Why We Built This
US electricians and DIY homeowners searching for NEC-compliant calculators often encounter tools using Australian/NZS standards, manufacturer-biased tools, or generic calculators without proper NEC context. We built the NEC Electrical Calculator Suite to provide free, accurate, US-specific electrical calculators that reference the National Electrical Code (NFPA 70) directly.
How Our Formulas Work
Voltage Drop Calculator
Uses the NEC recommended formula: VD = (2 × K × I × L) / CM where K is the
resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is
one-way circuit length in feet, and CM is conductor area in circular mils.
Conductor resistance values reference NEC Chapter 9, Table 9 (AC resistance in conduit).
Wire Size Calculator
Iterates through AWG sizes from smallest to largest, verifying two constraints per NEC: (1) NEC Table 310.15(B)(16) 75°C ampacity ≥ load current, and (2) calculated voltage drop ≤ your selected max drop tolerance (3% or 5%). Returns the smallest wire that satisfies both constraints.
kWh Calculator
Calculates energy consumption: kWh/day = (Watts / 1000) × Hours/day.
Monthly kWh = daily kWh × days/month. Yearly kWh = daily kWh × 365.
Electricity Cost Calculator
Multiplies monthly kWh by rate ($/kWh) for monthly cost; multiplies by 12 for yearly cost. Default state rates sourced from EIA (US Energy Information Administration) residential averages.
Conduit Fill Calculator
Sums conductor cross-sectional areas from NEC Chapter 9, Table 5, then compares to the allowed 40% fill area from NEC Chapter 9, Table 4. Fill % = (total wire area / allowed area) × 100. Compliance badge: green ≤ 40%, amber ≤ 50%, red > 50%.
Full Disclaimer
All results produced by this site are for informational and educational purposes only. They do not constitute licensed engineering advice, electrical design certification, or code compliance verification.
Always verify all electrical calculations against the current edition of the National Electrical Code (NEC / NFPA 70) and any local amendments adopted by your jurisdiction. Electrical work must be performed by a licensed electrician where required by law. Do not rely solely on these tools for safety-critical decisions.
Formula values reference NEC 2023 (NFPA 70-2023) unless otherwise noted. Local jurisdictions may adopt older or newer editions. Always confirm the edition adopted by your authority having jurisdiction (AHJ).
Contact & Feedback
Found a bug or want to suggest a new calculator? We welcome feedback from electricians and electrical engineers who use this tool on the job. Reach us at: feedback@electricalcalctools.com