CSLB General Building (B) — All Questions

A load draws 80 kW of real power and 100 kVA of apparent power. What is the power factor?

• a.1.0
• b.0.8✓
• c.0.6
• d.0.9

Power factor = real power / apparent power = 80 kW / 100 kVA = 0.8.

For a load of 80 kW real and 100 kVA apparent, what is the reactive power?

• a.80 kVAR
• b.100 kVAR
• c.60 kVAR✓
• d.40 kVAR

Reactive power = sqrt(kVA^2 - kW^2) = sqrt(100^2 - 80^2) = sqrt(3,600) = 60 kVAR.

What is the real power of a three-phase load at 480 V, 50 A, and 0.85 power factor?

• a.24,960 W
• b.41,568 W
• c.30,000 W
• d.35,326 W✓

P = 1.732 x V x I x PF = 1.732 x 480 x 50 x 0.85 = 35,326 W.

In a balanced wye (star) three-phase system, the line current is:

• a.Equal to the phase current✓
• b.1.732 times the phase current
• c.0.577 times the phase current
• d.2 times the phase current

In a wye connection the same conductor carries phase and line current, so line current equals phase current.

In a balanced delta three-phase system, the line current equals:

• a.The phase current
• b.1.732 times the phase current✓
• c.0.577 times the phase current
• d.3 times the phase current

In a delta connection the line current is the square root of 3 (1.732) times the phase (winding) current.

In a 208Y/120-V wye system, what is the phase (line-to-neutral) voltage?

• a.208 V
• b.240 V
• c.120 V✓
• d.277 V

Line-to-neutral voltage = line-to-line / 1.732 = 208 / 1.732 = 120 V.

In a 480Y/277-V system, what is the line-to-neutral voltage?

• a.120 V
• b.240 V
• c.208 V
• d.277 V✓

Line-to-neutral = 480 / 1.732 = 277 V.

An 80-kW load at 0.80 power factor is to be corrected to 0.95. How many kVAR of capacitance are required (tan 36.87 = 0.75, tan 18.19 = 0.329)?

• a.33.7 kVAR✓
• b.60 kVAR
• c.48 kVAR
• d.20 kVAR

Qc = kW x (tan(theta1) - tan(theta2)) = 80 x (0.750 - 0.329) = 80 x 0.421 = 33.7 kVAR.

In a three-phase, four-wire wye system serving nonlinear loads, which harmonic order adds arithmetically in the neutral?

• a.2nd
• b.3rd✓
• c.5th
• d.7th

Triplen harmonics, principally the 3rd, are in phase across all three phases and add in the neutral rather than canceling.

On a feeder with significant harmonic (nonlinear) load, the neutral conductor is treated as:

• a.Not a current-carrying conductor
• b.A half current-carrying conductor
• c.A current-carrying conductor✓
• d.An ignored conductor

Where the major portion of the load is nonlinear, the neutral carries harmonic current and must be counted as a current-carrying conductor for adjustment.2023 NEC 310.15(E)

A three-phase panel has phase loads of A = 40 A, B = 30 A, C = 20 A. What is the ideal balanced current per phase?

• a.40 A
• b.20 A
• c.25 A
• d.30 A✓

Total = 40 + 30 + 20 = 90 A; balanced across three phases = 90 / 3 = 30 A per phase.

A single-line (one-line) diagram represents an electrical system by:

• a.One line for each circuit or set of conductors✓
• b.Each individual conductor drawn separately
• c.The physical layout of raceways
• d.Only the branch-circuit wiring

A one-line diagram uses a single line and standard symbols to represent circuits and equipment, simplifying analysis of the power system.

A transformer steps 480 V down to 120 V. What is its turns (voltage) ratio?

• a.2:1
• b.4:1✓
• c.1:4
• d.8:1

Turns ratio = primary voltage / secondary voltage = 480 / 120 = 4:1.

A transformer has a 4:1 turns ratio and a 10-A primary current. What is the secondary current (ideal)?

• a.10 A
• b.2.5 A
• c.40 A✓
• d.20 A

Current is inversely proportional to the turns ratio: Is = Ip x (Np/Ns) = 10 x 4 = 40 A.

An AC circuit at 240 V draws 12 A. What is the circuit impedance?

• a.2 ohms
• b.0.05 ohms
• c.288 ohms
• d.20 ohms✓

By Ohm's law for AC, Z = V / I = 240 / 12 = 20 ohms.

A continuous load draws 40 A. At what minimum ampacity must the branch-circuit conductors and overcurrent device be rated?

• a.50 A✓
• b.40 A
• c.32 A
• d.45 A

Continuous loads require 125% sizing: 40 A x 1.25 = 50 A for the conductors and overcurrent device.2023 NEC 210.20(A)

The demand factor of a system is defined as:

• a.A value always greater than 1
• b.Maximum demand divided by total connected load✓
• c.Connected load divided by maximum demand
• d.Always equal to 1

Demand factor = maximum demand / total connected load; it is 1 or less and reflects load diversity.

What are the commonly recommended maximum voltage-drop limits for a branch circuit and for the combined feeder plus branch circuit?

• a.5% and 8%
• b.2% and 4%
• c.3% and 5%✓
• d.1% and 3%

The informational recommendation is 3% maximum on a branch circuit and 5% maximum for feeder plus branch circuit combined.