Electrical & Plumbing Calculators

NEC-based sizing tools for circuits, conduit, and plumbing planning — wire gauge, voltage drop, breaker sizing, water heater sizing, and more.

Last updated: August 2026

Safety first: Electrical work done wrong can cause fires and electrocution. These calculators follow common NEC (National Electrical Code) tables, but local codes vary and amendments are common. Use these tools for planning and material estimates — then have a licensed electrician or plumber review and perform (or at minimum inspect) the work. Permits are required for most electrical and plumbing work, and unpermitted work can void insurance coverage and complicate a home sale.

Plan the Circuit Before You Pull Wire

Electrical and plumbing projects punish guesswork more than any other kind of home improvement. A paint estimate that's off by a gallon costs you a store run; a wire gauge that's off by one size is a code violation at best and a fire hazard at worst. That's why every number on these pages comes from the same place an electrician's does — the NEC ampacity, conduit fill, and box fill tables — rather than rules of thumb. Before you buy a spool of wire, trench a conduit run to a detached garage, or commit to a tankless water heater, run the numbers. Ten minutes of planning tells you exactly what gauge to buy, whether your run is long enough to need upsizing for voltage drop, how many conductors legally fit in the conduit you already own, and whether your panel can even support the load you're adding.

The same logic applies on the plumbing side. Water heaters are sized by household demand — peak-hour gallons for tanks, simultaneous flow rate for tankless — and buying the wrong size means cold showers or wasted money every month for the next decade. Pipe volume matters for winterizing, chemical dosing, recirculation sizing, and knowing how much water you're about to drain onto the basement floor.

Why Wire and Breaker Sizing Go Together

The single most important concept in residential electrical work: the breaker protects the wire, not the appliance. A circuit breaker's job is to trip before the conductor behind your walls overheats. That means the breaker size and the wire gauge are a matched pair — a 20-amp breaker requires 12 AWG copper or heavier, a 30-amp breaker requires 10 AWG, and putting a bigger breaker on a smaller wire removes the very protection the breaker exists to provide. When you size a circuit, you work in one direction: figure the load in watts, convert to amps, apply the 125% continuous-load factor where it applies, pick the breaker, then pick the wire that breaker is allowed to protect. Long runs add one more step — voltage drop — which is why our wire size calculator checks distance too, and will tell you when a 12 AWG circuit needs to be pulled in 10 AWG just because of how far it travels.

Browse Electrical & Plumbing Calculators

Pick the tool that matches your project. Each calculator is built on the relevant NEC table or standard sizing method and explains the code context behind its answer.

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