What is the lighting demand load (before derating) for a house with outside dimensions of 30 ft 48 ft on the first floor and 22 ft 42 ft on the second floor? A one-family dwelling has a 175-ampere, 240-volt service that is fed with 75C (167F) conductors. What is the minimum size copper grounding electrode conductor required?
The correct answer and explanation is :
Lighting Demand Load Calculation:
To calculate the lighting demand load for the house, we use the National Electrical Code (NEC) guidelines for lighting load calculations. For a dwelling, the general lighting load is based on the square footage of the house.
- Calculate the total square footage of the house:
- First floor:
The first floor has dimensions of 30 feet by 48 feet.
Total area of the first floor = 30 ft × 48 ft = 1,440 square feet. - Second floor:
The second floor has dimensions of 22 feet by 42 feet.
Total area of the second floor = 22 ft × 42 ft = 924 square feet. - Total square footage:
Total area = 1,440 sq ft (first floor) + 924 sq ft (second floor) = 2,364 square feet.
- Lighting load based on NEC Table 220.12:
According to the NEC, the general lighting load for a dwelling is calculated at 3 volt-amperes (VA) per square foot.
- Lighting load = 2,364 sq ft × 3 VA/sq ft = 7,092 VA.
Thus, the lighting demand load for the house (before derating) is 7,092 VA or approximately 7.1 kVA.
Minimum Size Copper Grounding Electrode Conductor (GEC):
The grounding electrode conductor size is based on the service size and conductor temperature rating, per NEC Table 250.66.
- Service Size:
The house has a 175-amp, 240-volt service. - Temperature Rating of Conductors:
The conductors are rated at 75°C (167°F). - Find the minimum size GEC using NEC Table 250.66:
According to Table 250.66, for a 175-amp service and 75°C conductor rating, the minimum required grounding electrode conductor size is based on the ampacity of the service. The value for 175 amps corresponds to a 6 AWG copper conductor for the grounding electrode.
Therefore, the minimum size copper grounding electrode conductor required is a 6 AWG copper wire.
Summary:
- Lighting demand load: 7,092 VA or 7.1 kVA.
- Minimum size copper GEC: 6 AWG.