Article 220 — Load Calculations for Residential Occupancies

Article 220 — Load Calculations for Residential Occupancies

Why Load Calculations Matter

Before a single wire is pulled in a new home, an electrician must answer one critical question: how large does the service entrance need to be? Size it too small and the service will trip breakers, overheat conductors, and violate the NEC. Size it too large and the homeowner overpays for equipment they do not need. Article 220 of the National Electrical Code (NEC) provides the rules for answering that question correctly and legally.

Every building inspector, utility company, and plan reviewer expects to see a documented load calculation. Learning to do this accurately is a core job skill — and it is tested on both NCCER Level 2 assessments and many state journeyman licensing exams.

Safety Note Before You Start

Load calculation work is design and planning work — you are not yet touching energized equipment. However, the numbers you produce directly affect safety downstream. An undersized service entrance conductor can overheat and start a fire. Always double-check arithmetic, use current code tables, and never guess at demand factors. If you are ever making changes to an existing service, follow lockout/tagout (LOTO) procedures before touching any service equipment, and wear appropriate PPE including safety glasses and insulated gloves rated for the voltage involved.

The Standard Method: Article 220 Parts I–III

The standard (non-optional) calculation method is found in Article 220 Parts I through III. Work through it in this order.

Step 1 — General Lighting and Receptacle Load

For a dwelling unit, the general lighting load is calculated at 3 volt-amperes (VA) per square foot of living area (NEC Table 220.12). Measure the outside dimensions of the structure, excluding open porches, garages, and unfinished spaces not adaptable for future use. Multiply the total square footage by 3 VA/ft².

Step 2 — Small Appliance and Laundry Circuit Loads

The NEC requires a minimum of two 20-ampere small appliance branch circuits for kitchen and dining areas. Each is calculated at 1,500 VA regardless of the actual number of outlets. Add at least one 1,500 VA laundry branch circuit load. These loads are added to the general lighting total before demand factors are applied.

Step 3 — Apply Demand Factors to Lighting and Small Appliance Loads

Table 220.42 allows you to reduce the combined lighting and small appliance load. The first 3,000 VA is taken at 100%. The next portion — from 3,001 VA to 120,000 VA — is taken at 35%. Anything above 120,000 VA returns to 100%. Add the results to get your demand-adjusted lighting load.

Step 4 — Fixed Appliance Loads

List every fixed appliance: dishwasher, garbage disposal, water heater, well pump, and similar loads. Add their nameplate VA (or watts, treated as VA for resistive loads). If there are four or more fixed appliances (not counting ranges, dryers, AC, or heat), you may apply a 75% demand factor to the total.

Step 5 — Dryer Load

Use 5,000 VA or the nameplate rating, whichever is larger, for each electric clothes dryer (NEC 220.54 and Table 220.54). For a single dryer in a dwelling the demand factor is 100%.

Step 6 — Range and Cooking Equipment Load

For household electric ranges rated over 1¾ kW, use NEC Table 220.55. A single range rated up to 12 kW uses a maximum demand of 8,000 VA. Nameplate ratings over 12 kW require adjustments described in the table notes.

Step 7 — Heating or Air Conditioning (Largest Load)

Include only the larger of the heating load or the cooling load — not both — because they do not operate simultaneously. This is the non-coincident load rule.

Step 8 — Total the VA, Convert to Amperes

Add all demand loads together. Divide the total VA by the service voltage to get amperes. For a standard single-phase 120/240 V residential service, divide by 240 V.

> Amperes = Total Demand VA ÷ 240 V

Select the next standard service size at or above the calculated ampere value. Common sizes are 100 A, 150 A, 200 A, and 400 A.

The Optional Method: Article 220 Part IV

Part IV provides a simplified optional calculation. Instead of applying separate demand factors to each load category, it combines all loads and applies a single demand factor from Table 220.84 based on the total connected load and the number of dwelling units. For a single-family dwelling, the optional method often produces a smaller calculated load than the standard method, which can reduce service size and installation cost. However, the optional method is only available when the dwelling has electric heat or air conditioning.

Reading the Tables Accurately

Mistakes in load calculations almost always come from misreading a table. Always confirm you are using the correct table number, reading across the right row, and applying the correct voltage. Carry units (VA, kW, A) through every line of your work so errors are easier to catch.

Connecting to Your Credentials

NCCER Electrical Level 2 includes a full module on load calculations using the same Article 220 framework taught here. State journeyman exams routinely include load calculation problems — some states allow an open-code book, others do not, so know both the method and the table locations. OSHA 10 training reinforces that correctly sized services protect workers and building occupants from electrical fire hazards.