VoltCalc

Sources and code tables

Every safety-related number on this site comes from a published table, not from memory. Here is where each one comes from and how the tools were checked.

National Electrical Code (NFPA 70), 2023 edition

  • Table 310.16: allowable ampacity of insulated conductors, 60/75/90 °C, copper and aluminum, 14 AWG to 1000 kcmil. Values cross-checked against Turn2Engineering's republished chart (checked August 2026) and manufacturer data sheets.
  • Table 310.15(B)(1) ambient temperature correction and Table 310.15(C)(1) adjustment for more than three current-carrying conductors.
  • 110.14(C) terminal temperature limits; 240.4(D) small-conductor overcurrent limits; 334.80 NM cable at 60 °C.
  • 310.12 single-phase dwelling services and feeders (83% rule).
  • 210.20(A), 215.3 125% of continuous load; 240.6(A) standard overcurrent device ratings.
  • Chapter 9 Table 8 conductor properties: DC resistance at 75 °C for stranded uncoated copper and aluminum, and the Note 2 temperature equation (from the published table as distributed by NECA-IBEW).
  • Chapter 9 Tables 1, 4, 5: fill percentages, EMT and PVC Schedule 40 internal areas, THHN/THWN-2 areas.
  • Tables 430.248, 430.250 motor full-load currents; 430.6(A)(1), 430.22, 430.52 motor circuit sizing.
  • Informational Notes to 210.19 and 215.2: 3% / 5% voltage-drop recommendations.

The 2026 NEC has been published and some jurisdictions are adopting it. Ampacity values for the sizes covered here are unchanged; 2026 adds 16 AWG copper and 14 AWG copper-clad aluminum rows to Table 310.16.

Other standards and data

  • IEC 60062: resistor and capacitor marking codes (color bands, 3-digit codes).
  • EIA Short-Term Energy Outlook, September 2026: U.S. residential average electricity price, used as the default rate.
  • FAA / IATA lithium battery guidance: the 100 Wh and 160 Wh thresholds for spare batteries.

How the formulas were tested

  • Voltage drop: 20 A on 12 AWG copper over 100 ft reproduces the published result of 7.92 V.
  • Ampacity adjustment: 6 AWG copper, 90 °C insulation, 75 °C terminals, 36–40 °C ambient, 4–6 conductors reproduces 75 × 0.91 × 0.80 = 54.6 A.
  • Conduit fill: 16 × 12 AWG THHN in 3/4 in. EMT gives 39.9% (fits); 17 gives 42.4% (doesn't), matching published fill charts.
  • Standard circuit sizes (15 A → 14 AWG, 20 A → 12 AWG, 30 A → 10 AWG, 40 A → 8 AWG, 50 A → 6 AWG at 60 °C) match Table 310.16 and 240.4(D).

Limits

The NEC is a model code. States and cities adopt a particular edition, often with amendments, and the authority having jurisdiction interprets it. Outside the U.S., other codes apply (CEC in Canada, BS 7671 in the UK, IEC 60364-based rules elsewhere). Use these calculators to plan and to learn, and have a licensed electrician design, install and inspect the work.

Spotted an error? That matters on a site like this. The tables are in plain view on each page, so you can check any figure against your own code book.