Service Size Matters: Calculating Electrical Load the Right Way

Correctly sizing the electrical service is a cornerstone of a safe, efficient, and code-compliant electrical installation. Undersizing a service can lead to nuisance trips, overheating, and potential safety hazards, while oversizing results in unnecessary costs. Understanding how to calculate the service load according to the National Electrical Code (NEC) is essential for engineers, electricians, and designers.

This article walks through the principles of service load calculation, discusses NEC requirements, and highlights best practices for commercial and industrial projects.

What Is Electrical Service Size?

The service size refers to the rating (in amperes) of the service conductors, equipment, and overcurrent protection devices that deliver power from the utility to the building’s electrical system. It determines the maximum continuous current the system can safely carry.

Why Accurate Service Sizing Is Important

• Safety: Prevents overheating and reduces fire risk

• Reliability: Ensures the electrical system meets load demands without nuisance breaker trips

• Code Compliance: NEC mandates minimum sizing and proper calculation methods

• Cost Efficiency: Avoids overspending on oversized equipment and wiring

NEC Guidelines for Service Load Calculations

The NEC primarily addresses service load sizing in Article 220, with related provisions in Articles 230 and 240. There are two main calculation methods: the Standard Method and the Optional Method (mainly for residential).

The Standard Method (NEC 220.82)

The Standard Method involves a detailed tally of all loads on the premises, including:

• General lighting and receptacles

• Appliances and fixed equipment

• Heating and air conditioning loads

• Motors and special loads

• Demand factors applied to certain loads to reflect realistic usage

This method is typically used for commercial and industrial buildings.

Commercial Load Calculation Example

Consider a small commercial office building with the following data:

• General lighting and receptacles: 10,000 sq ft at 1.5 VA per sq ft (per NEC Table 220.12)

• Small appliance loads: 1,500 VA (break room equipment)

• Heating, Ventilation, and Air Conditioning (HVAC): 30,000 VA total

• Elevator load: 5,000 VA

• Motors (three motors at 10 HP each): Use NEC 430 to determine full-load amps (FLA) and calculate demand

Step 1: Calculate General Lighting and Receptacles
10,000 sq ft × 1.5 VA/sq ft = 15,000 VA

Step 2: Add Small Appliance Loads
Small appliances = 1,500 VA

Step 3: Add HVAC Load
HVAC equipment = 30,000 VA

Step 4: Calculate Motor Load Demand (Using NEC 430)
Assuming 10 HP motors running at 230 V, three-phase:

• Full Load Amps per motor (from NEC Table 430.250): approx. 28 A

• Total motor load amps = 3 × 28 A = 84 A

• Convert amps to VA:
  VA = 1.732 × 230 × 84 ≈ 33,422 VA

Apply demand factor per NEC 220.53(C)(1) for three or more motors:

• Demand factor for 3 motors = 100% of largest motor + 40% of the rest

• Largest motor = 28 A

• Other two motors = 2 × 28 A = 56 A

• Demand amps = 28 + (0.4 × 56) = 28 + 22.4 = 50.4 A

• Convert back to VA:
  1.732 × 230 × 50.4 ≈ 20,066 VA

Step 5: Sum Total Load
Lighting & Receptacles: 15,000 VA
Small Appliances: 1,500 VA
HVAC: 30,000 VA
Motor Demand: 20,066 VA
Total Load: 66,566 VA

Step 6: Convert VA to Amperes
Assuming a 480 V, three-phase service:
I = 66,566 ÷ (1.732 × 480) ≈ 80.1 A

Step 7: Apply Continuous Load Factor
NEC requires adding 125% for continuous loads:
80.1 × 1.25 = 100.1 A

Step 8: Final Service Size Selection
Select the next standard size breaker and conductors. In this case, a 100 A or 125 A service rating would be appropriate based on local utility standards and equipment availability.

NEC Compliance and Equipment Selection

• Service Conductors: Must be sized based on calculated load per NEC 310.15(B)

• Service Equipment Rating: Per NEC 230.79, the service equipment rating must not be less than the calculated load

• Overcurrent Protection: NEC Article 240 mandates sizing breakers and fuses based on load and conductor ampacity

• Voltage Drop: Best practice limits voltage drop to 3% for feeders and services to ensure efficiency and longevity

Conclusion

Accurate service size calculation is vital for commercial electrical systems to guarantee safe operation, code compliance, and cost-effectiveness. By applying NEC demand factors and carefully tallying all loads—including lighting, appliances, HVAC, and motors—engineers and electricians can confidently specify the right equipment and conductors for the job.

References

• NEC 2023:

  • Article 220 – Branch-Circuit, Feeder, and Service Calculations

  • Article 230 – Services

  • Article 240 – Overcurrent Protection

  • Article 310 – Conductors for General Wiring

  • Article 430 – Motors, Motor Circuits, and Controllers

• IEEE Green Book – IEEE Guide for Electric Power Distribution

• NFPA 70B – Recommended Practice for Electrical Equipment Maintenance

Disclaimer:
The information, tools, and resources provided on this website are for general informational purposes only. While Empower Engineering strives to ensure accuracy and relevance, no guarantee is made regarding the completeness or applicability of the content to specific projects. Users are responsible for verifying all calculations, code interpretations, and design decisions with licensed professionals and local authorities. Empower Engineering assumes no liability for any loss, damage, or code violation resulting from the use of this website’s content.

Empower Engineering | July 15, 2025