The rules of the National Electrical Code are written for people who have a pre-existing knowledge of electricity. In order to make sense of the Code, you must first understand basic electrical concepts such as voltage, amperage, resistance, Ohm's law, wattage, circuit theory and others.
This series of articles on electrical theory is designed to be a refresher course for the electrical professional. In these articles, we will go through all of the basic electrical concepts.
An obvious foundation for all electrical installations is a thorough knowledge of the laws that govern the operation of electricity. The general laws are few and simple, but are applied in an unlimited number of ways.
THE THREE PRIMARY FORCES
The three primary forces in electricity are voltage, current flow and impedance (resistance). They are the fundamental forces that control every electrical circuit everywhere.
Voltage is the force that pushes the current through electrical circuits. The scientific name for voltage is electromotive force, and is represented in formulas with the capital letter “E” (sometimes also represented as V). It is measured in volts. The scientific definition a volt is the electromotive force necessary to force one ampere of current to flow through a resistance of one ohm.
Voltage is comparable to water pressure. The higher the pressure, the faster water will flow through a system. With electricity, the higher the voltage (electrical pressure), the more current will flow through a system.
Current (which is measured in amperes) is the rate of flow of electrical current. The scientific description for current is intensity of current flow, and is represented in formulas with the capital letter “I.” The scientific definition of an ampere is a flow of 6.25 × 1023 electrons (called one coulomb) per second.
“I” compares with the rate of flow in a water system, which is typically measured in gallons per minute. In simple terms, electricity is thought to be the flow of electrons through a conductor. Therefore, a circuit that has 12A flowing through it will have three times as many electrons flowing through it as a circuit that has a current of 4A.
Impedance is the total opposition to the flow of electricity. Impedance is measured in ohms and represented by the letter “Z.” The scientific definition of an ohm is the amount of resistance that will restrict 1V of potential to a current flow of one ampere. Ohms are represented by the capital Greek letter omega (Ω).
It is important to differentiate between impedance and resistance. Resistance is the more frequently used term in the electrical industry. Unfortunately, it is also the less-accurate term. Impedance better describes the flow of electricity. Resistance is a fine term for a circuit that has no reactance, where voltage and current remain in-phase. In actual use, though, almost all circuits have some reactance; and impedance is almost always the better term. Like impedance, resistance is also measured in ohms, and represented by the letter “R.”
Impedance, a term for total resistance in an alternating current circuit, is very similar to resistance and is measured in ohms. An alternating current circuit contains normal resistance but may also contain certain other types of resistance called reactance, which are found only in AC (alternating current) circuits. This reactance comes mainly from the use of magnetic coils, called inductive reactance; and capacitors, called capacitive reactance. The general formula for impedance is as follows:
Z = √(R2 + [XL-XC]2)
This formula applies to all circuits, and specifically those in which resistance, capacitance and inductance are all present.
The general formula for impedance when only resistance and inductance are present is:
Z = √(R2 + XL2)
The general formula for impedance when only resistance and capacitance are present is:
Z = √(R2 + XC2)
Reactance is the part of total resistance that appears in AC circuits only. Like other resistance, it is measured in ohms. Reactance is represented by the letter “X.” The two types of reactance are inductive reactance and capacitive reactance. Inductive reactance is signified by XL; capacitive reactance by XC.
Inductive reactance is the resistance to current flow in an AC circuit, due to the effects of inductors in the circuit. Inductors are coils of wire, especially those that are wound on an iron core. Transformers, motors and fluorescent ballasts are the most common types of inductors. The effect of inductance is to oppose a change in current in the circuit. Inductance tends to make the current lag the voltage in the circuit. When the voltage begins to rise in the circuit, the current does not begin to rise immediately, but lags behind the voltage. The amount of lag depends upon the amount of inductance in the circuit. In a purely inductive circuit, this would be a 90 deg. lag.
The cosine of the angle between the voltage and current sine waves is the power factor. The formula for inductive reactance is as follows:
XL = 2π FL
In this formula, “F” represents the frequency (measured in Hertz), and “L” represents inductance, measured in Henrys. The higher the frequency, the greater the inductive reactance. Inductive reactance is much more of a problem at high frequencies than at the 60Hz level.
In many ways, capacitive reactance is the opposite of inductive reactance. Inductors resist a change in current, and capacitors resist a change in voltage. The unit of measure for capacitance is the farad. Technically, one farad is the amount of capacitance that would allow you to store one coulomb (6.25 × 1023) of electrons under a pressure of 1V. Because the storage of one coulomb under a pressure of 1V is a tremendous amount of capacitance, the capacitors commonly used are rated in microfarads (millionths of a farad), or picofarads (billionths of a farad). Figure 1 (page 34) illustrates current lead and lag.
Capacitance tends to make current lead voltage in a circuit, while inductance tends to make current lag. This is why capacitors are use to correct power factor in industrial circuits, which are predominantly inductive.
This word play will help you remember current lead and lag: ELI the ICE man. “E” (symbolizing voltage) is ahead of I (symbolizing current). The center letter is L (symbolizing inductance). In an inductive circuit, E leads I. So, the meaning of the “ICE” portion is I leads E in a capacitive circuit.
Capacitors are made of two conducting surfaces (generally some type of metal plate or metal foil), which are just slightly separated from each other. They are not electrically connected. Thus, capacitors can store electrons, but cannot allow them to flow from one plate to the other.
To a DC circuit, a capacitor gives almost the same effect as an open circuit. For the first fraction of a second, the capacitor will store electrons, allowing a small current to flow. But after the capacitor is full, no further current can flow because the circuit is incomplete. However, if the same capacitor is used in an AC circuit, it will store electrons for part of the first alternation, and then release its electrons and store others when the current reverses direction. Because of this, a capacitor, even though it interrupts a circuit, can store enough electrons to keep current moving in the circuit. In a purely capacitive circuit, I leads E by 90 deg. The formula for capacitive reactance is as follows.
XC = 1/2πFC
F is frequency, and C is capacitance, measured in farads.