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Electrical Calculators

Electrical relationships derived from physics rather than from a code table — Ohm's law, the power relationships, voltage drop and three-phase conversions. Everything here shows its working, and everything here stops short of telling you what a code requires.

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Physics here, code elsewhere

Every calculator in this section is built from relationships that belong to nobody: Ohm's law, the power identities, the voltage-drop formula, and the AWG geometric progression that gives conductor area. Circular mils are computed from the gauge definition rather than looked up, so the figures are exact for any size.

What you will not find here is ampacity, conduit fill or derating tables. Those are published in the National Electrical Code, they are copyrighted by the NFPA, and reproducing them would be both an infringement and a poor service — code requirements vary by jurisdiction, edition and installation conditions in ways a web page cannot track.

Voltage drop is not ampacity

These are two separate questions and conflating them is the commonest sizing mistake. Ampacity asks whether a conductor can carry the current without overheating — a safety limit. Voltage drop asks whether enough voltage survives the journey to run the load properly — a performance limit.

On short runs ampacity governs. On long ones, voltage drop almost always governs first: a conductor can be perfectly safe and still deliver unusable voltage at the far end. Three per cent on a branch circuit and five per cent overall is the widely used design target, and a motor fed below its rated voltage draws more current, runs hotter and fails early.

Where this stops

Nothing here tells you what size conductor a code requires, what overcurrent protection to fit, or whether an installation complies. Those depend on the code edition in force where you are, on installation conditions like ambient temperature and conductor bundling, and on inspection.

Undersized conductors cause fires. Use these to understand the physics, to check a design's reasoning, or to scope a job — then have the work specified and inspected by a licensed electrician.

Frequently asked questions

How do I calculate voltage drop?

For single phase, drop equals 2 × K × length × current ÷ circular mils, where K is 12.9 for copper and 21.2 for aluminium. Three phase substitutes √3 for the 2. Enter the one-way distance — the formula already accounts for the return path.

What voltage drop is acceptable?

Three per cent on a branch circuit and five per cent overall is the widely used design target. It is a performance recommendation rather than a hard limit, but motors fed low voltage draw more current, run hotter and die early.

Why do you not include ampacity tables?

They are published in the NEC and copyrighted by the NFPA. Beyond the legal point, ampacity depends on ambient temperature, conductor bundling, insulation rating and the code edition in force locally — a static table on a web page would be wrong for many real installations.

What is the difference between kVA and kW?

kVA is apparent power and kW is real power; power factor is the ratio between them. Transformers are rated in kVA because apparent power is what heats the windings, regardless of how much useful work the load extracts.

Can I use these to wire my house?

No. These explain the physics and let you check reasoning or scope a job. Conductor sizing, overcurrent protection and compliance belong with a licensed electrician working to the code in force where you are.