Electrical Calculators

Electrical work is the area where a small mistake is dangerous, so these tools are planning aids only. They estimate wire size, voltage drop, conduit fill, loads and generator size from standard practice, and your local electrical code and a licensed electrician decide what is allowed.

Which tool answers which question

• Conduit Fill Calculator: Conductor area and count against conduit area to percent fill and max conductors.
• Electrical Load Calculator: Connected watts to load current and a recommended breaker size, with the 80% continuous-load rule.
• Generator Size Calculator: Running and starting watts of your essential loads to a recommended generator size.
• Voltage Drop Calculator: Wire gauge, length and load to voltage drop and percent drop on a circuit.
• Wire Gauge Calculator: Load current and wire run length to a recommended wire gauge, keeping drop under 3%.

Wire size, voltage drop and conduit fill

Wire size is chosen so the conductor can carry the current without overheating, and long runs need a larger wire to keep the voltage drop low. Common guidance keeps voltage drop in a branch circuit to about 3 percent. Conduit has a fill limit, commonly 40 percent of its area for three or more conductors, so that wires can be pulled and can shed heat.

Loads and generators

To size a generator, add up the running watts of everything you want to power and allow for the higher starting surge of motors. A generator must be connected to a house only through a proper transfer switch, and never run indoors or in a garage, because of carbon monoxide.

Common mistakes

• Working on a live circuit. Turn off and test the power.
• Sizing wire on current alone. Length and temperature matter.
• Backfeeding a house with a generator. Use a transfer switch.
• Skipping the permit and inspection. Electrical work is usually inspected.