Master electrician exam
There are a number of differences between the Journeyman test and the Master electrician exam. Generally speaking, the test for the Master electrician’s exam has more questions, lasts longer and includes more advanced electrical fundamentals and formulas. I have already briefly touched on some of these theories in some of the earlier technical articles, but now it’s time to build on the basics.
A master electrician has to be capable of determining the proper size for grounding conductors.
Grounding conductor sizes are provided in Table [250-122] below. Branch circuit and feeder conductors are protected by fuses or a circuit breaker, and the size of the grounding conductor is based on the rating of the fuse or circuit breaker.
Minimum Size Equipment Grounding Conductor for Grounding Raceway and Equipment
|Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)||Copper||Size (AWG or kcmil)|
Aluminum or Copper-Clad Aluminum*
- Note! Where necessary to comply with Section 250-2(d), the equipment grounding conductor shall be sized larger than this table.
- * See installation restrictions in Section 250-120.
Solve example #1
Let’s look at an example: If a 150 ampere circuit breaker protects a set of #1/0 AWG copper feeder conductors with insulation rated at 758 degrees C, then the equipment grounding conductor shall not be smaller than which of the following:
- #6 AWG copper
- #8 AWG copper
- #10 AWG copper
- #12 AWG copper
In this problem, you start by looking up 150 amperes in Table [250.122] above. Since there is no listing for 150 amperes, you have to use 200 amperes, and choose #6 copper.
When conductors are run in parallel, and each set of conductors is run in a separate nonmetallic raceway, then an equipment grounding conductor is required with each circuit conductor in the raceway.
Solve example #2
Look at the another example below and determine the correct answer:
If a feeder is protected by a set of 300 amp fuses, and runs from one panel to another as two parallel sets of #1/0 AWG copper conductors with 758 degree C insulations, and terminates in separate non-metallic raceways, and a copper equipment grounding conductor runs with each parallel set of conductors, then the minimum equipment grounding size shall be no less than which of the following:
- #4 AWG
- #6 AWG
- #8 AWG
- #12 AWG
Look up the 300 amp overcurrent device in Table [250.122] to find that it would be necessary to run #4 AWG copper grounding conductors in each of the raceways.
Potential difference or voltage drop
One of the next variables you are likely to find not only on the exam, but in the field as well as a master electrician, is Voltage Drop. Voltage drop is the reduction in voltage between the source of power in an electrical circuit and a device that utilizes the power.
Voltage drop is present in all electrical circuits powering any device and must be taken into consideration in circuit design.
The NEC sets guidelines for the maximum voltage drop that is allowed in branch circuits, conductors and feeders. Sections [210.19(A)(1) FPN #1] and [215.2(A) FPN #2] require branch circuit conductors and feeders individually to be sized to prevent a voltage drop larger than 3 percent at the farthest outlet of power or 5 percent combined.
Ohm’s law is used to calculate current, potential difference, and resistance. The potential difference is the voltage drop. Alternating current continually reverses direction in a circuit at 60 hertz (60 cycles per second). The voltage drop in an alternating current (AC) circuit is the product of the current and the impedance (Z) of the circuit.
The equation is:
I (Current) = V (Voltage) / R (Resistance)
|V = I x R||Voltage = Current x Resistance (Ohms)|
|I = V / R||Current = Voltage / Resistance (Ohms)|
|R = V / I||Resistance (Ohms) = Voltage / Curent|
The formula for voltage drop is based on single-phase or 3-phase systems:
- Single phase voltage drop: VD = 2 × I (Current) × R (Resistance)
- Three phase voltage drop: VD = 1.73 × (for 3-phase) × I × R
Resistance in a conductor
Resistance in a conductor is represented by “K ” multiplied by the length of a conductor divided by the cross-sectional area of the conductor, which is represented in circular mils by CM. K can be 12.9 ohms for copper or 21.2 ohms for aluminum.
- K is direct-current constant and it represents the DC resistance for a 1,000-circular mils conductor that is 1,000 feet long, at an operating temperature of 75 degrees C.
- I = Amperes: The load in amperes at 100%
- L = Distance: The distance of the load from the power supply. When calculating conductor distance, use the length of the conductor—not the distance between the equipment connected by the conductor.
- CM = Circular-Mils: The circular mils of the circuit conductor as listed in NEC Chapter 9, Table 8.
The formula for calculating resistance is also based on single-phase or 3-phase.
- Single phase resistance: 2 × K (Resistance) × L (length) 4CM
- Three phase resistance: 1.73 × K (Resistance × L (length) 4CM
Current flows through the non-zero resistance of a practical conductor and produces a voltage across that conductor.
The DC resistance of a conductor depends on the conductor’s length, cross-sectional area, type of material, and temperature. The impedance in an AC circuit depends on the spacing and dimensions of the conductors and the frequency of the current. Electrical impedance, like resistance, is expressed in ohms and opposes the current flow in a circuit.
Electrical impedance is the vector sum of electrical resistance, capacity reactance, and inductive reactance (as illustrated below).
Z = √(R2+XL2)
- Z – Impedance (ohms)
- R – Resistance (ohms)
- XL – Inductance Reactance (ohms)
Reactance (Inductive and Capacitive)
Reactance is the part of total resistance that appears in AC circuits only. Like other resistance, it is measured in ohms. Reactance produces a phase shift between the electric current and voltage in the circuit. Reactance is represented by the letter “X.”
The two types of reactance are Inductive Reactance and Capacitive Reactance.
- If X > 0, the reactance is said to be inductive.
- If X = 0, then the circuit is purely resistive (in other words it has no reactance).
- If X < 0, it is said to be capacitive.
The relationship between impedance, resistance, and reactance is illustrated by the equation:
Z = R + jX
- Z – Impedance in ohms
- R – Resistance in ohms
- X – Reactance in ohms
- “j” – Imaginary unit of √-1
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, typically wires that are wound on an iron core. Transformers, motors and fluorescent ballasts are the most common types of inductors. Inductance opposes a change in current in a circuit and creates a lag in the voltage in the circuit. When the voltage begins to rise in the circuit, the current does not begin to rise immediately.
Instead it lags behind the voltage. It is like when you turn the heat on in your car – the fan starts blowing right away but the warm air takes a minute to start coming out. The amount of lag depends on the amount of inductance in the circuit.
The reactance formula looks like this:
X = XL − XC
The reactance present is proportional to the frequency, which is why there is zero reactance in DC. There is also a phase difference between the current and the actual voltage that gets applied.
The formula for inductive reactance is:
XL = ωL = 2πL
- XL – the inductive reactance measured in ohms.
- ω – the angular frequency, also known as angular speed, which is the measurement of how fast an object (in this case electricity) is rotating, measured in radians per second.
- f – the frequency, which is measured in Hertz.
- L – the inductance, which is measured in Henries. If the rate of change of current in a circuit is one ampere per second and the resulting electromotive force is one volt, then the inductance of the circuit is one henry.
- π – the symbol for Pi. Pi is a mathematical constant equal to approximately 3.1415926535897932.
Capacitive reactance (symbol XC) exists because electrons cannot pass through a capacitor, but effectively alternating current (AC) can pass!
There is also a phase difference between the alternating current flowing through a capacitor and the potential difference across the capacitor’s electrodes. The potential difference is the voltage present between two points, or the voltage drop over an impedance, and represents the energy that would be required to move a unit of electrical charge from one point to the other against any electrostatic field that might exist.
The formula for Capacitive Reactance is the following:
Xc = 1 / ωC = 1 / 2πfC
- XC – the capacitive reactance, which is measured in ohms
- ω – the angular frequency, which is measured in radians per second
- f – the frequency, which is measured in hertz
- C – the capacitance, which is measured in farads. A capacitor has a value of one farad when one coulomb of stored charge creates a potential difference of one volt across the capacitor terminals.
At this point, you might feel like you just completed an electrical engineering course!! While the theories we just covered are not something you are going to have to use everyday in the field, they are the building blocks of how electrical energy moves, so you need to be able to understand them!
Reference // Electrician’s Exam Study Guide (Purchase at Amazon)
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