Guide to Service Size Calculation Questions on NEC Electrical Exams

Service size calculation is a crucial skill for electricians, as it ensures that electrical systems are properly sized to handle the expected load safely and efficiently. This knowledge is essential for passing NEC electrical exams and for real-world applications in electrical design and installation.

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Example Service Size Calculation Questions on NEC Electrical Exams

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A single-family dwelling has the following loads: general lighting (3,500 VA), kitchen appliances (5,500 VA), laundry (1,500 VA), electric heating system (12,000 VA), HVAC (10,000 VA), and an electric vehicle charging station (7,200 VA). Calculate the minimum service size required for this dwelling.

An office building has a calculated load of 85,000 VA. What is the minimum ampere rating for the service-entrance conductors supplying this building, assuming a 208Y/120V, three-phase system?

A residential property has a total calculated load of 27,500 VA. If the local utility provides 240/120V single-phase service, what is the minimum size overcurrent protection device required for the service disconnect?

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How to Identify a Service Size Calculation Question on NEC Electrical Exams

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Key phrases to look out for in service size calculation questions:

• "Calculate the minimum service size"
• "Determine the service rating"
• "Size the main breaker"
• "Ampere rating for service-entrance conductors"
• Multiple load types listed (lighting, appliances, HVAC, electric heating, etc.)

When you spot these elements:

• Confirm it's a service size calculation question, not a branch circuit or feeder calculation
• Refer to NEC Article 220 for load calculations and Article 230 for services

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Service Size Calculation Articles: NEC 220 and 230

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To correctly apply NEC Articles 220 and 230, focus on these main elements:

• Article 220.40: Basic Calculation
• Article 220.42: General Lighting Load
• Article 220.51: Fixed Electric Space Heating
• Article 220.53: Appliance Load
• Article 220.54: Electric Clothes Dryers
• Article 220.55: Electric Ranges and Other Cooking Appliances
• Article 220.60: Noncoincident Loads
• Article 230.42: Minimum Size and Rating
• Article 230.79: Rating of Service Disconnecting Means
• Article 240.6: Standard Ampere Ratings for Overcurrent Protection

Summary of how to use the codebook:

1. Calculate the total load using Article 220
2. Apply any demand factors as specified in the relevant sections
3. Convert the total VA to amperes
4. Use Article 230 to determine the minimum service size 
5. Use Article 240 to determine overcurrent protection rating

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Walkthrough for a NEC Electrical Exam Service Size Calculation Question

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Question: A single-family dwelling has the following loads: general lighting (3,500 VA), kitchen appliances (5,500 VA), laundry (1,500 VA), electric heating system (12,000 VA), HVAC (10,000 VA), and an electric vehicle charging station (7,200 VA). Calculate the minimum service size required for this dwelling.

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Step 1: Identify and sum up all loads

• General lighting: 3,500 VA
• Kitchen appliances: 5,500 VA
• Laundry: 1,500 VA
• Electric heating system: 12,000 VA
• AC: 10,000 VA
• EV charging station: 7,200 VA Total load: 39,700 VA

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Step 2: Apply demand factors

• General lighting, laundry, and small appliances: 220.52(A) and (B) permit the small appliance and laundry loads to be included with general lighting and subjected to the demand factors found in 220.45.
• General Lighting Load: Lighting Load + Small Appliance Load + Laundry = 3,500 VA + 6,000 VA + 1,500 VA = 11,000. Now apply 220.45 demand factors. 11,000 VA  - 3000 VA = 8,000 VA. 7,500 VA x 35% = 2,800 VA. 2,800 VA + 3,000 VA = 5,800 VA 
• Electric heating system: 100% (NEC 220.51)
• AC: Can be eliminated from calculation due to being noncoincident with heat (NEC 220.60) 
• EV charging: 7,200 VA minimum

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Step 3: Add all of the loads together

• General Lighting Load: 5,800 VA
• Heating System: 12,000 VA
• Electric Vehicle Charger: 7,200 VA

5,800 VA + 12,000 VA + 7,200 VA = 25,000 VA

Adjusted total: 25,000 VA

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Step 4: Convert VA to amperes

Assuming 240V single-phase service: 25,000 VA ÷ 240V = 104.2 A

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Step 5: Determine minimum service size

According to NEC 230.79, then select  the next higher standard size using NEC 240.6 which is 110 A.

Therefore, the minimum service size required for this dwelling is 110 A.

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