Sources & methodology

Every calculator on AllJobsCalc is built from a specific code section or published reference table, checked against independent sources, and scoped so it only answers what that source supports. This page lists all of it, calculator by calculator.

How we build a calculator

One named source per rule. Each formula or table is tied to a code section (NEC, IPC, IRC, state septic rules) or a published reference chart, named on the calculator page.
Cross-checked, not copied once. Table values are compared against at least one independent source, and every calculator is tested against a worked example done by hand.
Honest confidence levels. Code tables are marked high confidence. Rules of thumb (HVAC load, septic drainfields, shingle waste) are marked medium and presented as a starting point.
Narrow scope on purpose. If a rule depends on details a simple form cannot capture, the calculator says so and leaves it out instead of guessing.

Calculator by calculator

Voltage Drop Calculator

Confidence: High
Based on: K-factor method: VD = 2 × K × I × D ÷ CM (×1.732 instead of 2 for three-phase), K = 12.9 copper / 21.2 aluminum; circular mils from NEC Chapter 9, Table 8; 3% / 5% guidance from the informational notes to NEC 210.19(A) and 215.2(A). Metric mode: simplified method of the Spanish Guía Técnica de Aplicación del REBT, Anexo 2 (e = 2·P·L/(γ·S·U) single-phase, P·L/(γ·S·U) three-phase, reactance neglected up to 120 mm², γ from its Table 3) and the REBT limits of its Table 6.
How it was checked: Formula confirmed in several independent electrical references; tested on 12 AWG copper, 20 A, 100 ft at 120 V (7.9 V, about 6.6%).
Not covered: AC reactance and power factor on very large conductors.

Wire & Breaker Ampacity

Confidence: High
Based on: NEC Table 310.16 (60 / 75 / 90 °C columns, copper and aluminum), ambient correction from 310.15(B)(1) and bundling adjustment from 310.15(C)(1).
How it was checked: The 90 °C copper column matched an independent reference value for value, from 14 AWG to 300 kcmil.
Not covered: The small-conductor breaker limits of NEC 240.4(D) and other overcurrent rules are flagged on the page, not fully modelled.

Conduit Fill Calculator

Confidence: High
Based on: NEC Chapter 9: Table 1 fill limits (53 / 31 / 40%), Table 4 conduit areas, Table 5 conductor areas.
How it was checked: EMT areas for 1/2", 3/4", 1" and 2" and the 12 AWG THHN area matched an independent source exactly; three 12 AWG THHN in 1/2" EMT checked by hand.
Not covered: Conduit types and insulation types not in the menus, and nipples under 24".

Electrical Box Fill

Confidence: High
Based on: NEC 314.16: Table 314.16(A) box volumes, Table 314.16(B) volume per conductor and the 314.16(B)(1)–(5) counting rules for devices, grounds and clamps.
How it was checked: Box volumes confirmed against an independent listing (e.g. 4" round/octagonal × 1-1/2" = 15.5 in³).
Not covered: Support fittings, fixture studs, hickeys and non-standard box volumes (add those allowances per NEC 314.16(B)).

Motor Full-Load Current

Confidence: High
Based on: NEC Table 430.248 (single-phase), Table 430.249 (two-phase, 4-wire) and Table 430.250 (three-phase, including the synchronous-type unity power factor columns and the 1.1 / 1.25 footnote multipliers for 90% / 80% power factor) full-load current; 125% conductor sizing per NEC 430.22. European mode: I = P / (√3 × U × η × cos φ) with the IE3 minimum efficiencies of Commission Regulation (EU) 2019/1781, Annex I.
How it was checked: Three-phase values cross-checked against an independent table and for consistency across voltage columns. The formula reproduces four manufacturer datasheets (ABB, Siemens; 15 to 160 kW at 400 V) within 1.5 %, as unit tests.
Not covered: Overload and short-circuit protection sizing, multi-motor circuits.

Transformer Sizing

Confidence: High
Based on: kVA = V × I ÷ 1000 (single-phase), × √3 for three-phase, rounded up to standard distribution transformer ratings: US sizes, or in the European mode the ratings of EU Regulation 548/2014, Annex I, Table I.1 (25–3 150 kVA); kW are turned into kVA with kVA = kW ÷ cos φ.
How it was checked: Pure power arithmetic, tested against hand calculations.
Not covered: Load diversity, motor inrush, harmonics, ambient derating and NEC Article 450 protection.

Dwelling Load Calculator

Confidence: High
Based on: NEC 220.82 optional method: 3 VA per sq ft, 1,500 VA per small-appliance and laundry circuit, first 10,000 VA at 100% and the rest at 40%, plus the larger of heating or cooling.
How it was checked: Two independent explanations of 220.82 agreed on the 10,000 VA / 40% rule; tested on a 2,000 sq ft example (22,000 VA, 100 A service).
Not covered: Heat pumps with supplemental heat, four or more separately controlled heaters, and multi-family dwellings (220.84).

EV Charger Circuit Sizing

Confidence: High
Based on: NEC 625.42 (continuous load, dedicated circuit), 625.41 (overcurrent protection at 125%) and 210.19(A)(1) for conductors; wire sizes from Table 310.16.
How it was checked: Confirmed in two independent NEC 625 summaries; a 40 A charger gives a 50 A breaker, matching published examples.
Not covered: Panel capacity, multiple chargers on one circuit and energy management systems.

BTU Load Estimator

Confidence: Medium: a starting point
Based on: Rule-of-thumb sizing: about 20 BTU per sq ft in a moderate climate, scaled by climate zone, ceiling height (about 12.5% per foot above 8 ft), insulation, sun, room type and occupants.
How it was checked: Baseline and adjustments confirmed in several HVAC references; they are common approximations, not a single standard.
Not covered: It is not an ACCA Manual J load calculation, which equipment sizing for a permit usually needs.

Superheat & Subcooling

Confidence: High
Based on: Measured superheat and subcooling from gauge readings, using pressure–temperature tables for R-410A, R-22, R-32 and R-454B (midpoint of the glide for R-454B).
How it was checked: R-410A table agreed with two independent sources within about 1.5 psi at every point. R-22 and R-32 come from one source, checked at known reference points, so they are medium confidence.
Not covered: Manufacturer target superheat and subcooling, which depend on the metering device and conditions.

Drain Pipe Sizing

Confidence: High
Based on: IPC Table 709.1 fixture drainage units and IPC Table 710.1(2) horizontal branch capacity, including the 3" minimum for any drain serving a water closet. European mode: EN 12056-2 System I, discharge units (Table 2), frequency factor K (Table 3), Qww = K × √ΣDU, and stack capacity with primary ventilation (Table 11).
How it was checked: Capacity table re-verified against the published code text on 23 September 2026, which corrected the 2" limit (see the log below). EN 12056-2 values were taken from two manufacturer planning guides that reproduce the standard's tables (Jung Pumpen, Geberit) and checked against each other and a worked example.
Not covered: Vent sizing, building drains and stacks, and local plumbing code amendments.

Septic Tank & Drainfield Sizing

Confidence: Medium: a starting point
Based on: A common tank-size pattern by bedroom count, shown next to three state rules: Minnesota (Minn. Rules 7080.1930), Massachusetts (Title 5, flat 1,500 gal) and Virginia (12VAC5-610-815). European rules: France, fosse toutes eaux per the arrêté of 7 September 2009 (3 m³ up to five main rooms, +1 m³ per extra room); Italy, Imhoff tank per the national resolution of 4 February 1977 (40–50 l/person sedimentation, 100–120 or 180–200 l/person sludge).
How it was checked: Each of the three state rules was read in the regulation text itself. Drainfield area is interpolated between two sourced percolation endpoints. The French rule was read in the decree text itself; the Italian figures match two municipal copies of the 1977 text and manufacturer sheets (medium confidence, regions may differ).
Not covered: A real design: drainfields need a percolation test and county approval.

Stair Stringer Calculator

Confidence: High
Based on: IRC R311.7.5.1 (maximum riser 7-3/4") and R311.7.5.2 (minimum tread 10"), with the rise split evenly so every riser is identical. The metric mode uses the user's own maximum riser (limits differ by country) and checks the step-length rule 2h + g ≈ 63 cm (comfortable at about 60–65 cm).
How it was checked: Checked against the 2021 IRC text and a city building department stair handout; both agreed exactly.
Not covered: Headroom, handrails, guards, winders and landings.

Roof Pitch & Shingle Calculator

Confidence: Medium: a starting point
Based on: Pitch factor √(1 + (rise ÷ 12)²) converts plan area to roof area; 100 sq ft per square; bundles per square and waste allowance chosen by the user.
How it was checked: Pitch factor matched independent sources (6/12 = 1.118). Bundles and waste figures are industry conventions that vary by product.
Not covered: Roofs with several pitches (add each plane separately); hips, valleys and dormers only through the waste allowance; manufacturer-specific coverage.

MIG/Stick Amperage & Wire Speed

Confidence: Medium: a starting point
Based on: MIG settings for ER70S-6 wire with 75/25 argon–CO₂, flat position; stick amperage ranges for E6010/E6011, E6013 and E7018 electrodes. The metric view converts the same rows (1 in = 25.4 mm, IPM × 0.0254 = m/min) and shows the nearest metric wire and electrode sizes.
How it was checked: MIG rows for 1/8" and 1/4" steel and the 1/8" E6011 range matched independent references exactly.
Not covered: TIG, out-of-position welding and materials other than mild steel.

CNC Feeds & Speeds

Confidence: Medium: a starting point
Based on: RPM = SFM × 3.82 ÷ diameter (Machinery's Handbook) and feed = RPM × flutes × chip load; carbide end-mill SFM ranges by material. Metric mode: n = vc × 1000 ÷ (π × d), vf = n × z × fz (milling) or n × f (turning, drilling); the same SFM ranges converted to m/min.
How it was checked: Formula cross-checked against tool-maker calculators; SFM ranges taken from sources that agreed with each other, discarding one inconsistent chart.
Not covered: Cutting speeds for turning and drilling (enter the tool maker's value); tapping; tool-specific chip loads (the default is editable).

Corrections and updates

What changed in the calculators and when. Layout-only changes are left out.

2026-10-25 Box fill: grounding conductors now follow NEC 2020/2023 314.16(B)(5) (up to four count as one allowance, each further one adds a quarter); new field for the number of grounds. Every calculator page now lists common mistakes, limits and primary sources.
2026-10-20 Voltage drop: metric mode following the Spanish REBT technical guide. Also metric modes for CNC speeds (with turning and drilling), roof pitch, superheat/subcooling, welding, transformer sizing (EU 548/2014 ratings) and stairs.
2026-10-20 Motor current: two-phase motors (Table 430.249), synchronous columns with the 1.1 / 1.25 power-factor multipliers, 115 V and 2,300 V, up to 500 hp.
2026-10-18 Correction to box fill: four box volumes fixed (4" square × 1-1/2" = 21.0 in³; 3×2" device boxes 2-1/2", 2-3/4", 3-1/2" deep = 12.5, 14.0, 18.0 in³). Added EMT conduit fill and box fill charts.
2026-10-14 Septic sizing: added the French (fosse toutes eaux) and Italian (Imhoff) rules, the default on the French and Italian pages.
2026-10-14 Motor current: added the European method (IEC motors in kW at 50 Hz), the default on the German, Italian, French, Spanish and Greek pages.
2026-10-14 Pipe sizing: added the European method (EN 12056-2), the default on the German, Italian, French, Spanish and Greek pages, with translated appliance names.
2026-10-10 Every calculator page now has a worked example, frequently asked questions and, where useful, a reference table computed with the same formula.
2026-09-24 Added dwelling load, EV charger, superheat/subcooling, septic, stair stringer, roofing, welding and CNC calculators. The R-410A table, the stair limits and the three state septic rules were checked against their sources before publishing.
2026-09-23 Correction to drain pipe sizing: a 2" horizontal branch now carries at most 6 drainage fixture units (IPC Table 710.1(2)); it previously allowed 16. The unverified 1-1/4" row was removed, and any branch serving a water closet now needs at least 3".
2026-09-22 First electrical, HVAC and plumbing calculators published.