Introduction
Every electrical project in a home — adding a circuit, upgrading a panel, planning an EV charger — starts with the same question: how much electrical load does this building actually carry? Size the service too small and breakers trip, the inspector fails the job, or the utility won’t approve the connection. Size it far larger than needed and you pay for capacity that never earns its keep. A load calculation answers with the National Electrical Code’s own arithmetic: tally the loads the Code assigns to floor area and appliances, apply demand factors, and convert the result to a service size.
The Electrical Load Calculator handles the core calculation: enter floor area, branch circuits, and major appliances, and it returns total demand load in volt-amperes plus the recommended service size. This guide explains everything behind that number — the terminology, the NEC Article 220 formulas, a full worked example, and the judgment calls the calculator can’t make for you.
What an Electrical Load Calculation Is and Where It’s Used
A load calculation estimates the maximum electrical demand a dwelling, feeder, or circuit is expected to place on the power system. It is the Code’s standardized prediction of the worst realistic case — built from floor area, required circuits, and appliance nameplates — not a measurement of current usage. The people who need it:
- Homeowners planning additions, EV chargers, heat pumps, hot tubs, or workshop subpanels.
- Electricians sizing new services, panels, feeders, and branch circuits.
- Builders deciding between 100A, 200A, or larger services before rough-in.
- Inspectors and utilities, who expect a calculation on file before approving new services and upgrades.
The method changes with building type, but the principle doesn’t: start from connected load, discount for diversity with demand factors, size equipment for the result. For single circuits, the Breaker Size Calculator is the more focused tool.
Key Terms and Variables
- Volt-amperes (VA) — apparent power (volts × amperes). The NEC sizes services in VA, so all of Article 220 speaks VA. For resistive loads VA equals watts; for motors and electronics they differ.
- Amperes — current draw. Services are sold in amps (100A, 200A), so the final step converts VA to amps at the service voltage.
- Demand factor — the Code’s recognition that not everything runs at once. NEC 220.42 counts the first 3,000 VA of general load at 100% and the remainder at 35%. Derived in full in the formula-explained guide.
- Continuous load — runs at maximum current 3+ hours (EV chargers, water heaters). Conductors and breakers must be sized at 125% of the load — the most commonly missed adjustment in DIY calculations.
- Service / feeder / branch circuit — the service runs from utility to building; a feeder runs to a subpanel; a branch circuit is the final run to outlets. This guide sizes the service; the same method sizes a feeder.
The NEC Article 220 Framework
| NEC section | What it covers | Key figure |
|---|---|---|
| 220.12 (Table 220.12) | General lighting, dwellings | 3 VA/ft² (32.3 VA/m²) |
| 220.52(A) and (B) | Small-appliance and laundry circuits | 1,500 VA per circuit |
| 220.42 (Table 220.42) | General demand factors | First 3,000 VA at 100%, remainder at 35% |
| 220.53 | Fixed appliances (4+) | 75% demand factor |
| 220.54 | Electric dryer | 5,000 VA or nameplate, greater |
| 220.55 (Table 220.55) | Household range | 8,000 VA demand for one 12 kW range |
| 220.60 | Noncoincident loads | Larger of heating vs. cooling only |
| 220.82 / 220.83 | Optional methods | Simplified paths for qualifying dwellings |
Confirm which NEC edition your AHJ adopted — section numbers are occasionally reorganized, and the AHJ’s edition governs. See the code-compliance guide.
The Formulas Involved
Step 1 — General lighting: floor area (ft²) × 3 VA/ft², from outside dimensions, excluding porches, garages, and unfinished non-adaptable spaces. Metric: m² × 32.3.
Step 2 — Required circuits: (small-appliance circuits × 1,500) + (laundry circuits × 1,500); minimum two and one respectively. Add to the lighting load.
Step 3 — General demand: 3,000 × 100% + (subtotal − 3,000) × 35%. This typically cuts the general load nearly in half.
Step 4 — Major appliances: dryer (5,000 VA or nameplate), range per Table 220.55, water heater at nameplate, fixed appliances at 75% if four or more. Only the larger of heating and cooling.
Step 5 — Service amps: total demand VA ÷ 240V, rounded up to the next standard size (100A, 125A, 150A, 200A, 400A).
Worked Example: A 2,000 ft² Home
2,000 ft² (about 186 m²), two small-appliance circuits, one laundry circuit, 5 kW dryer, 12 kW range, 4.5 kW water heater, 5 kW AC:
Step 1: 2,000 × 3 = 6,000 VA (metric check: 186 × 32.3 = 6,008 VA ✓).
Step 2: 3 × 1,500 = 4,500 VA → general subtotal 10,500 VA.
Step 3: 3,000 + (7,500 × 0.35) = 5,625 VA.
Step 4: 5,000 (dryer) + 8,000 (range) + 4,500 (water heater) + 5,000 (AC) = 22,500 VA → total 28,125 VA.
Step 5: 28,125 ÷ 240 = 117A → 200A service is the common choice, with headroom for an EV charger or heat pump later.
Common Variations
- Apartments: calculate each unit separately, then apply multifamily demand provisions.
- Additions: recalculate the whole dwelling with new area and appliances; verify the existing service still fits.
- EV chargers: a 48A Level 2 charger is 11,500 VA continuous (60A of panel capacity at 125%). Run the numbers with and without it — the top reason 100A services need upgrading.
- Workshops/garages: calculate separately for feeder and subpanel sizing; motor starting current needs separate attention.
For the inventory method behind the inputs, see the estimating guide.
Mistakes That Distort Results
- Skipping demand factors — nearly doubles the answer and triggers unneeded upgrades.
- Forgetting the 1,500 VA circuits — understates the general load by 4,500 VA.
- Confusing watts and VA — fine for heaters, wrong for motors and electronics.
- Dividing by 120V instead of 240V — doubles the apparent service size.
- Ignoring the 125% continuous-load rule — a 40A EV charger needs a 50A circuit.
- Counting heating and cooling together — carry only the larger.
Full catalogue with fixes: the common mistakes guide.
Related Guides
- Formula Explained — every Article 220 formula derived step by step.
- Estimating Guide — the room-by-room inventory method.
- Cost Guide — panel upgrades, circuits, and EV installs priced.
- Pro Tips — future-proofing and headroom planning.
- Code Compliance — permits, inspections, AHJ requirements.
- Load Calculation Checklist — measurement to final inspection.
Frequently Asked Questions
What is an electrical load calculation?
The NEC Article 220 method for estimating a building’s maximum demand: floor area × 3 VA/ft² plus required circuits and appliance nameplates, reduced by demand factors, converted to service amps.
How many amps does a typical house need?
Most modern single-family homes calculate 100–200A. A 200A service is the common new-construction standard for EV and heat-pump headroom.
What is the 3 VA per square foot rule?
NEC Table 220.12’s minimum general lighting load for dwellings (32.3 VA/m²), measured from outside dimensions, excluding garages and porches.
How do I convert VA to amps?
Amps = VA ÷ volts. At 240V single-phase, 28,125 VA ≈ 117A. The Amperage Calculator converts directly.
Is a load calculation required by code?
The NEC requires services and feeders to be sized from one, and most AHJs want it with permit applications for new services, upgrades, and major additions.
Get Your Number
The shape is simple: measure the area, add the required circuits, discount for diversity, add the big appliances, convert to amps. The Electrical Load Calculator runs that sequence — gather your floor area and nameplates, run the numbers, and compare against your existing service before planning any addition.
Disclaimer: planning estimates only. Calculator results are estimates for planning purposes — verify your final numbers with a licensed electrician and your local authority having jurisdiction (AHJ) before buying equipment or scheduling work. This guide is not engineering advice.
