Before You Start: Understanding Electrical Capacity -- Guide to Basic Wiring Techniques

The wires used for conducting electricity offer low resistance to the flow of electric current, but the friction produced by the electrical flow produces heat. A small amount of heat is normal, but when it reaches a level where the insulation around the wire begins to fail the current must be reduced or turned off to avoid a fire.

Vocabulary of Electricity

To understand electrical capacity, you need to know the terminology that describes the various components of electric power. These words are amperes, volts, and watts. They de scribe common functions and are interrelated in precise ways.

To comprehend this relationship, think of electricity as similar to water flowing through a hose. Like the water, the flow of electricity must be contained, in this case, in a wire. The moving electricity is a current, called amperes, and it moves under pressure, called volts. When you multiply these two elements, amperes (current) X volts (pressure), you get the number of watts, which is a measurement of the energy be mg used, or to state it another way, a measurement of the work being done.

Amperes, typically called amps, is the measurement of the number of electrons flowing past a given point each second. It takes 6.28 billion billion electrons passing a given point each second to produce 1 amp, which is barely enough to ring your doorbell.

Volts measure how much pressure is being used to push electricity through the wire. Thus, the amount of current flowing through a given circuit depends on the amount of voltage being applied to that circuit. Because voltage is a measurement of pressure, it's subject to resistance. This resistance can be measured as well; it's stated in ohms. Voltage alone can't produce work, but when it's applied to a circuit, a current flows and work is produced.

The voltage arriving at a house can fluctuate from 114 to 126 volts. That is why you may see references elsewhere to household voltages other than 120 and 240. Like the NEC, this book refers to voltage as being 120 and 240, the medians of the fluctuations.

Watts measure the amount of power being used at any given moment. Taken separately, neither amperes nor volts can tell you how much power is being used; they work together to produce power, which is expressed in watts. To calculate watts, simply multi ply volts by amps. For example, a standard light bulb drawing 1/2 ampere from a 120-volt circuit uses 60 watts of power (120 volts X 0.50 amps = 60 watts). Because watts are the result of an equation, variable factors can produce the same wattage. For example, that 60 watts in the light bulb is equivalent to the 60 watts produced when a car headlight draws 5 amps from a 12-volt battery (12 volts X 5 amps = 60 watts).

Knowing these concepts and the simple amps X volts = watts formula can help you in many ways as you undertake a wiring project. For instance, if you need to install a new electric clothes dryer, you will have to know what size wire to use and what size circuit breaker to choose to protect the wire. If the clothes dryer is 240 volts and is rated at 7,200 watts, you could calculate the required amperage by changing the A x V = W formula to W / V = A, or 7200 / 240 = A. The answer is 30, so you would use a wire size able to carry 30 amps, which would be No. 10 copper wire, protected by a 30-amp breaker.

Preventing Circuit Overload

Knowing how amps, volts, and watts interrelate can help you prevent the dangerous condition known as circuit overload. When a circuit breaker trips or a fuse blows, it means too much current is being drawn on that circuit. You can calculate the current a circuit will draw simply by adding up the watts of all the lights and appliances being used on it at the same time. (The wattage stated on each of these items). Divide the total watts by 120 volts to get the amperage drawn on that circuit. The resulting value should not exceed the amperage rating marked on the corresponding fuse or circuit-breaker handle. For example, 1,800 watts is the maximum power that can be delivered by a 15-amp, 120-volt circuit (15 amps X 120 volts = 1,800 watts). Exceeding 1,800 watts will overload the circuit. Until that overload is relieved, the overcurrent-protection de vice for that circuit will keep tripping or blowing, protecting you and your property. If you ignore these warnings and re place a blown fuse with one with a higher amperage rating than indicated for the circuit, you create potential conditions for overheating wires.

Of course, this situation can not occur in a house where the circuit breakers or fuses properly correspond to the wire sizes being used. Whichever overcurrent-protection device you have would open the circuit long before there was any danger, warning you some thing was wrong. Don’t ignore that warning.

Just as a wire must be protected by a properly sized circuit breaker or fuse, the wire itself must be large enough to handle the load being served by that circuit. This point is crucial and is discussed in greater detail in the “Rough Wiring” section.