Licensed Electrical Contractor Code, Theory, and Calculations 4

Question 1

According to NEC 430.52, what is the maximum rating of an inverse-time circuit breaker used as short-circuit protection for a 10 HP, 230 V, single-phase motor with a full-load current of 50 A?

Accepted Answer

150 A

Answer

NEC Table 430.52 allows inverse-time breakers at 250% of FLC for single-phase motors; 50 A × 250% = 125 A, rounded up to the next standard size of 150 A.

Question 2

What does the term 'demand factor' mean in electrical load calculations?

Accepted Answer

The ratio of the maximum demand to the total connected load

Answer

Demand factor is the ratio of the maximum demand of a system to the total connected load, accounting for the fact that not all loads operate simultaneously at full capacity.

Question 3

Under NEC 700.12, emergency system power sources must be arranged to supply loads within:

Accepted Answer

10 seconds

Answer

NEC 700.12 requires that emergency power sources supply power within 10 seconds of normal power failure.

Question 4

Two capacitors of 20 µF and 30 µF are connected in series. What is the total capacitance?

Accepted Answer

12 µF

Answer

Series capacitance: 1/Ct = 1/20 + 1/30 = 3/60 + 2/60 = 5/60, so Ct = 12 µF.

Question 5

NEC 310.15(B)(3)(a) requires conductor ampacity correction when more than 3 current-carrying conductors are in a raceway. How does adding a 4th current-carrying conductor affect the allowed ampacity?

Accepted Answer

Reduce to 80% of the base ampacity

Answer

NEC Table 310.15(B)(3)(a) requires a 0.80 correction factor (80%) when 4 to 6 current-carrying conductors are bundled together.

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Practice Test

Licensed Electrical Contractor Code, Theory, and Calculations 4