Spark Tools

Electrical calculators

Seven calculators that print the arithmetic underneath the answer. Useful on a job, and useful the week before an exam.

These are planning tools built on the standard tables. The code edition adopted where you work and the inspector standing in front of you are the authority, not this page.
On this page

Voltage drop

How much voltage a run loses on the way to the load, against the three percent target for a branch circuit.

Wire size for a load

The smallest conductor that carries the load, bumped up when the run is long enough that voltage drop takes over.

Conduit fill

Whether the conductors fit, and the smallest pipe that takes them. Areas are for THHN, which is the common case.

Box fill

The cubic inches a box needs for what is going in it. Every allowance uses the largest conductor in the box.

Dwelling load calculation

The standard method for a home, so you can size a service before quoting a panel or a car charger.

Ohm law and watts

Fill any two boxes and the other two follow. Clear a box to solve for something else.

Car charging time and cost

How long a charge takes on a given circuit, what it costs, and the circuit size the charger wants.

Every term on this page, explained Why wire size and breaker size have to match