Power Calc Toolsv0.2 · beta

Methodology

Which code edition, table and formula each calculator uses, what it assumes by default, and where it does not apply. Every number on this site can be traced back to one of these sources — and checked against published examples on the verification page.

Voltage DropCable SizingElectrical LoadPower Factor CorrectionTransformer SizingVerification cases →

Voltage Drop

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NEC 2023 (NFPA 70) Chapter 9; IEC 60364-5-52:2009 Annex G

Reference data

  • NEC: Chapter 9 Table 9, AC resistance and effective impedance for copper and aluminum in steel conduit, 14 AWG – 1000 kcmil.
  • NEC conductor loss: Chapter 9 Table 8 DC resistance at 75 °C, uncoated stranded copper / stranded aluminum.
  • IEC: 60364-5-52 reference R (at 70 °C) and X for PVC-insulated single-core cables, reference method C, 1.5 – 630 mm².

Calculation

  • Vd = b × I × L × Ze / 1000 / sets, with b = 2 for single-phase and √3 for three-phase, L one-way.
  • Ze = R cosθ + X sinθ (Table 9 Note 2). NEC: R is the 1.0-PF column and X is recovered from the 0.85-PF column, so both published columns are reproduced exactly. Below 0.85 PF the result is never lower than the 0.85 column.
  • Pass/fail against 3 % (branch) and 5 % (feeder + branch) — NEC 210.19 / 215.2 Informational Notes, IEC Annex G.

Defaults

  • Steel conduit (highest reactance of the Table 9 raceways — conservative)
  • 75 °C conductor temperature (NEC), 70 °C (IEC)
  • Stranded conductors, balanced load, one circuit per raceway

Does not cover

  • Conductor temperatures far from 75 °C — resistance changes about 0.4 % per °C.
  • Cables in free air with wide spacing, or multicore cable geometry very different from the reference method.
  • Motor starting (inrush) voltage dip, harmonics, or unbalanced loads.
  • Multi-segment feeders — calculate each segment and add the drops.

Cable Sizing

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NEC 2023 (NFPA 70) Articles 110, 210, 215, 240, 250, 310

Reference data

  • Table 310.16 ampacities (60 / 75 / 90 °C columns), 14 AWG – 1000 kcmil, copper and aluminum.
  • Ambient correction from the 310.15(B) equation F = √((Tc − Ta) / (Tc − 30)), which generates Table 310.15(B)(1)(1).
  • Table 310.15(C)(1) adjustment for more than three current-carrying conductors.
  • 240.4(D) small-conductor limits, 240.6(A) standard breaker ratings, Table 250.122 equipment grounding conductors.

Calculation

  • Check 1: terminal-column ampacity ≥ 125 % of continuous + 100 % of non-continuous load (210.19(A) / 215.2(A)).
  • Check 2: insulation-column ampacity × correction × adjustment ≥ the actual load; the result is capped at the terminal rating (110.14(C)).
  • Check 3: the OCPD sized at check 1 protects the conductor (240.4, next size up ≤ 800 A).
  • Then voltage drop at the real length; the larger of the ampacity size and the voltage-drop size is recommended.

Defaults

  • 90 °C insulation (THHN/THWN-2), 75 °C terminals
  • 30 °C ambient, 3 current-carrying conductors
  • Parallel sets only for 1/0 AWG and larger (310.10(G))

Does not cover

  • Cable types with their own ampacity limits (for example NM cable at 60 °C, 334.80) — set insulation to 60 °C.
  • Conductors in free air, underground duct banks or over 2000 V (Tables 310.17 – 310.21, 311).
  • Motor circuits (Article 430), which have their own sizing rules.
  • The EGC increase required by 250.122(B) when conductors are upsized for voltage drop — apply it by hand.

Electrical Load

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NEC 2023 (NFPA 70) 220.82 optional method; 220.57; 230.79(C); 310.12

Reference data

  • 3 VA/ft², 1500 VA per small-appliance and laundry circuit, appliance nameplate VA.
  • Standard service ratings 100 – 800 A; Table 310.16 75 °C column for the service conductor.

Calculation

  • General load: first 10 kVA at 100 %, remainder at 40 %.
  • Plus the largest heating or cooling option from 220.82(C) at its own factor (100 / 65 / 40 %).
  • EVSE at the larger of 7200 VA and nameplate (220.57), in the general load by default or at 100 % if your AHJ requires.
  • Amps = VA / 240, rounded up to a standard service of at least 100 A; service conductor at 83 % of the rating (310.12).

Defaults

  • One dwelling unit, 120/240 V single-phase
  • EVSE in the 220.82(B)(3) general load (contested — switch to 100 % if your AHJ requires)

Does not cover

  • The standard method (Part III), multifamily (220.83 / 220.84) or commercial occupancies.
  • Existing services sized from measured demand (220.87).
  • NEC 2026, which moves dwelling load calculations to Article 120 with different values.

Power Factor Correction

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Circuit theory; installation per NEC 2023 Article 460 / IEC 60831

Reference data

  • Common low-voltage capacitor bank ratings (kvar).

Calculation

  • Qc = P × (tan φ1 − tan φ2); rounded up to the next standard bank, with the resulting PF reported.
  • Capacitance: C = Q / (ω V²) single-phase; delta C = Q / (3 ω V_LL²); wye three times the delta value.
  • Current falls by PF_old / PF_new at constant kW; I²R loss with its square.

Defaults

  • Balanced sinusoidal load at full load
  • Fixed bank, no detuning reactors

Does not cover

  • Systems with significant harmonics (VFDs, rectifiers) — resonance needs a harmonic study and detuned banks.
  • Light-load conditions where a fixed bank causes leading PF.
  • Correction of individual motors, which is limited by the motor manufacturer’s maximum kvar.

Transformer Sizing

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NEC 2023 (NFPA 70) 110.9, 110.10, 450.3(B); ANSI/IEEE C57.12 ratings

Reference data

  • ANSI/IEEE C57.12 standard kVA ratings, single- and three-phase.
  • Table 450.3(B) protection percentages; 240.6(A) standard ratings.

Calculation

  • kVA required = load × (1 + growth); next standard rating.
  • FLA = kVA × 1000 / (√3 × V) three-phase, kVA × 1000 / V single-phase.
  • Isc = FLA / (%Z / 100), optionally with %Z reduced 10 % for nameplate tolerance (point-to-point method, infinite source).

Defaults

  • Infinite (zero-impedance) primary source
  • Bolted three-phase fault at the secondary terminals
  • ≤ 1000 V for the 450.3(B) protection result

Does not cover

  • Fault current downstream of the transformer — cable impedance lowers it; use a point-to-point calculation.
  • Arc-flash studies, which need minimum as well as maximum fault current.
  • Motor contribution to fault current.

How changes are checked

  • Every calculator has reference cases worked out by hand from the cited tables; they run as automated tests on every change.
  • A change to calculation code or table data cannot be deployed without a new hand-verified test that cites its source.
  • Published worked examples from independent sources are re-run on every build — see Verification.
  • Every correction that changes a result is listed in the Changelog.

Results are for planning and checking. They do not replace a licensed electrician or engineer; the Authority Having Jurisdiction has the final word on code interpretation.