Electrical installation calculators

Size and check an installation to BS 7671, IEC 60364 or the NEC — cable sizing and voltage drop, load and supply, protection, containment and running cost. Every result names the standard it came from and the date it was last reviewed.

Free calculators: 32
Standards: BS / IEC / NEC
Registration: None
Engineering input: 4k7

32 calculators

Ohm’s Law

Solve for voltage, current, resistance or power. Get all four values at once, plus the resistor wattage you actually need.

R1R2

Voltage Divider

Calculate output voltage, ratio, current and output impedance — with the loading effect built in, not hidden on another page.

Power & Energy

Power from any two of voltage, current and resistance — then energy in kWh and what it costs to run.

4-6

Resistor Color Code

Decode 4, 5 and 6-band resistors, or enter a value to see its bands. Tolerance range and temperature coefficient included.

104

Capacitor Code

Decode ceramic capacitor markings — 104 is 100 nF, not 104 of anything — or work backwards from a value to the printed code.

LED

LED Resistor

Size the current-limiting resistor for any LED. Rounds up to the nearest E24 value, because rounding down overdrives the LED.

RC Filter

Cutoff frequency, time constant and rise time for a single-pole RC low-pass or high-pass, with the gain and phase at any test frequency plotted on a live Bode chart.

555

555 Timer

Frequency, period, duty cycle and pulse width for the NE555 from R1, R2 and C — with the duty-cycle trap and the pin 7 current limits called out.

mAh

Battery Life

How long a load really runs from a given cell — duty cycle, regulator topology and self-discharge included, which is exactly where the naive mAh ÷ mA answer goes wrong.

ΔV

Voltage Drop

Voltage drop for DC, single-phase and three-phase runs in copper or aluminium, sized in mm² or AWG, to BS 7671, IEC 60364 or NEC limits — with the BS 7671 Appendix 4 mV/A/m figure shown alongside.

Cable Current Capacity

Derated current-carrying capacity to BS 7671 and IEC 60364 — Iz = It × Ca × Cg × Ci × Cf, with the ambient, grouping, thermal insulation and BS 3036 factors applied and the Ib ≤ In ≤ Iz condition checked.

S=√(I²t)/k

Adiabatic Equation

Minimum conductor size to survive a fault, S = √(I²t) ÷ k, to BS 7671 Regulation 543.1.3 — with k derived from the conductor material and its permitted temperature rise rather than looked up.

AWG

AWG Wire Gauge

Diameter, cross-section, resistance and current rating for any AWG size — with separate free-air and in-conduit ampacity, because they differ by a factor of two.

AWG

Wire Size

What size wire you need, from the load, the run length and the conditions — sized against both NEC ampacity and voltage drop, with the terminal temperature rule that stops 90 °C wire giving 90 °C ampacity.

Earth Fault Loop Impedance

Zs = Ze + (R1 + R2) against the maximum permitted for the protective device, to BS 7671 Chapter 41 — with the maximum derived from U0 × Cmin ÷ Ia rather than looked up, and the cold-measurement rule applied separately.

Ipf

Prospective Fault Current

The highest current a fault could produce, from a loop impedance, a transformer nameplate or your meter readings — checked against the breaking capacity of the device that has to interrupt it.

EGC

Ground Wire Size

Equipment grounding conductor size from the breaker rating — including the 250.122(B) proportional increase almost every other calculator leaves out when a run is upsized for voltage drop.

IPC

PCB Trace Width

Minimum trace width for a given current and temperature rise, to IPC-2221B — in mils and mm, with copper area, resistance, voltage drop and final conductor temperature.

kVA

Electrical Load

Size a house service the way an electrician does: NEC Article 220 standard calculation, with the demand factors that stop a 40 kW house needing a 400 A service.

⚡

Panel Capacity

Whether your existing panel has room for a new circuit — in amps and in breaker spaces, which are different constraints and only one of them is visible through the door.

40%

Conduit Fill

How many conductors fit in a conduit, and what size you need. EMT, IMC, RMC and PVC with the real Chapter 9 table areas — including the 31% rule that catches people out.

∠

Conduit Bending

Where to put the marks, for any angle rather than the five on the bender — plus the shrink figure that gets forgotten and lands the pipe short of the box.

in³

Box Fill

Whether your conductors, devices and clamps fit the box — with the three counting rules that produce nearly every wrong answer: pigtails count zero, all grounds count one, each device counts two.

kWh

Appliance Wattage

What every household appliance draws, what it costs to run for a year, and where it ranks against everything else in the house.

kWh

Electricity Bill

Your bill from meter readings or kWh — including the standing charge, tiered blocks and tax that most calculators leave out, plus the effective rate you actually pay per unit.

EV Charging

How long a charge really takes and what it really costs — with the DC taper that makes 80–100% slower than 10–80%, and the charging losses you pay for but never receive.

Generator Sizing

What size generator you actually need — running watts plus the largest single starting surge, derated for altitude and heat. Not the sum of every surge, which is how most calculators oversize you.

3Ø

Three-Phase Power

kW, kVA and kVAr from line voltage and current, with star and delta phase values and what power factor correction would save you.

FLA

Motor Full Load Amps

Full load current from motor kW or hp, including efficiency — the term most calculators drop, which understates the current by 10–15%.

Motor Breaker

Breaker, overload relay and conductor size for a motor circuit — three different percentages of two different currents, which is why a 40 A breaker on 12 AWG is correct rather than a violation.

kVA

Transformer Sizing

kVA from the load, full-load current on both windings, and the overcurrent devices — including why the primary is allowed 250% once the secondary is protected.

Solar & Battery Sizing

Array watts and battery bank capacity from your daily load — with system losses and depth of discharge included, which is where most sizing goes wrong.

Browse by category

Each hub collects the calculators for one area, with the background you need to choose between them.

Supply & Demand — How much power the installation actually needs, and what size supply, panel and transformer will carry it.
Cables & Voltage Drop — Choosing a conductor that carries the current without overheating and delivers the voltage the load needs.
Protection & Earthing — What protects the circuit, whether it disconnects fast enough, and how the installation gets to earth.
Containment — Whether the cables physically fit — conduit, boxes and the bends between them.
Motors & Equipment — Circuits for the things on the end of the wiring — motors, welders, generators and special locations.
Energy & Running Cost — What it draws, what it costs to run, and what a solar or EV setup actually delivers.
Electronics — Bench and circuit-design calculators — Ohm’s law, component codes, LEDs, filters and timers.

Why ElectroCalculators?

Engineering notation input. Type 4k7, 2M2 or 100n directly — no unit dropdowns to fight with.
Standard component values. Every computed resistor or capacitor value is snapped to the nearest E-series part, with the resulting error shown.
Power ratings, not just numbers. Results include the dissipation and the wattage you should actually specify.
Shareable and embeddable. Every result has its own URL, and every calculator can be embedded in your own site.
Verified formulas. Cross-checked against IEC, IEEE and NEC and standard reference texts, with unit tests against textbook worked examples.