Guide on General Lighting Load Calculations
Understanding general lighting load calculations is crucial for electrical professionals. These calculations determine the minimum electrical capacity required for a dwelling's lighting and general-use receptacles. Accurate calculations ensure safe and adequate power distribution, preventing overloads and potential hazards. This topic frequently appears on NEC electrical exams, making it essential for aspiring electricians, contractors, and electrical engineers to master.
Example General Lighting Load Questions on NEC Electrical Exams
Calculate the demand load, in VA, on the ungrounded service-entrance conductors for the general lighting and receptacle loads in a single-family dwelling to be constructed that has 3,500 square feet of livable space with two (2) small-appliance branch circuits and one (1) branch circuit for the laundry room
Calculate the general lighting load for a 4,200 square foot single-family home with three small appliance branch circuits and one laundry circuit. What is the total demand load in VA?
A triplex has 2,000 square feet of living space per unit. Each unit has two small appliance branch circuits and one laundry circuit. What is the combined general lighting load for all three units in VA?
How to Identify a General Lighting Load Question on NEC Electrical Exams
Key phrases to look out for in the general lighting load question:
- "General lighting load"
- "Demand load for lighting and receptacles"
- "Square footage" or "livable space"
- "Small appliance branch circuits"
- "Laundry circuit"
When you spot these elements:
- Confirm it's a general lighting load question, not a specific circuit or appliance load question
- Refer to NEC sections 220.41, 220.45, 220.52
General Lighting Load Articles: NEC 220.41, 220.45, 220.52 (NEC 2023)
To correctly apply NEC Articles 220.41, 220.45, and 220.52, focus on these main elements:
- General Lighting Load (220.41): 3 VA per square foot for dwelling units
- Demand Factors (220.45): Apply demand factors to the calculated load
- Small Appliance and Laundry Loads (220.52): Include these in the calculation
Summary of how to use the codebook:
- Use 220.41 to determine the unit load per square foot (3 VA for dwellings)
- Apply Table 220.45 for demand factors
- Add loads for small appliance and laundry circuits as per 220.52
Walkthrough for a NEC Electrical Exam General Lighting Load Question
Question:
Calculate the demand load, in VA, on the service-entrance conductors for the general lighting and receptacle loads in a two-family dwelling to be constructed, given the following information:
- 3,500 square feet of livable space per unit
- 500 square foot patio
- 1,500 square foot shared basement
- 800 square foot shared garage
- Two (2) small-appliance branch circuits and one (1) branch circuit for the laundry room per unit
Step 1: Determine the total square footage
- Include livable space for both units and the basement
- (3,500 sq. ft. x 2) + 1,500 sq. ft. = 8,500 sq. ft.
- Note: Garage and patio are not included in this calculation (NEC 220.5)
Step 2: Calculate the general lighting load (NEC 220.41)
- Multiply total square footage by 3 VA/sq. ft.
- 8,500 sq. ft. x 3 VA/sq. ft. = 25,500 VA
Step 3: Add small appliance and laundry circuit loads for both units (NEC 220.52)
- Four small appliance circuits: 4 x 1,500 VA = 6,000 VA
- Two laundry circuits: 2 x 1,500 VA = 3,000 VA
- Total additional load: 6,000 VA + 3,000 VA = 9,000 VA
Step 4: Calculate total connected load
- General lighting load + Small appliance and laundry loads
- 25,500 VA + 9,000 VA = 34,500 VA
Step 5: Apply demand factors (NEC 220.45)
- First 3,000 VA at 100%: 3,000 VA
- Remaining load (34,500 VA - 3,000 VA = 31,500 VA) at 35%: 31,500 VA x 0.35 = 11,025 VA
Step 6: Calculate final demand load
- Sum of demand factors: 3,000 VA + 11,025 VA = 14,025 VA
Therefore, the demand load on the service-entrance conductors for the general lighting and receptacle loads is 14,025 VA.