Dwelling Unit Load Calculator
Calculate dwelling service load using all three NEC Article 220 methods — Standard (220.82 Part III), Optional (220.82), and Existing Dwelling (220.83) — with side-by-side comparison, service size ladder, and five preset scenarios including an undersized service case.
Overview
Dwelling unit service sizing is one of the most common NEC calculations performed by residential contractors and engineers — and one of the most frequently misapplied because NEC Article 220 provides three distinct methods that can produce meaningfully different results for the same home. This calculator runs all three in parallel: the Standard Method (NEC 220 Part III — general lighting at 3 VA/ft², appliance branch circuits, range demand from Table 220.55, dryer demand from 220.54, and heating/cooling with noncoincident rule), the Optional Method (NEC 220.82 — simplified two-tier demand factor approach for new dwellings), and the Existing Dwelling Method (NEC 220.83 — used when adding load to an existing service). Inputs are slider-based for rapid exploration: square footage (400–6,000 ft²), water heater VA (0–9,000), range/oven VA (0–20,000), dryer nameplate VA (0–12,000), cooling/AC VA (0–20,000), and electric resistance heat VA (0–40,000). The Results tab shows a Service Utilization gauge for the currently active method against a user-selected service size (100–400 A), a Load Composition chart, a Method Comparison summary, and a Service Size Ladder. The Trace tab provides separate step-by-step traces for each of the three methods. A Method Comparison Summary card shows all three results side by side and identifies when the Optional Method produces a more favorable result — typical for small to medium all-electric homes. Five presets cover Small, Medium, Large All-Electric, Optional-Favorable, and Undersized service scenarios. A Design Challenge tab and a Self-Test engine harness are included. Supports NEC 2017, 2020, and 2026.
App preview
A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.
Inside the lab
What you'll do
- 01Apply the NEC Standard Method under Article 220 Part III to a single-family dwelling, calculating general lighting load, small appliance branch circuits, range demand, dryer demand, and HVAC loads in sequence
- 02Apply the Optional Method under NEC 220.82 to the same dwelling and explain when it produces a lower calculated load than the Standard Method
- 03Identify when the Existing Dwelling Method under NEC 220.83 applies and explain what additional load information is required that is not needed for new-dwelling calculations
- 04Apply the range demand factor from NEC Table 220.55 to a nameplate VA value and explain why demand factors are permitted for ranges
- 05Apply the noncoincident load rule under NEC 220.60 to heating and cooling loads and determine which is included in the dwelling service calculation
- 06Use the Service Size Ladder and Method Comparison Summary to select the minimum standard service size that satisfies all three NEC methods for a given dwelling configuration
Who it's for
Tags
Keep exploring
Panelboard & Feeder Load Calculation
Build a multi-category panelboard or feeder load schedule with NEC Article 220 demand factors applied per category — general lighting, receptacles, kitchen, motors, HVAC, and more — then evaluate feeder conductor sizing and bus utilization.
General Lighting & Receptacle Branch-Circuit Calculator
Size general lighting and receptacle branch circuits under NEC Articles 210 and 220 — with occupancy VA/ft² lookup, custom VA entry, continuous-load factor, receptacle and dedicated-load configuration, and a circuit designer with self-test mode.
Continuous Load Sizing Basis Calculator
Enter continuous and noncontinuous load amperes for a branch circuit, feeder, or service and get the NEC sizing basis — with the 125% multiplier applied correctly, the 100%-rated assembly exception explained as a listing condition, and edition-specific notes for 2017, 2020, and 2026.