Methodology and sources

How each calculator works, which tables it uses and what it leaves out. Last checked October 10, 2026.

Sources

• NFPA 70 (National Electrical Code) 2023: Table 310.16 (ampacity), Chapter 9 Table 8 (conductor resistance), 240.4(B) and 240.4(D) (overcurrent protection of conductors), 240.6(A) (standard breaker sizes), 210.19, 210.20 and 215.2 (continuous loads and voltage drop notes), 334.80 and 340.80 (NM and UF cable ampacity), 110.14(C) (termination temperature), 625.41 (EV charging), 702 (optional standby systems). Ampacity values in Table 310.16 have been stable across the 2017–2023 editions.
• U.S. Energy Information Administration: Electric Power Monthly, Tables 5.6.A and 5.6.B, residential electricity prices by state, July 2026 data (released 2026-09), preliminary; and heat content of fuels.
• U.S. Department of Energy, Energy Saver: estimating appliance energy use (method and sample wattages).
• U.S. Consumer Product Safety Commission and CDC: generator carbon monoxide guidance.

Wire size

1. Design current = load, ×1.25 if continuous.
2. Breaker = next standard size ≥ design current.
3. Conductor = smallest size whose ampacity in the column allowed by the wiring method (60 °C for NM-B, UF and unmarked terminations ≤ 100 A; 75 °C for conductors in conduit on 75 °C terminations) is at least the design current, where either the ampacity is at least the breaker rating or the breaker is the next standard size above it (240.4(B)), and the 240.4(D) small-conductor limits are met.
4. Then upsized until voltage drop is within the chosen limit.

Not covered: ambient temperature correction, adjustment for more than three current-carrying conductors, motor circuits (Article 430), dwelling service and feeder sizing (310.12), parallel conductors.

Voltage drop

Vdrop = k × L × R × I ÷ 1,000, with k = 2 for single-phase and DC, √3 for three-phase; R from Chapter 9 Table 8 at 75 °C (uncoated copper; aluminum). Reactance is ignored, which slightly understates drop for large AC conductors in steel conduit.

Generator size

Running total = sum of running watts. Peak = running total + the largest single (starting − running) difference, assuming motors are started one at a time. Recommended running capacity = running total ÷ 0.8. Appliance running and starting watts are typical values for US appliances compiled from nameplate ranges; every value can be edited in the calculator, and nameplate values should be preferred. A soft-start kit is modeled as cutting compressor inrush to 35%.

Power station and battery runtime

Runtime = rated Wh × (1 − reserve) × efficiency ÷ (load × duty cycle). Defaults: 85% for AC output, 95% for DC. Home battery: kWh needed = daily kWh × days ÷ efficiency ÷ usable fraction; units = the larger of the count by energy and the count by continuous power.

Generator fuel

Fuel per hour = (rated kW × load × 3,412 BTU/kWh) ÷ efficiency ÷ fuel heat content. Full-load efficiency: 17% below 4 kW, 19% from 4 to 13 kW, 22% above. Part-load factor: 0.62 at ≤ 25% load, rising linearly to 0.75 at 50%, 0.90 at 75% and 1.0 at 100%. This curve was checked against published manufacturer fuel and run-time figures for inverter, portable and standby generators and agrees within about ±20%. It is an estimate; spec sheets win.

Electricity cost

Cost = watts × hours ÷ 1,000 × price. State prices are the EIA's residential average for January–July 2026, which smooths out summer and winter swings better than a single month. EV cost assumes 90% wall-to-battery charging efficiency.

Reference tables used

Testing

The calculation code has automated tests against worked answers from the tables above. Run them yourself: the calculation code is served at /js/calc.js and the tables at /js/tables.js.