Electrical & Plumbing Calculators

Wire size, voltage drop, pipe volume and loads.

Electrical and plumbing sizes come from code tables, not guesswork: NEC ampacity and voltage drop for wire and conduit, water supply fixture units for pipe, first hour rating for water heaters. The calculators here apply those published tables so you can plan a circuit, a feeder, a supply line or a generator before calling a licensed trade.

Every result on these pages is a planning estimate. Ampacity derating, termination temperatures, local amendments and inspection all change the final answer, and this work is permitted and inspected in most jurisdictions. Use the numbers to understand a quote, not to skip the electrician or plumber.

All electrical & plumbing calculators

Wire Size CalculatorFind the wire gauge (AWG) for a circuit from the load in amps, run length, voltage and allowed voltage drop, using NEC ampacity tables for copper and aluminum.Generator Size CalculatorAdd up running and starting watts for a refrigerator, sump pump, furnace, well pump, air conditioner and lights to find the generator size in kW you need.Voltage Drop CalculatorCalculate the voltage drop in volts and percent for a given wire size, load, run length and voltage, plus the longest run that stays inside the NEC 3% recommendation.Electrical Load CalculatorCalculate the service load of a house in VA and amps with the NEC 220.82 optional method from floor area, appliances, heating and cooling, and see what service size fits.Conduit Fill CalculatorCheck conduit fill percent for THHN conductors in EMT or PVC with NEC Chapter 9 tables, and find how many wires fit a conduit or the smallest conduit for your wires.Pipe Volume CalculatorCalculate how much water a pipe holds in gallons and cubic feet from its inside diameter and length, with copper, PEX and PVC sizes built in, plus the hot water wait.Pipe Size CalculatorSize the water supply main for a house by the fixture unit method: count bathrooms and appliances for WSFU, convert to peak gpm, and pick a pipe for the pressure and run.Septic Tank Size CalculatorEstimate the septic tank size a house needs from its bedroom count, plus the design daily flow, drain field absorption area and trench length for your soil type.Sump Pump Size CalculatorSize a sump pump from how fast water rises in the pit, the vertical lift and the discharge run, giving inflow in gpm, total dynamic head and the pump horsepower you need.

A typical project, in order

Carry each result into the next tool.

1Electrical Load CalculatorService size for the whole house
2Wire Size CalculatorGauge for a circuit by load and distance
3Voltage Drop CalculatorCheck the drop on a long run
4Conduit Fill CalculatorConduit size for the conductors
5Generator Size CalculatorBackup power for the essentials
6Pipe Size CalculatorWater main by fixture units
7Water Heater Size CalculatorTank or tankless by first hour rating

Electrical & Plumbing questions

What size wire do I need for a 20 amp circuit?

12 AWG copper at minimum. Past about 45 ft one way at 120 V, step up to 10 AWG to keep voltage drop under 3%.

What is the 3% voltage drop rule?

An NEC recommendation to keep the drop on a branch circuit under 3% and the feeder plus branch under 5%, so motors run cool and lights do not dim.

Can I use these calculators instead of an electrician?

No. They help you plan and check a quote. Wiring and plumbing work needs a permit and inspection in most places and must follow the current code and local amendments.