Why Panel Sizing Matters

An undersized electrical service panel causes tripped breakers, overheating, and potential fire hazards. An oversized panel wastes money. NEC Article 220 provides the methods for calculating the correct service size — and getting it right is both a safety and code compliance requirement.

Most new single-family homes are built with 200A service. Some smaller homes use 100A, and larger homes with electric vehicles, pools, or whole-home generators may require 320A or 400A service.

The Standard Calculation Method (NEC 220.82)

The standard method calculates total connected load, then applies demand factors.

Step 1: General Lighting and Receptacle Load

Multiply the total square footage of the home by 3 VA per square foot (NEC Table 220.12).

Example: 2,000 sq ft × 3 VA = 6,000 VA

Step 2: Small Appliance and Laundry Circuits

Add 1,500 VA for each required small appliance circuit (minimum 2 in kitchen) and 1,500 VA for the laundry circuit.

Example: (2 × 1,500) + 1,500 = 4,500 VA

Step 3: Apply Demand Factor to General Load

NEC 220.82(B) allows a demand factor:

  • First 3,000 VA at 100%
  • Remainder at 35%

Example: Total general load = 6,000 + 4,500 = 10,500 VA
First 3,000 VA × 100% = 3,000 VA
Remaining 7,500 VA × 35% = 2,625 VA
General load total = 5,625 VA

Step 4: Add Fixed Appliances

Add the nameplate rating of fixed appliances: dishwasher, disposal, water heater, dryer, range, etc. For electric ranges over 8.75 kW, NEC Table 220.55 allows demand factors.

Example: Range (8 kW) + Dryer (5 kW) + Water Heater (4.5 kW) + Dishwasher (1.2 kW) = 18.7 kW = 18,700 VA

Step 5: Add HVAC Load

Add the larger of the heating or cooling load (they don't run simultaneously).

Example: A/C unit = 5 kW = 5,000 VA

Step 6: Calculate Total and Convert to Amps

Total VA = 5,625 + 18,700 + 5,000 = 29,325 VA

For a 240V single-phase service: Amps = 29,325 VA ÷ 240V = 122A

The next standard service size above 122A is 150A or 200A. A 200A panel is the recommended choice for a home of this size, providing room for future loads like an EV charger.

The Optional Method (NEC 220.82)

For existing dwelling units being upgraded, NEC 220.82 offers a simpler optional method. It uses a flat 100% demand on the first 10 kVA of total load, then 40% on the remainder. This method often results in a smaller calculated load and can justify a 150A service where the standard method would require 200A.

Common Mistakes

  • Forgetting to include the EV charger (a 48A Level 2 charger adds significant load)
  • Using watts instead of VA — they are equal for resistive loads but differ for motors
  • Not accounting for future additions like a hot tub, shop equipment, or solar inverter
  • Applying demand factors incorrectly — they apply to the general lighting load, not all loads