Journeyman Electrician’s Exam Journeyman Electrician's Service and Feeder Calculations 4 — Questions and Answers
Question 1: A 3-phase, 4-wire, 208Y/120V service supplies a balanced 3-phase load of 90 kVA. What is the line current?
- 180 A (Correct answer)
- 250 A
- 150 A
- 216 A
Correct answer: 180 A
I = VA ÷ (V × √3) = 90,000 ÷ (208 × 1.732) = 249.9 ÷ 1.38 ≈ 249.9; corrected: 90,000 ÷ 360.3 = 249.9 ≈ 250 A — answer B.
Question 2: Per NEC 230.79, a single-family dwelling service must have a minimum rating of what amperage?
- 60 A
- 100 A (Correct answer)
- 150 A
- 200 A
Correct answer: 100 A
NEC 230.79(C) requires a minimum 100-ampere service for single-family dwellings.
Question 3: When using the optional calculation for an existing dwelling per NEC 220.82, which loads are NOT included in the total connected load?
- Air conditioning
- Electric range
- Small appliance circuits (Correct answer)
- Electric space heating
Correct answer: Small appliance circuits
Under NEC 220.82, small appliance branch circuits are included as a set load, but the calculation uses nameplate ratings and specific listed loads — all mandatory loads are included.
Question 4: A feeder neutral is allowed a 70% demand factor on that portion of the unbalanced load exceeding 200A per NEC 220.61(B). This reduction applies to what types of loads?
- Non-linear loads only
- Balanced three-phase loads (Correct answer)
- Discharge lighting and similar loads
- Resistive heating loads
Correct answer: Balanced three-phase loads
NEC 220.61(B) allows the 70% neutral reduction on the portion exceeding 200A for other than non-linear loads and discharge lighting.
Question 5: A 480V, 3-phase motor has a full-load current of 27A. What is the minimum feeder ampacity required for this motor alone?
- 27 A
- 30 A
- 33.75 A (Correct answer)
- 40 A
Correct answer: 33.75 A
Per NEC 430.24, the feeder must be at least 125% of the largest motor FLC: 27A × 1.25 = 33.75A.
Question 6: When calculating the service load for a dwelling with a 10 kW heat pump and a 6 kW electric furnace, which load is used in the service calculation?
- Both loads added together
- The heat pump load only
- The larger of the two loads (Correct answer)
- The smaller of the two loads
Correct answer: The larger of the two loads
NEC 220.82(C)(5) requires that the larger of the heating or cooling load be used, not both simultaneously.
Question 7: A 120/240V single-phase service feeder supplies two 240V loads of 8,000W each and a 120V load of 3,000W. What is the neutral current?
- 0 A
- 12.5 A (Correct answer)
- 25 A
- 37.5 A
Correct answer: 12.5 A
The 240V loads place no current on the neutral; the 120V load = 3,000W ÷ 120V = 25A on one leg, so neutral carries 12.5A unbalanced current.
Journeyman Electrician’s Exam Journeyman Electrician's Service and Feeder Calculations 4
A 3-phase, 4-wire, 208Y/120V service supplies a balanced 3-phase load of 90 kVA.
What is the line current?