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

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

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

To correctly apply NEC Articles 220 and 230 in NEC 2023, focus on these main elements:

  • Article 220.41-45: General Lighting Loads
  • Article 220.51: Fixed Electric Space Heating
  • Article 220.52: Small Appliance and Laundry Loads
  • 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

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 and overcurrent protection

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Walkthrough for a NEC Electrical Exam Service Size Calculation Question

Question: A 900 sq ft. single-family dwelling has the following loads: four kitchen appliances (2,500 VA each), range (15 kW), two small appliances, laundry, electric heating system (10,000 VA), HVAC (12,000 VA), and an electric vehicle charging station (6,000 VA). Calculate the minimum service size required for this dwelling.

Step 1: Identify and sum up all loads

  • General lighting: 900 x 3 VA = 2,700 VA
  • Kitchen appliances: 4 x 2,500 VA = 10,000 VA
  • Range: 1 x 15,000 kW = 15,000 VA
  • Small appliances: 2 x 1500 VA = 3000 VA
  • Laundry = 1 x 1500 VA  = 1500 VA
  • Electric heating system: 10,000 VA
  • HVAC: 12,000 VA
  • EV charging station: 6,000 VA

Step 2: Apply demand factors

  • General lighting, laundry, and small appliances:
    • First 3,000 VA at 100%, 3001 to 120,000 VA at 35% (NEC 220.45)
    • Total General lighting, laundry & small appliances = 2700 + 3000 + 1500 = 7,200 VA
    • Load with demand factors: 3000 + (7,200 - 3000) x 0.35 = 4,470 VA
  • Kitchen appliances:
    • More than four appliances means apply demand factor of 75% on total load (220.53)
    • Load with demand factors = 10,000 VA x 0.75 = 7,500 VA
  • Range
    • More than 8.75 kW apply demand factors based on Table 220.55 Column C and more than 12 kW apply demand factors based on Table 220.55 Note 1 where the maximum demand in Column C shall be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating of individual ranges exceeds 12 kW.
    • Base demand is 8 kW.
    • Additional demand: 5% x (15 / 12) = 15%
    • Load with demand factors is 8,000 x 1.15 = 9,200 VA
  • Electric heating system & HVAC
    • Non coincidental loads where they will not run at the same time so select the largest of the two (220.60)
    • Load with demand factors for both heat and AC: 12,000 VA
  • EV charging:
    • Either 7200 watts (volt-amperes) or the nameplate rating of the equipment, whichever is larger (220.57)
    • Since 6000 VA less than 7200 VA, use 7,200 VA

Step 3: Add up all of the calculated loads

  • General lighting, laundry, and small appliances: 4,470 VA
  • Kitchen appliances: 7,500 VA
  • Range: 9,200 VA
  • Electric heating system & HVAC: 12,000 VA
  • EV charging: 7,200 VA

Adjusted total: 40,370 VA

Step 3: Convert VA to amperes

Assuming 240V single-phase service: 40,370 VA ÷ 240V = 168 A

Step 4: Determine minimum service size

According to NEC 240.6, the next standard size up is 175 A. Minimum service size is 175 A.

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