Whether your city or state uses companies like Prometric, Pearson Vue, Continental Testing or the International Code Council to administer your examination, sharpening your test taking skills is the key to passing your electrical exam.
You’ll need to know how to:
Use formulas that may not be referenced in your code book to answer exam questions.
Example:
What is the horsepower rating for a 208-volt three-phase induction type motor with and efficiency rating of 80%, a power factor of .85 and a current rating of 30 amps?
Solution:
Identify the Hp formula required when the voltage, amperage, efficiency and power factor are known.
Hp = I x E x 1.732 x Eff x Pf/746 watts*
*746 watts = 1 Hp
Plug known values provided in the question into the formula.
Hp = 30 amps x 208 volts x 1.732 x .80 x .85/746 watts
Solve the equation.
Hp = 30 amps x 208 volts x 1.732 x .80 x .85/746 watts
Hp = 9.85
Use the key word or phrase in a question to locate the answer in your code book.
Example:
What is the minimum size inverse time breaker required to protect a circuit supplying a 5 hp 230 volt single-phase a.c. motor?
Key word: Motor
This key word reveals that the code rules found in Article 430 will apply because when it comes to motor calculations the rules found in Article 430 take precedence over rules found in other parts of the code.
Key phrase: Inverse time breaker
This key phrase reveals that what you are really looking for is the overcurrent protection rating for the breaker that will protect this motor branch circuit.
Key phrase: Minimum size
This word indicates that there is a minimum sizing method for determining the size of overcurrent protection required.
Solution:
Refer to NEC Article 430-52 (B) Rating or Setting for Individual Motor Circuit-All Motors. “The motor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor.”
Since the starting or locked-rotor current of a motor is six (6) times its f.l.c. (full load current) and the f.l.c for this motor is 28 amps, the locked rotor current for this motor is 168 amps.
Since a breaker will allow three (3) times its rating to flow for starting purposes, dividing 168 amps by 3 = 56 amps. Since 56 amps is not a standard size breaker, the next higher standard size is 60 amps.
Preparation is the key to being able to answer these kinds of questions quickly and easily. We’ll help you to understand exam question terminology and develop the test taking skills you’ll need to succeed. Call 888-919-3926 to start preparing to pass your exam today.