 watts. We speak of watts, not watts per hour or kilowatts per hour, just as we say that an automobile engine delivers 200 hp, not 200 hp per hour.

Watt-hours and kilowatt-hours : Watts and kilowatts measure the rate at which power is being used at any given moment. Watt-hours and kilowatt-hours, units of energy, measure the total amount of power that has been used during any specified interval of time. One watt used for 1 hour is 1 watthour. Multiplying the watts used by the number of hours gives the watthours. A 60-watt lamp used for 6 hours consumes 360 watthours (60 x 6 = 360). A 2,000-watt room air conditioner used for 2 hours consumes 4,000 watthours (2,000 x 2 = 4,000).

A watthour is a very small amount of energy, so it’s common to speak of kilowatt- hours. A kilowatt-hour is 1,000 watthours. The air conditioner mentioned in the previous paragraph, consuming 4,000 watthours in 2 hours, consumes 4 kilowatt- hours (abbreviated “kWh”). Electric energy from your power supplier is measured and paid for by the kilowatt-hour.

One kilowatt-hour will operate the average clothes washer for about 3 hours. It will operate a 1-hp motor for about an hour or pump about 1,000 gallons of water. It will operate the average stereo about 15 hours, a 50-watt lamp for about 20 hours, or an electric clock for about 3 weeks. One kilowatt-hour costs from 6 to 12 cents depending on your location.

RESIDENTIAL ELECTRICAL POWER

The amount of electricity that you and other consumers of electric power use leads to the cost you pay for electric service from your power company. Average 1998 rates for electric power are given on the next page. You can check your consumption on your electric meter (installed by your power company) and on your monthly statements. Methods to reduce costs include using energy-saving lamps and energy-efficient appliances. Examples of power consumption by various household appliances are given in Table 3-1 below.

Reading your meter : Some meters being installed today provide a digital readout of the total kilowatt-hours used by the household. You read a digital meter just as you do the odometer on your car.

Most meters have dials as shown in Fig 3-1. Two of the pointers move in one direction, the others in the opposite direction. Four dials are shown, but you may have five dials on your meter. To read the dial meter : Assume it’s the beginning of the month; going from left to right simply write down the number that each pointer has passed, as on a clock. The total of the meter in FIG. 3-1 is 1,642 kWh.

FIG. 3-2 shows the same meter a month later. One of the pointers points directly to the 7. Before writing down 7, look at the pointer on the dial to the right; it has not quite reached the zero. Therefore, even though the pointer seems to point directly to 7, it has not actually reached the 7, so write down 6 instead, for a total reading of 2,269 kWh. (If the last pointer were just past the zero, the total would be 2,270.) The difference between the two readings, 627 in this case, represents the number of kilowatt-hours of electricity used during the month.

FIG. 3-1: To read this meter, write down the numbers the pointers have passed.

FIG. 3-2: The meter shown in FIG. 3-1, but now a month later.

Power rates : The rate charged for domestic electric power averages about 8.51 per kilowatt-hour in the United States , but the rate varies greatly. Before the energy crisis of the 1970s, rate schedules were stepped so the more power you used, the lower the average cost per kWh. Now, to encourage conservation of energy, most rates are flat after an initial step, and some are inverted—the more power you use, the higher the average cost per kWh.

Here is an example of an inverted rate structure:

First 100 kWh used per month … 4.8 cents per kWh

All over 100 kWh used per month … 9.2 cents per kWh

The meter reading in Figs. 3—1 and 3—2 showed 627 kWh consumed during the month. The bill is figured this way:

100 kWh at 4.8 cents … $4.80

27 kWh at 9.2 cents … $48.49

627 kWh … Total $53.29

Average per kWh … 8.5 cents

Considering the rate of inflation and fluctuating fuel costs, these rates may not be typical at the time you are reading this.

Operating cost per hour : To find out how much it costs to operate any electrical load for one hour, multiply the watts that the item consumes by the rate in cents per kWh, placing the decimal point five places from the right to arrive at the cost in dollars per hour.

Some examples of operating costs:

ELECTRICAL LOAD | WATTS X RATE | OPERATING COST IN DOLLARS PER HOUR

60-watt lamp at 8.5 cents per kWh | 60 x 8.5 = 510 | $0.0051 (slightly more than a half-cent per hour)

600-watt appliance at 8.5C per kWh | 600 X 8.5 = 5100 | $0.05 1 (5.1 cents per hour)

How long does it take a particular electrical load to consume one kWh? To find out, simply divide 1,000 by the wattage of the load to get the number of hours, as in the following examples.

ELECTRICAL LOAD | 1000 ÷ WATTS | TIME IT TAKES TO CONSUME ONE KILOWATTHOUR

40-watt lamp 1000 ÷ 40 = 25 | 25 hours

600-watt appliance | 1000 ÷ 600 = 1.666 | 1.666 hours (about 1 hr 40 mm)

Electric clock using about 2 watts | 1000 ÷ 2 = 500 | 500 hours

Watts consumed : The following table will aid you in estimating power required (watts consumed), or the operating cost, for various appliances. The figures are only approximate, and new energy-saving appliances are appearing all the time. Appliances often have their total wattage (representing the load on the circuit) listed on their nameplates.

Table 3-1 WATTS CONSUMED BY VARIOUS APPLIANCES

TYPES OF ELECTRIC CURRENT

Current is either alternating or direct. On batteries, one terminal is always positive (+) and the other terminal is negative (-). The type of current characterized by each wire being always of the same polarity—either positive or negative—is known as direct current (dc). Current from a battery is always dc. The current coming into your home or farm is alternating current (ac).

Alternating current (ac) : In alternating current each wire changes or alternates continually between positive and negative. The change from positive to negative and back again to positive is a “cycle.” This takes place 60 times every second, and such current is known as 60-hertz or 60-Hz current. The term “hertz” is used instead of “cycles per second?’ It’s named for the German scientist Heinrich Hertz who discovered the cyclical nature of electrical waves.

Sixty times every second each wire is positive, and 60 times every second it’s negative, and 120 times every second there is no voltage at all on the wire. The voltage is never constant but is always gradually changing from zero to a maximum of usually about 170 volts, but averaging 120 volts, and such current is known as 120-volt current (see FIG. 3-3).

FIG. 3-3 : One cycle of 120-volt alternating current. If it’s 60-Hz current, all the changes shown take place in 1/60 second.

You might wonder why lights don’t flicker if there is no current in the wire 120 times every second. The filament in the lamp does not cool off fast enough to create flickering, but very small lamps used on 25-Hz current (where there is no voltage on the wire 50 times every second) do have an annoying flicker.

Single-phase and three-phase current : The current described in the previous paragraph is single-phase current. Remember that if 120-volt, 60-Hz current flows in a pair of wires, 120 times every second the wires are dead (no voltage at all); 120 times every second the voltage is about 170 volts; at all other times it’s somewhere in between, but averaging 120 volts. The voltage is always changing, but the changes are so rapid that for most purposes it can be considered a steady 120-volt current. Single-phase power may be supplied with either two or three wires from the power supplier’s transformers.

Three-phase power is used in factories and commercial establishments where there are many motors. It’s seldom found in homes or farms. To understand how three- phase power is generated, imagine three separate electric generators all on a single shaft, arranged so the voltage reaches its maximum at different times in each of the three generators : first in one, then in the second, then in the third, then again in the first, and so on. A pair of wires would run from each of the three generators. The three generators together then are said to deliver three-phase power (although the power from any one generator is still single-phase). In actual practice, the three generators become a single generator with three coils of wire (windings); the three pairs of wires become three wires.

Three-phase power requires three-wire instead of two-wire high-voltage trans mission lines and three transformers instead of one. (Three-phase power may be supplied with either three or four wires from the transformers.) Don’t be misled into thinking that because there are three wires in the service entrance, the result must be three-phase power. On farms and in homes, the presence of three wires almost always means single-phase, three-wire, 120/240-volt service. If you are fortunate enough to have three-phase power available, by all means use three-phase motors for greater efficiency. If you do have a 240-volt, three-phase supply, consult the power supplier to determine whether you also have 120/240-volt, single-phase power for lighting and small appliances without installing your own transformer.

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Monday, 2008-12-29 16:59 PST