Electrical Calculations Guide

The four questions every circuit must answer

Designing an electrical circuit is answering four questions in order. First: how much current does the load draw, continuously? Second: what conductor carries that current without overheating? Third: does that conductor deliver acceptable voltage at the load end? Fourth: what overcurrent device protects the conductor? Each question has a calculator, and skipping one is how circuits end up non-compliant.

The load current comes from the device nameplate or from power divided by voltage — the kVA-to-amps and motor current calculators handle both patterns. Continuous loads (running three hours or more) multiply by 1.25 before anything else happens; that single rule drives most of the sizing that follows.

Ampacity: the conductor's heat budget

A conductor's ampacity is the current it can carry without its insulation exceeding its temperature rating. The NEC table value assumes 30 °C ambient and three current-carrying conductors; real installations routinely violate both assumptions. Attics reach 50 °C, conduit fills hold a dozen conductors, and both corrections stack multiplicatively — a 10 AWG THHN circuit derated for a hot attic with nine conductors drops from 35 A to under 22 A effective ampacity.

The wire size calculator walks the standard copper conductor series; the derating calculator applies the real-world corrections. Do both before trusting any ampacity figure.

Voltage drop: the performance limit

Code ampacity keeps the conductor from burning; voltage drop keeps the load working. The NEC's informational guidance suggests 3% for branch circuits and 5% total. Exceed it and motors lose torque, lights dim, and electronics brown out — and the energy wasted as I²R heat in the cable is paid for every hour the circuit runs.

Drop grows with length and current: doubling either doubles the drop. The voltage drop calculator computes it for single and three-phase circuits with NEC conductor data; the cable reactance calculator refines it for large conductors, where inductive reactance rivals resistance.

Power, power factor and the kVA world

Real power (kW) does work; apparent power (kVA) is what wires and transformers must carry. The gap between them is reactive power, quantified by power factor. Motors at part load and heavily loaded transformers can pull power factor down to 0.7 — meaning 30% more current in every conductor than the useful power requires.

Utilities penalize industrial customers below roughly 0.90 PF; the power factor correction calculator sizes the capacitor bank that fixes it. The three-phase power calculator shows the full power triangle for balanced loads.

Energy: where electricity becomes money

Power is instantaneous; energy is power × time, and energy is what you buy. A 1,500 W heater running 4 hours consumes 6 kWh — at $0.15/kWh, 90 cents per evening, $328 per year. Multiplying small daily costs by 365 is how efficiency decisions get made rationally.

The energy cost calculator handles appliances and fleets of devices; the EV charging and cooling cost calculators apply the same math to their domains.

Questions about electrical calculations

What is the difference between kW and kVA?

kW is real, usable power. kVA is what the conductors must carry — real and reactive current combined. At power factor 1.0 they're equal; at PF 0.8, a 10 kVA supply delivers only 8 kW of work.

Should I size wire for ampacity or voltage drop?

Both — whichever demands the larger conductor. Short, high-current circuits are ampacity-limited; long, modest-current circuits are drop-limited. Check both on every run.

Is voltage drop a code requirement?

The 3%/5% figures appear as informational notes, not enforceable sections — but equipment malfunction from excessive drop creates real liability, and some jurisdictions adopt them as mandatory.