NEC Load Calculator: Sizing an Electrical Service

The NEC load calculator works out the minimum electrical service size (in amps) for a dwelling or building using the National Electrical Code (NFPA 70) standard method. You enter the floor area, the number of small-appliance and laundry circuits, and the major appliance loads (or their VA ratings), and it returns the total calculated load in volt-amperes (VA) and the recommended service size in amperes at 120/240V. This is the same math an electrician uses to decide whether a 100A, 150A, or 200A panel is code-compliant for a house.

Formula

The NEC standard method for a dwelling (Article 220, Part III) adds up loads in this order:

• General lighting and receptacle load: 3 VA per square foot of floor area (exclude unfinished basements, garages, and open porches).
• Small-appliance and laundry circuits: 1,500 VA for each 20-amp small-appliance circuit in the kitchen, plus 1,500 VA for the laundry circuit.
• Appliance loads: nameplate VA for the range, dryer, water heater, dishwasher, disposal, and so on. The largest motor (usually the A/C compressor or heat pump) gets a 25% bump per 430.24.
• First 8,000 VA of the total: take 100%. The next 12,000 VA: take 40%. Anything above 20,000 VA: take 40% as well (the demand factor).

In our calculator, the expression 3000 + a * 3 + na * 1500 + max(la, oa) is a simplified version: it applies the 3 VA/sq ft, adds the 1,500 VA circuits, and takes the larger of the motor or other loads (since the 25% motor bump is already included in the largest-motor value). It does not apply the 40% demand factor to the portion above 8,000 VA, so it overestimates for large homes—use the full NEC method for a final design.

Worked Example

Using the default values: floor area = 2,000 sq ft, 3 small-appliance/laundry circuits, largest motor = 1,200 VA, other loads = 8,000 VA.

1. General lighting: 2,000 × 3 = 6,000 VA
2. Small-appliance + laundry: 3 × 1,500 = 4,500 VA
3. Largest motor (A/C): 1,200 VA (with 25% bump already included)
4. Other loads: 8,000 VA
5. Total = 6,000 + 4,500 + 1,200 + 8,000 = 19,700 VA
6. Demand factor: first 8,000 at 100% = 8,000; remaining 11,700 at 40% = 4,680; total = 12,680 VA
7. Minimum service amps = 12,680 / 240 = 52.8 A → round up to 60 A

The calculator’s simplified formula gives 19,700 VA and 82.1 A (since it skips the demand factor), which is conservative—safe but oversized. For a real 2,000 sq ft house with typical appliances, the NEC method usually lands around 100–150 A.

When to Use the NEC Load Calculator

Use this calculator when you’re planning a new service, upgrading an old panel, adding a large load (EV charger, heat pump, hot tub), or checking if an existing service is overloaded. It’s also what a home inspector or permit reviewer expects to see on a load calculation sheet. The result tells you the minimum service size—not the recommended size. Many electricians install one size larger (e.g., 200A instead of 150A) to allow for future additions.

How to Read the Result

The output is the minimum amperage at 240V. Standard service sizes are 60, 100, 125, 150, 200, 400, and 600 amps. If your calculated load is 82 A, you need at least a 100 A service. If it’s 110 A, you need 125 A. The calculator gives a decimal; always round up to the next standard size. For a typical 2,000 sq ft home with electric range, dryer, and central A/C, the NEC method usually yields 100–150 A—so a 200 A service is common for new builds.

Common Mistakes and Misconceptions

• Mixing up VA and watts: For resistive loads (heaters, ranges), VA = watts. For motors and A/C, use the nameplate VA or multiply watts by the power factor (often 0.8–0.9).
• Forgetting the 25% motor bump: The largest motor gets an extra 25% on its nameplate rating per NEC 430.24.
• Ignoring demand factors: The 40% factor on the portion above 8,000 VA is what keeps the service from being absurdly large. Skipping it overestimates by 20–40%.
• Using the optional method when you shouldn’t: NEC 220.82 allows a simpler optional method for dwellings with only one feeder—it’s less conservative but still code-compliant. Our calculator uses the standard method.
• Thinking the breaker size is the load: The main breaker rating is the maximum, not the calculated load. A 200 A panel can have a calculated load of only 120 A.

Limits of the Calculator

This simplified tool does not apply the full demand factors from NEC Table 220.42 (which reduces the general lighting load for larger homes) or the specific demand factors for ranges (Table 220.55) and dryers (220.54). For a real installation, you must use the complete NEC method, including the nameplate ratings of every appliance. Also, the calculator assumes a 120/240V single-phase service—common in North America. If you have 120/208V (apartments) or three-phase, the voltage divisor changes.

Frequently Asked Questions

What does NEC stand for?

NEC stands for National Electrical Code, also known as NFPA 70. It’s the standard for safe electrical design, installation, and inspection in the United States. Most local jurisdictions adopt it with amendments.

How many amps does a typical house need?

A small apartment or condo might need 60–100 A. A typical 2,000–3,000 sq ft house with electric appliances usually needs 150–200 A. All-electric homes with heat pumps, EV chargers, or electric water heaters often require 200 A or more.

Can I use this calculator for a commercial building?

No. The dwelling standard method (Article 220 Part III) is for single-family homes and individual units. Commercial buildings use different load calculations (Article 220 Part IV) with demand factors for lighting and receptacle loads that are beyond this simple tool.

What is the difference between the standard and optional NEC methods?

The standard method (220.82) is more detailed and conservative—it adds up every load with specific demand factors. The optional method (220.82) is simpler and allows a single demand factor on the total connected load, often resulting in a smaller service. Both are code-compliant, but the optional method is only for dwellings with one feeder.

Why does the calculator show a higher amp than the NEC method?

Because it omits the 40% demand factor on loads above 8,000 VA. For a 2,000 sq ft home, that factor reduces the load by about 30%. The calculator is a conservative check—if it says you need 200 A, the full NEC method will likely say 150 A or less.

Do I need a permit for a service upgrade?

Yes, in most jurisdictions, changing the service size requires a permit and inspection. The load calculation is part of the permit application. Check with your local building department—requirements vary.

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in a | Floor area | sq ft | 2000 | 0
in na | Small-appliance + laundry circuits | circuits | 3 | 0
in la | Largest motor (e.g., A/C, disposal) | VA | 1200 | 0
in oa | Other loads (dryer, range, water heater, etc.) | VA | 8000 | 0
out load | Total calculated load | VA | 3000 + a * 3 + na * 1500 + max(la, oa) | 0
out amps | Minimum service at 240V | A | load / 240 | 1