Electrical & MEP Calculators
23 free electrical & mep calculators — standards-based, with formulas, worked examples, and no sign-up.
Most of this category is the National Electrical Code turned into arithmetic, article by article. Service and feeder loads follow the NEC Article 220 demand-factor method through to panel schedules; conductor ampacity applies the Table 310.16 base ratings with temperature and conduit-fill derating for copper and aluminum; voltage drop covers single- and three-phase runs against the NEC and IEC recommended limits; box fill itemizes every counted conductor, device, and clamp per 314.16(B); outlet spacing walks walls and countertops through the 210.52 rules; cable tray and conduit fill follows Article 392; and EV charging circuits carry the 125% continuous-load factor the code requires.
The power-systems tools go beyond the NEC into IEEE practice. Fault current is computed from transformer impedance with point-to-point reduction and motor contribution — the number breaker interrupting ratings must exceed. Arc flash follows IEEE 1584 to incident energy and PPE category. Harmonics report THD and transformer K-factor against IEEE 519 limits; grounding resistance uses the Dwight/IEEE 142 rod and grid formulas; and motor starting estimates locked-rotor kVA and the voltage dip that makes lights flicker across the facility. Transformer, generator, and power-factor-correction sizing complete the distribution chain.
For design work, lighting layouts use the zonal cavity method to luminaire counts, grids, and power density, and the solar PV yield tool estimates array output per IEC 61853 — the electrical end of the renewable story, which continues in the Renewable Energy category. The conduit bending calculator is the one purely field tool: offset multipliers, saddle marks, and stub-up take-up, straight from bender practice.
All Electrical & MEP Tools
Electrical Load Calculator
NEC Article 220 demand factor calculations with service sizing, cable selection, and panel schedule generation.
Lighting Design Calculator
Luminaire layout using zonal cavity method with lux levels, uniformity ratio, and power density.
Solar PV Yield Calculator
Panel array sizing and annual energy yield estimation with tilt and azimuth optimisation.
Cable Pulling Tension Calculator (Straight Runs & Bends)
Segment-by-segment cable pulling tension by the Southwire Power Cable Installation Guide Chapter 7 method: horizontal straights add w·μ·W·L, vertical risers add or subtract W·L, and conduit bends multiply by e^(wμφ) per the manual's published approximation — checked against the governing allowable tension (verified Table 7-4 conductor stress: 0.008 lb/cmil soft copper and hard aluminum, 0.006 for 3/4-hard and AA-8000 aluminum, times kcmil × 1,000 × conductors; or the device — pulling eye ≤ 10,000 lb, basket grip 1,000 lb) and sidewall pressure T/R at every bend against the Table 7-7 limits (500 lb/ft through 15 kV, 300 above and for interlocked armor, editable), with lubricated-friction guidance from Table 7-6 (0.35–0.5 typical), a weight-correction input for multi-cable pulls, a per-segment tension table, and a full pull-direction reversal comparison — the direction with the bends near the feed always wins. Planning estimate; jamming/clearance out of scope.
Cable Tray & Conduit Sizing
Fill ratio and derating calculations for cable trays, conduits, and electrical raceways.
Voltage Drop Calculator
Single and three-phase conductor voltage drop with NEC 3%/5% and IEC 60364 compliance checks.
Fault Current Calculator
Available short-circuit current at a transformer and downstream points via the point-to-point method.
Transformer Sizing Calculator
Required kVA, next standard rating, and full-load currents for single and three-phase transformers.
Power Factor Correction Calculator
Capacitor kVAR sizing, current and loss reduction, and demand-charge savings for PF correction.
Grounding Conductor Size Calculator (NEC 250.66 & 250.122)
Both NEC grounding tables in one tool: the grounding electrode conductor from NEC 2023 Table 250.66 keyed to the largest ungrounded service-entrance conductor (copper or aluminum, parallel sets by the table-note equivalent circular-mil area), and the equipment grounding conductor from Table 250.122 keyed to the OCPD rating (15–3000 A, next-higher-row rounding), both answered in copper and aluminum, plus the 250.122(B) proportional upsize that scales the EGC by the installed-to-required circular-mil ratio from Chapter 9 Table 8 and rounds up to the next standard size — with the 250.66(A)–(C) rod/pipe/plate and concrete-encased reliefs stated as caveats.
Grounding Resistance Calculator
Earthing resistance of driven rods, parallel rods, and ground grids against the 25-ohm NEC target.
Generator Sizing Calculator
Standby/prime genset kW and kVA from running load and motor starting with derating and spare margin.
Arc-Flash Calculator
IEEE 1584 arcing current, incident energy, arc-flash boundary, and NFPA 70E PPE category.
Motor Starting & Voltage Dip Calculator
Three-phase motor full-load and locked-rotor current, bus voltage dip, and reduced-voltage starting method comparison.
Lightning Protection (Rolling Sphere)
Rolling-sphere method of IEC 62305-3 for sphere radius, protected radius and area, mesh size, and protection angle.
EV Charging Load Calculator
Electric-vehicle charging (EVSE) load sizing per NEC Article 625: per-charger continuous load at 125%, single- and three-phase circuit current, total connected demand in amps and kVA, and an energy-management diversity factor for the recommended feeder/service ampacity.
Harmonic Distortion & K-Factor Calculator
Total harmonic distortion (THD), true RMS and transformer K-factor from a measured harmonic spectrum per IEEE 519 and IEEE C57.110, with the recommended K-rated transformer and distortion-limit guidance.
Conductor Ampacity Calculator (NEC 310.16)
NEC Table 310.16 wire sizing for copper and aluminum conductors: ampacity by insulation temperature rating with ambient-temperature correction and conductor-bundling adjustment, continuous-load 125% sizing, and termination temperature limit checks.
Conduit Bending Calculator (Offset, Saddle, Stub-Up)
EMT hand-bender layout for the three everyday bends: two-bend offsets with the exact 1/sin(angle) multiplier and shrink, 3-point saddle center and outer marks, and 90 degree stub-ups using the bender take-up deduct per trade size, with mark positions from the obstruction and a bend-spacing check.
Box Fill Calculator (NEC 314.16)
Minimum outlet, device, and junction box volume per NEC 314.16(B): Table 314.16(B) allowances for conductors by AWG size, internal clamps, support fittings, device yokes at two allowances each, and equipment grounding conductors, with every standard box that fits listed with its utilization percentage.
Outlet Spacing Calculator (NEC 210.52)
Minimum dwelling-unit receptacle counts per NEC 210.52, verified against the published 2023 code text (every encoded value is identical in the 2020 edition): wall spaces entered in feet under the 210.52(A) 6-ft rule — no point along the floor line more than 6 ft from a receptacle, so receptacles at most 12 ft apart, with any space 2 ft or wider counting and doorways, fireplaces, and fixed cabinets breaking the wall line — and wall countertop segments entered in inches under the 210.52(C) 24-in rule with the 12-in minimum width, each list giving per-segment minimums plus wall, countertop, and grand totals, documented as code minimums that real layouts exceed, with the 2023 island/peninsula change (receptacles now optional with a future-provision requirement) and GFCI/AFCI protection covered in prose.
Ohm's Law Calculator — Volts, Amps, Ohms & Watts
The full Ohm's-law/power-law relationship from any two known quantities: pick the known pair among voltage, current, resistance, and power (all six combinations) and get all four via V = I·R and P = V·I with the derived forms P = V²/R, P = I²·R, R = V²/P, V = √(P·R), and I = √(P/R) — DC / purely resistive AC basis (power factor 1) with division-by-zero and range guards, and a worked-examples table covering one case from each common known pair.
Three-Phase Power Calculator — kW, kVA, Amps & HP
The four everyday AC power conversions for single- and three-phase systems: kW to amps via I = P·1000/(√3·V·PF) with line-to-line voltage, kVA to amps via I = S·1000/(√3·V) (no power factor — a kVA rating already is √3 × V × I, which is why transformer nameplate amps hold at any load PF), amps to kW, and motor HP to amps treating nameplate horsepower as shaft output with electrical input kW = HP × 0.7456999/η (mechanical hp = 745.6999 W per NIST SP 811) at an entered motor efficiency — with power factor 0.05–1, voltage presets 120/208/230/240/400/480/600 V plus free entry to 1,000 V, the full power triangle in every result, and a kVA-to-amps chart at 400 and 480 V.
Frequently Asked Questions
Which comes first — ampacity, voltage drop, or breaker size?
Ampacity is the code minimum: the conductor must carry the load after 310.16 derating, protected by its breaker. Voltage drop then frequently governs on long runs — a conductor can be perfectly legal and still deliver poor voltage at 200 ft. Practical order: size the breaker from the load (with 125% on continuous loads), pick the minimum legal conductor from the ampacity tool, then run voltage drop and upsize if you exceed the 3%/5% recommendations.
Why does fault current matter if my breakers already match the load?
Load rating and interrupting rating are different specs. A breaker sized perfectly for 20 A of load may be rated to safely interrupt only 10 kA of fault current — if the available fault at that point (transformer size and impedance, service length) exceeds it, the breaker can fail violently instead of clearing. The fault current tool gives the available fault at each point; the arc flash tool turns the same physics into worker-safety PPE categories per IEEE 1584.
The box fill numbers seem stricter than what I see in old work. Has the rule changed?
The 314.16(B) counting rules are stricter than folk practice: every conductor entering counts, each device counts double, all grounds together count once (two in the 2023 cycle for larger boxes), and internal clamps count. Old overfilled boxes are grandfathered until touched. The calculator itemizes each rule line so you can see exactly what consumed the cubic inches — usually the device yokes.