Methodology & Data Sources

This page documents the formulas, code references, and data sources used in VoltFlow calculators. If you want to verify our math against your own work, this is where to start.

Wire Sizing (NEC US)

Reference: NEC 2023, Article 310, Tables 310.16 and 310.17.

Method: We start with the load amperage, apply the 125 percent continuous load factor (NEC 210.19) where applicable, and select the smallest conductor whose ampacity at 75 degrees C insulation rating meets or exceeds the calculated current. We apply temperature correction factors from NEC Table 310.15(B)(1)(1) and adjustment factors for more than three current-carrying conductors per NEC Table 310.15(C)(1).

Default assumptions: 30 degrees C ambient, copper conductors, THWN-2 insulation, raceway installation. These can be changed in calculator inputs.

Voltage Drop

Formula: Voltage drop (V) = 2 × K × I × L / CM (single-phase) or √3 × K × I × L / CM (three-phase)

Where K is the conductor constant (12.9 for copper, 21.2 for aluminum at 75 degrees C), I is current in amps, L is one-way length in feet, CM is the conductor cross-sectional area in circular mils.

For DC circuits and AC circuits at unity power factor, this is exact. For AC circuits with significant power factor displacement, our calculator includes the option to apply the impedance method using NEC Table 9 effective Z values.

Conduit Fill

Reference: NEC 2023, Chapter 9, Tables 1, 4, 5.

Maximum fill is 53 percent for 1 conductor, 31 percent for 2 conductors, and 40 percent for 3 or more conductors. Cross-sectional areas come directly from NEC Chapter 9, Table 5 for each conductor type.

Breaker Sizing

Reference: NEC 240.4, 210.20, 215.3.

Breakers are sized at 125 percent of continuous load plus 100 percent of non-continuous load, then rounded up to standard breaker sizes per NEC 240.6(A). Standard sizes used: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400 amps.

Solar System Sizing

Production estimate: Annual kWh = system kW × peak sun hours × 365 × system derate factor (default 0.80).

Peak sun hours: Sourced from NREL TMY3 data by location.

Derate factor: 0.80 default accounts for inverter efficiency, soiling, wiring losses, mismatch, and module age.

Payback: Net cost (after 30 percent federal ITC and applicable state incentives) divided by year-one electricity savings, with no compounding for rate inflation. We provide a separate inflation-adjusted lifetime calculation.

Battery Sizing

Nominal capacity: kWh nominal = (daily energy need in kWh) / depth of discharge (DoD).

Default DoD: 80 percent for LiFePO4, 50 percent for lead-acid (per industry-standard cycle life curves).

Round-trip efficiency: 95 percent LFP, 82 percent AGM lead-acid.

Electricity Cost

Source: EIA Form-861, Form-826, Monthly Electric Power Industry Report. We use residential rate averages by state.

Calculation: Daily cost = wattage × hours of use × (1 / 1000) × rate per kWh. Monthly cost = daily cost × 30. Annual cost = daily cost × 365.

Limitation: Average rates do not reflect tiered or time-of-use pricing. Customers on TOU rates should multiply by their applicable peak/off-peak rate for the hours of use specified.

EV Charging Cost

Calculation: Cost per mile = (kWh per 100 miles / 100) × electricity rate × charging efficiency factor (default 0.88 for AC L2). Charging efficiency accounts for losses in the AC-DC conversion at the vehicle.

International Standards

For non-US calculations, we use:

• UK: BS 7671 (18th Edition Wiring Regulations)
• Europe: IEC 60364, EN 50525 cable standard
• Australia/NZ: AS/NZS 3000:2018, AS/NZS 3008.1.1 cable selection
• France: NF C 15-100
• Germany: DIN VDE 0100
• Spain: REBT 2002 (Reglamento Electrotécnico de Baja Tensión)
• Mexico: NOM-001-SEDE-2012

Each international calculator page indicates the specific standard used.

Data Update Schedule

If you find a calculation that does not match the methodology above, or if you have access to a more current data source than we are using, please let us know.