Journeyman Electrician’s Exam Journeyman Electrician's Service and Feeder Calculations 4

Question 1

A 3-phase, 4-wire, 208Y/120V service supplies a balanced 3-phase load of 90 kVA. What is the line current?

Accepted Answer

180 A

Answer

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?

Accepted Answer

100 A

Answer

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?

Accepted Answer

Small appliance circuits

Answer

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?

Accepted Answer

Balanced three-phase loads

Answer

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?

Accepted Answer

33.75 A

Answer

Per NEC 430.24, the feeder must be at least 125% of the largest motor FLC: 27A × 1.25 = 33.75A.

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Practice Test

Journeyman Electrician’s Exam Journeyman Electrician's Service and Feeder Calculations 4