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Electrical Service Size Calculator (NEC 220)

Service size estimate by load calculation. User enters NEC demand factors — verify with NEC 220 Part III.

warning

ESTIMATE ONLY — not a stamped engineering design. Verify with a licensed PE or licensed electrician before procurement or construction. This tool does not contain any NEC table values. You enter every demand factor yourself by referencing your current adopted edition of NFPA 70 (National Electrical Code), Article 220.

For your reference only — does not change any calculation. Refer to the matching section of NEC Article 220 for your occupancy type.

Connected Loads (VA — user-entered)

Per NEC 220.12, general lighting for dwellings is calculated as a unit load per square foot (you look up the current per-SF value in your adopted NEC edition and multiply by floor area).

Per NEC 220.52(A), dwelling small appliance branch circuits are calculated at a fixed VA per circuit, with a minimum number of circuits required.

Per NEC 220.52(B), dwelling laundry branch circuit has a minimum required VA.

Read off appliance nameplate. NEC 220.55 has demand factors / column tables for cooking appliances — enter your applicable demand factor below.

User-entered — see NEC 220.55 Table for cooking equipment demand factors.

Per NEC 220.54, the dryer load is the nameplate or a minimum VA, whichever is greater. Enter the larger value here.

User-entered — see NEC 220.54.

Per NEC 220.60, use the larger of A/C or heating noncoincident loads.

Sum of dishwasher, disposal, microwave, etc. nameplate VAs.

Per NEC 220.53, you may apply a reduced demand factor when four or more fixed appliances (other than ranges, clothes dryers, A/C, or space heating) are served by the same feeder/service. Enter the applicable percentage from the current NEC.

Lighting Demand Factor Tiers (NEC Table 220.42 — user-entered)

See NEC Table 220.42 for dwelling unit lighting demand factors. The table provides tier breakpoints (in VA) and the demand factor to apply within each tier. Enter the breakpoints and percentages from your current adopted NEC edition below. These apply to the combined total of general lighting + small appliance + laundry.

Recommended Service Size

100 A

Calculated demand: 0.0 A at 240 V

Total connected load0 VA
Lighting demand (after tier factors)0 VA
Range demand0 VA
Dryer demand0 VA
Water heater demand0 VA
HVAC demand0 VA
Other fixed appliances demand0 VA
Total demand load0 VA
Calculated amps (demand VA ÷ V)0.0 A
Next standard service size100 A
info

Standard sizes shown (100, 125, 150, 200, 225, 320, 400, 600, 800, 1000, 1200 A) are common service rating increments — they are not NEC code values themselves. Local AHJ may require a larger size due to voltage drop, future expansion provisions, or solar/EV upgrades.

Methodology

For each load category you enter the connected VA and the demand factor (%) you read from the relevant NEC section. The tool sums (connected VA × DF/100) for every category to get total demand VA, then divides by service voltage to get amps, then rounds up to the next standard service size. For dwelling lighting (NEC 220.42), the combined general lighting + small appliance + laundry load is tiered: portions below each user-entered breakpoint get the user-entered tier factor. No NEC table values, demand factors, unit loads, or tier breakpoints are hardcoded in this tool. You supply every code-specific number yourself based on your adopted edition of NFPA 70. This calculator implements the Standard Method (NEC 220 Part III). For the Optional Method for dwellings (NEC 220.82), the structure is different — consult the code directly.

Frequently Asked Questions

What is a demand factor?
A demand factor is the ratio of the maximum demand on a system to the total connected load. NEC allows the use of demand factors because not all loads operate at full capacity simultaneously — for example, a dwelling will not run every lighting circuit, range burner, and dryer at the same instant. Applying a demand factor (less than 100%) to the connected load gives a realistic estimate of the actual load on the service, which avoids oversizing conductors, panels, and service equipment.
What is the difference between the Standard and Optional NEC dwelling calculations?
NEC Article 220 Part III is the Standard Method — it walks through each load category (lighting, small appliance, range, dryer, HVAC, etc.) applying separate demand factors from individual sections (220.42, 220.52, 220.54, 220.55, 220.60). NEC 220.82 is the Optional Method for one-family dwellings, which uses a single combined demand calculation with one tier breakpoint and a remainder factor, with HVAC handled noncoincidentally. The Optional Method usually yields a smaller service size, but both methods are code-compliant — consult the current NEC for the exact tier limits and percentages.
When do I need a load study from a licensed electrical engineer?
You need an EE-stamped load study for any commercial or industrial service, any service over 400 A in a dwelling, any service requiring utility coordination (new transformer, service drop upgrade), and any project where the AHJ requires it. Solar PV interconnections, EV charging additions to existing services, and multifamily feeders also commonly require stamped calculations. This tool is a rule-of-thumb estimator only and is not a substitute for an EE-stamped load study.
How often does the NEC update?
The National Electrical Code (NFPA 70) is published on a three-year cycle. The most current editions in wide adoption are NEC 2017, NEC 2020, NEC 2023, and (as state codes catch up) NEC 2026. Adoption is set by each state, county, or city, and many jurisdictions lag the latest edition by one to two cycles. Always confirm which NEC edition your AHJ has adopted before applying any value — demand factors, unit loads, and table breakpoints have changed multiple times between editions.