NEC Calculations That Show Their Work
Every voltage drop, wire size, and load calculation with the NEC article reference that passes inspection. All editions from 2017 through 2026 — including the new Article 120.
Free during early access
The problem
You’re bouncing between Southwire for voltage drop, a random website for conduit fill, and hand-written calc sheets for the permit office. Enterprise tools like ETAP cost $5,000+/year — for a set of NEC table lookups you could do faster on your phone.
- Southwire’s voltage drop calculator doesn’t show the formula — the inspector asks how you got the number and you can’t explain it
- Your AHJ enforces NEC 2020, the next town uses 2023, and the new job requires 2026 — different tables, different article numbers, different results
- NEC 2026 moved Article 220 to Article 120 and cut general lighting from 3 VA/sq ft to 2 VA — every load calc template you have is wrong
- Hand-writing calculation sheets for permit applications takes 30 minutes per job and the inspector still calls with questions
- Calculating conduit fill for mixed conductor sizes means looking up 10+ entries in Chapter 9 Table 5 by hand
What it does
Voltage Drop Calculator
VD = 2KIL/CM for single-phase, 1.732KIL/CM for three-phase. Copper and aluminum, AWG 14 through 2000 kcmil. Pass/fail against the 3% branch / 5% total thresholds.
Wire Sizing with Multi-Factor Derating
Table 310.16 ampacity lookup with ambient temperature correction, conduit fill adjustment, and 125% continuous load multiplier. Shows which factor controls the final wire size.
Residential Load Calculation
NEC Article 220/120 optional method with demand factors. General lighting, small appliance circuits, range, dryer, HVAC, and EV charger loads — sized to the correct service.
Commercial Load Calculation
Occupancy-type demand factors per Table 220.42/120.42. Kitchen equipment, motor loads at 125% of largest, receptacle calculations, and multi-family dwelling factors.
Conduit Fill Calculator
EMT, PVC Schedule 40/80, RMC, IMC, and FMC. Mixed conductor sizes with Chapter 9 Table 4 and Table 5 lookups. Visual fill diagram with pass/fail and next size up.
NEC 2017–2026 Multi-Edition Support
Switch between NEC editions with one click. Handles the Article 220→120 renumbering, 2 VA/sq ft lighting (2026), and all table changes. Edition badge on every output.
Permit-Ready PDF Export
Project cover page, calculation sheets with full NEC references, summary table, and panel schedule — formatted the way permit offices expect to see them.
Show Your Work Engine
Every output includes the NEC article, formula, all inputs, intermediate steps, and the controlling factor. Expand any result to see the complete derivation.
Why switch
Transparent Methodology
Every calculation shows the NEC article, formula, and derivation steps. The inspector can verify your work, the apprentice can learn from it, and you can defend it at the permit counter.
All Calculations, One Tool
Voltage drop, wire sizing, load calcs, conduit fill, OCPD sizing, grounding, fault current, and motor circuits. Stop bouncing between five websites and a code book.
Multi-Edition NEC Support
NEC 2017, 2020, 2023, and 2026 with automatic article cross-referencing. Use the edition your AHJ enforces — not the one the free calculator was built for.
Free tools
Try these tools right now — no signup required.
Voltage Drop Calculator
Calculate voltage drop for single-phase and three-phase circuits per NEC 2026 standards.
Electrical Load Calculator
Calculate residential service size per NEC 2026 optional method with demand factor breakdown.
Wire Size Calculator
Calculate wire size per NEC Table 310.16 with temperature and conduit fill derating.
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