AmpSizer

2023 NEC · cited on every page

Size the conductor. Show the work.

Ampacity, derating, voltage drop, and conduit fill — worked from published tables with every step visible and the source cited. Free, private, and entirely in your browser. No account, nothing uploaded.

What size wire do I need?Browse the tables
6 AWG65 A at 75°C
21
Conductor sizes, 14 AWG to 750 kcmil
123
Ampacity values, copper and aluminum
426
Free reference pages
4
Calculators, free forever

Calculators

Every one of them free

Wire size calculatorEnter the load, the run length, and the conditions. Get the conductor that satisfies both ampacity and voltage drop, and see which one governs.Ampacity lookupAny conductor, any temperature column, derated for your actual ambient and conductor count — with every step of the arithmetic shown.Voltage drop checkerHow much voltage a run costs you, what reaches the load, and the smallest conductor that lands inside your limit.Conduit fillBuild a conductor set, check it against EMT, PVC, or RMC, and find the smallest trade size that takes the whole run.

Common lookups

Straight to the answer

12 AWG copper10 AWG copper8 AWG copper6 AWG copper4/0 aluminum20 A breaker50 A breaker100 A breaker200 A breaker50 A at 100 ft100 A at 150 ft1" EMT fill

Why the work is shown

Most wire-size tools hand you a number. That is fine right up until an inspector asks how you got it, or until the conditions on your job differ from whatever the tool silently assumed.

AmpSizer shows the published table value, then each correction applied to it — the ambient factor, the conductor-count adjustment, the small-conductor ceiling — and names which requirement ended up governing. If you disagree with an assumption, you can see exactly where to change it. And every page cites the specific table its figures came from, with the edition labelled, so you can check the source rather than trusting us.

The reference surface is never gated. Ampacity is safety-relevant information; putting it behind a paywall would be the wrong call regardless of the business case. Premium adds job-level tooling — a panel schedule, a printable job sheet, unlimited saved circuits — on top of a reference that stays open.

What AmpSizer is not

It is not the code, and it does not reproduce code text. It publishes numeric table data with the source cited — the requirements themselves, along with the exceptions and conditions those tables depend on, live in the code document, and you need that document to do this work properly.

It is not an electrical design. It does not model motor circuits, air conditioning equipment, paralleled sets, service calculations, or the load-specific articles that govern much real work. It assumes conditions that may not match your job.

And it is not a substitute for a licensed electrician. Jurisdictions adopt code editions years apart and amend them freely, so the edition cited here may not be the one enforced where you are. Treat every figure as a starting point for a conversation with the person who will sign off on the work.