Journeyman Electrician's Service and Feeder Calculations Flashcards
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A single-family dwelling has a 200-amp, 240V service. What is the minimum service entrance conductor size required using 75°C copper conductors per NEC Table 310.12?
Answer: 2/0 AWG copper
NEC Table 310.12 allows 2/0 AWG copper at 75°C for a 200-amp service entrance.
When calculating the demand load for electric ranges rated over 12 kW but not exceeding 27 kW in a dwelling, the NEC allows a maximum demand of 8 kW for one range, plus what percentage per additional kW over 12 kW?
Answer: 5%
Per NEC Table 220.55, for ranges over 12 kW, add 5% for each additional kW above 12 kW.
A commercial building has 50 kVA of fluorescent lighting. Using standard demand factors, what minimum amperage feeder is required on a 208Y/120V, 3-phase system?
Answer: 139 A
50,000 VA ÷ (208V × 1.732) = 138.8 A, so a 139 A minimum feeder is required.
Per NEC 220.82, the optional calculation for a single-family dwelling applies the first load increment of 10 kVA at 100%, then applies what demand factor to the remainder?
Answer: 70%
NEC 220.82(B) applies 100% to the first 10 kVA and 40% to the remainder of the total connected load.
What is the minimum feeder size for a 60A, 240V single-phase load that operates continuously for more than 3 hours?
Answer: 75 A
Continuous loads must be sized at 125%, so 60A × 1.25 = 75A minimum feeder rating.
In a multi-family dwelling with 20 units, each having a 12 kW electric range, what demand factor does NEC Table 220.55 Column C assign to 20 ranges?
Answer: 38%
NEC Table 220.55 Column C lists a 38% demand factor for 20 electric ranges.
When sizing a service for a 2,400 sq ft single-family dwelling using the standard calculation method, what is the minimum general lighting load at 3 VA per square foot?
Answer: 7,200 VA
2,400 sq ft × 3 VA/sq ft = 7,200 VA for the general lighting load.