Journeyman Electrician's Service and Feeder Calculations Flashcards

A feeder supplies a motor with a 40A full-load current and a 30A continuous non-motor load. What is the minimum feeder ampacity required?

Answer: 87.5 A

Motor feeder = 40A × 125% = 50A; continuous non-motor = 30A × 125% = 37.5A; total = 87.5A minimum.

What NEC section requires that the neutral conductor of a 3-wire, single-phase service be sized to carry the maximum unbalanced load?

Answer: NEC 220.61

NEC 220.61 addresses feeder and service neutral load calculations including the maximum unbalanced load.

A 240V, single-phase dryer rated at 5,000W is installed in a dwelling. What demand load does NEC 220.54 assign to a single dryer?

Answer: 5,000 W

NEC 220.54 requires the nameplate rating or 5,000W, whichever is larger, so 5,000W applies.

When multiple feeder conductors are installed in parallel, each conductor in the parallel set must be what minimum size?

Answer: 1/0 AWG

NEC 310.10(H) requires each parallel conductor to be a minimum of 1/0 AWG.

A commercial occupancy has 10,000 sq ft of office space. Using NEC Table 220.12, what is the general lighting load at 3.5 VA per square foot?

Answer: 35,000 VA

10,000 sq ft × 3.5 VA/sq ft = 35,000 VA for office general lighting.

For dwelling unit services, NEC 220.83 (optional calculation) requires that air conditioning or heat be included at what percentage when calculating the total load?

Answer: 100%

Under NEC 220.83, the larger of heating or cooling loads is included at 100% in the optional calculation.

What is the calculated feeder demand for six 10 kW household electric ranges in a multi-family building per NEC Table 220.55?

Answer: 24 kW

NEC Table 220.55 Column C assigns a demand factor of 43% for 6 ranges: 60 kW × 0.43 = 25.8, but the table directly lists 24 kW for 6 ranges.