Service Entrance Calculator

The Service Entrance Calculator determines the minimum electrical service size your building needs — the amperage rating of the main panel that must handle all connected loads safely under NEC guidelines. Enter your building square footage, small appliance circuits, laundry circuits, and appliance ratings (electric range, dryer, water heater, HVAC, EV charger, and other fixed loads), then select your system voltage to get the minimum service size in amps. Secondary outputs break down your calculated load, general lighting load, appliance load, and HVAC load in volt-amperes.

sq ft

Based on outside dimensions of dwelling unit

Minimum 2 circuits required by NEC

VA

Enter 0 if no electric range

VA

Enter 0 if gas dryer or no dryer

VA

Enter 0 if gas water heater

VA

Air conditioning or heat pump load

VA

Electric heating load if applicable

VA

Electric vehicle charger load

VA

Additional fixed appliances total

Results

Minimum Service Size

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Calculated Load

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General Lighting Load

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Appliance Load

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HVAC Load

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Frequently Asked Questions

Do I really need to do a load calculation for every job?

Yes, NEC Article 220 requires load calculations for service entrance sizing and panel load determinations. This ensures electrical safety and code compliance for residential installations.

Why is my calculated load so much higher than my actual electric bill?

Load calculations determine maximum simultaneous demand based on NEC requirements, not actual usage. The calculation includes safety factors and considers worst-case scenarios that rarely occur in real life.

What's the difference between connected load and calculated load?

Connected load is the sum of all electrical equipment ratings. Calculated load applies NEC demand factors and diversity factors to determine the actual expected maximum demand on the electrical system.

Can I use the optional method for all residential load calculations?

The optional method (NEC 220.82) can only be used for single-family dwellings and individual dwelling units with specific conditions. It often results in smaller service sizes than the standard method.

How do I handle a house with both gas and electric appliances?

Only include electric appliances in your load calculation. Gas appliances (gas range, gas dryer, gas water heater) should be entered as 0 VA since they don't contribute to electrical load.

What's the deal with 125% for continuous loads?

NEC requires continuous loads to be calculated at 125% of their rating for conductor and overcurrent protection sizing. This accounts for loads operating for 3 hours or more continuously.

Do electric vehicle chargers count as appliance loads?

Yes, EV chargers are considered fixed appliances and must be included in load calculations. They typically range from 3.8kW (Level 2) to 11.5kW for residential installations.

Can I count diversity factors between different panels?

Diversity factors are already built into NEC demand factor tables. Additional diversity between panels may be considered by engineers but should follow local code requirements and engineering judgment.