Sizing Electrical Wire Right, the First Time
Wire Calculator is a free engineering-grade wire sizing tool built on the National Electrical Code. Enter your load, your distance, and your installation conditions — get the correct AWG, the reason behind it, and the breaker that protects it, all computed instantly in your browser.
Code-aware wire sizing, in secondsPhoto: UnsplashWhat Is Wire Calculator?
Wire Calculator (wirecalculator.com) exists to answer one deceptively simple question accurately: what size wire does this circuit need? The question is simple to ask and surprisingly easy to get wrong. Undersized conductors run hot, waste energy as heat, trip breakers on ordinary afternoons, and in the worst cases start fires. Oversized conductors waste money on copper, refuse to fit lugs and raceways, and make every termination harder than it needs to be. Between those two failure modes sits a body of engineering — ampacity tables, temperature derating, small-conductor caps, and voltage-drop mathematics — that professionals navigate daily and DIYers meet for the first time, usually at the hardware store counter, holding two spools and a phone.
We built Wire Calculator to put that body of engineering in everyone’s pocket. The tool is deliberately narrow. It does not try to be a full electrical design suite, a load-calculation worksheet, or a permit filing system. It does one thing — match conductors to circuits, and circuits to conductors — and it does that one thing with the same table values and rules a licensed electrician would apply by hand: NEC Table 310.15(B)(16) and (17) ampacities, the 240.4(D) small-conductor rule, standard 240.6(A) overcurrent sizes, and the classic voltage-drop formula Vd = k × L × R × I ÷ 1000.
Everything runs client-side. When you type 240 volts, 48 amperes, and a 150-foot run, those numbers are evaluated by JavaScript in your browser and answered before your finger leaves the key. There is no server round-trip, no account, no history. That architectural choice is also a privacy promise: we literally cannot lose data we never receive.
NEC-Anchored Math
Every recommendation traces back to NEC Table 310.15(B)(16)/(17) ampacity data, the 240.4(D) small-conductor rule, and standard 240.6(A) breaker sizes — not to ad-hoc shortcuts.
Privacy by Design
Voltage, amperage, distance, and wire parameters are computed entirely in your browser. Nothing you type is transmitted, logged, or stored on our servers.
Instant Dual Direction
Solve forward from load to wire gauge, or in reverse from an installed AWG to the maximum safe amps and the largest standard breaker that protects it.
Free, No Sign-Up
No accounts, no paywalls, no export limits, no watered-down trial version. Open the page, enter your numbers, get your answer, and get back to work.
Two Calculators, One Wire-Sizing Engine
Real projects run in both directions. Sometimes you know the load and need the conductor; sometimes you have a spool of 10 AWG in the garage and need to know what it can safely carry on a 200-foot run. Wire Calculator handles both directions with the same underlying rule set, so answers stay consistent no matter which door you enter through.
Forward: Load → Gauge
Wire Size Calculator
- Enter circuit voltage, load amperes, and one-way run length in feet
- Choose single-phase or three-phase power
- Select an insulation temperature rating: 60°C, 75°C, or 90°C
- Pick copper or aluminum conductors
- Set the installation method: raceway, cable, direct burial, or free air
- Enforce a voltage-drop ceiling from 1% to 5%
Inverse: Gauge → Amps
Reverse Ampacity Calculator
- Start from wire you already own or have already pulled
- Enter the AWG or kcmil size, run length, and supply voltage
- Get the maximum continuous amps the conductor can safely carry
- See the precise limit before flooring, so borderline circuits are obvious
- Receive the largest standard breaker size that protects the conductor
- Learn which constraint binds: table ampacity, the small-conductor rule, or voltage drop
From load to lug — both directions coveredPhoto: UnsplashHow Our Calculations Work
Trust in a calculator comes from being able to check its work. That is why every result on Wire Calculator shows its limiting factor and its notes. Here is the exact logic the engine applies, in order, for every calculation.
Table Ampacity
The engine starts with NEC Table 310.15(B)(16) for conductors in raceway, cable, or direct burial, and Table 310.15(B)(17) for conductors in free air, filtered by your conductor material and 60°C / 75°C / 90°C insulation rating. These are the published ampacities for three current-carrying conductors at a 30°C (86°F) ambient.
Small-Conductor Cap
For 14, 12, and 10 AWG, NEC 240.4(D) hard-caps overcurrent protection regardless of how generous the insulation rating is: 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, with lower values for aluminum. The calculator applies the cap automatically and tells you when it forced a larger wire.
Voltage Drop
Drop is computed as Vd = k × L × R × I ÷ 1000, where k is 2 for single-phase or √3 for three-phase, L is one-way length in feet, and R is conductor resistance per 1,000 ft at a 75°C operating temperature. The result is compared against your selected ceiling, and the voltage actually arriving at the load is reported.
Limiting Factor
Whichever constraint is strictest wins. If ampacity binds, you get the smallest code-compliant gauge; if voltage drop binds, the tool upsizes until the drop at the load is within your target; if 240.4(D) binds, that is flagged explicitly. You always know why an answer is what it is.
Breaker Recommendation
The reverse calculator finishes with the largest standard overcurrent device from NEC 240.6(A) — 15, 20, 25, 30, 35, 40, 45, 50, 60 A and upward — that protects the conductor without nuisance tripping or leaving it unprotected.
A Worked Example
Suppose you need to feed a 48 A Level-2 EV charger over 150 feet at 240 V, single-phase, in conduit, with 75°C copper (THWN) and a 3% drop limit. Table ampacity alone suggests 8 AWG (50 A at 75°C) — but 8 AWG copper (0.764 Ω per 1,000 ft) would drop 2 × 150 × 0.764 × 48 ÷ 1000 ≈ 11.0 V, which is 4.6% of 240 V. The voltage-drop constraint binds, and the engine walks up to 6 AWG (0.491 Ω): 2 × 150 × 0.491 × 48 ÷ 1000 ≈ 7.1 V, about 2.9% — inside your limit. The result panel reports exactly this chain of reasoning, so you can defend the spec to an inspector or a supplier.
Illustrative example; continuous loads and terminal temperature limits still require evaluation by a licensed electrician.
Who Uses Wire Calculator
The same question — “what gauge wire do I need?” — is asked by very different people in very different situations. We designed the interface to be fast enough for daily professional use and clear enough for a first-time DIYer working through their first subpanel.
Licensed Electricians & Apprentices
Field-speed double checks between the truck and the panel. Confirm a feeder size, sanity-check a long run, or explain to a customer exactly why a job needs 6 AWG instead of 10 AWG — with the code basis spelled out.
Solar & EV Installers
PV string output, inverter feeders, and EVSE branch circuits are where voltage drop quietly kills performance. Model long DC and AC runs and see how quickly 10 AWG stops being enough.
DIY Homeowners
Planning a subpanel for a workshop, a circuit for a welder, or a new hot-tub feed? Understand what gauge the job actually needs before you buy a single roll of wire or schedule an inspection.
Engineers & Designers
Fast pre-drafting checks that mirror the limiting-factor logic you will formalize later in your load calc software. Compare copper versus aluminum economics at a glance.
Students & Educators
A transparent teaching tool. Every result shows its limiting factor and the note trail behind it, so apprentices can learn why the answer is right, not just what the answer is.
Inspectors & AHJs
A neutral, manufacturer-independent reference for reviewing submittals and answering homeowner questions during permit conversations.
Long PV and EV runs are where voltage drop bites hardestPhoto: UnsplashReal projects, real constraints
A workshop subpanel at the far end of a property. A well pump at the bottom of a long trench. A detached garage that needs its own feeder. A welder that only runs on weekends but draws serious current when it does. Each of these jobs stresses a different constraint — ampacity, drop, or the small-conductor rule — and each has a different correct answer.
That is precisely why a single “wire chart” is never enough, and why the calculator models your actual inputs instead of handing you a generic lookup row. Change any parameter and watch the answer move — the tool recalculates in real time, so building intuition is as easy as editing a number.
Why We Built Wire Calculator
The idea came from a familiar frustration. Wiring a detached workshop feeder, one of our founders found a dozen “wire size charts” online — and they disagreed with each other. Some ignored voltage drop entirely. Some quoted 90°C column ampacities for circuits terminated on 75°C lugs. None explained what to do when a 100-foot run made the tabulated answer unsafe. The authoritative source, the NEC itself, is a thick, copyrighted, deeply cross-referenced volume that is wonderful for professionals and impenetrable at the hardware store counter.
Meanwhile, the consequence of guessing wrong is not a wasted afternoon. Undersized conductors are a leading contributor to residential electrical fires, and even the “lucky” failure — chronic breaker trips and warm outlets — is expensive and frightening. We believed a calculator could be both radically simple and engineering-honest at the same time, if it showed its work: named tables, visible limiting factors, and stated assumptions instead of a bare number.
So Wire Calculator was built around three commitments. First, traceability: every answer names the table or rule that produced it. Second, direction-agnosticism: sizing and reverse-ampacity share one engine, so the two tools can never disagree. Third, privacy: a calculation about your home should not require sending your home’s details to a server, so the math runs entirely in your browser.
Today the site serves electricians double-checking a feeder between jobs, solar and EV installers modeling long runs, engineering students learning why 240.4(D) exists, and homeowners planning their first subpanel with confidence instead of crossed fingers. If the tool has saved you one Callback visit or one wrong spool of wire, it has done its job.
Show the work, earn the trustPhoto: UnsplashData Sources & Editorial Standards
A calculator is only as trustworthy as its sources and its maintenance. These are the standards every page and every table on Wire Calculator is held to.
Named Sources, Always
We cite the National Electrical Code (NFPA 70) — Tables 310.15(B)(16) and (17), Sections 240.4(D), 240.6(A), and the informational voltage-drop notes to 210.19(A). Where a value comes from a specific table, the page says so.
Conservative by Default
When two defensible interpretations exist, we ship the more conservative one and label the alternative in the notes. Conductors are expensive; callbacks and melted insulation are more expensive.
Assumptions Stated Up Front
Every table and tool states its baseline conditions: 30°C ambient, three current-carrying conductors, and no correction or adjustment factors. If your job exceeds those conditions, the notes tell you which derating rules apply next.
Reviewed on Every NEC Cycle
The NEC is revised on a three-year cycle and jurisdictions adopt different editions. When a table or section reference changes, we update the calculator and date-stamp the change instead of quietly hoping nobody notices.
Found an error? We treat every correction report seriously — send the page URL, your inputs, and the result you expected via our contact page, and we will verify against the current NEC tables and update the tool. Corrections that change results are documented on the page they affect.
The Principles Behind the Tool
Four commitments shape every feature decision on this site — what we build, what we refuse to build, and how we explain the results.
Free Forever
Electrical safety information should never sit behind a subscription. The calculators, tables, and guides are free to use, unlimited, without an account.
Local-First Privacy
Your inputs never leave your browser. There is no upload step, no history, and nothing to leak — because there is nothing stored.
Speed as a Feature
A calculator you use in the field has to load fast and answer in real time. Results update as you type, with no submit button and no page reloads.
Honest About Limits
We do not pretend a web tool replaces an electrician. Ambient-temperature corrections, conductor bundling, terminal temperature limits, and continuous-load rules are your electrician's and inspector's call — and we say so plainly.
Frequently Asked Questions About Wire Calculator
Is Wire Calculator free to use?▼
Yes — completely. There is no account, no trial, no usage cap, and no paywalled features. The site is supported by advertising, which is why you can run unlimited calculations without paying or signing up.
Which code edition does the calculator follow?▼
The calculation library is built on the National Electrical Code (NFPA 70), using Table 310.15(B)(16) for raceway, cable, and direct-burial installations, Table 310.15(B)(17) for free air, the 240.4(D) small-conductor rule, and the 240.6(A) standard overcurrent ratings. Because states and provinces adopt different NEC editions with local amendments, always confirm the edition your Authority Having Jurisdiction enforces.
Are my inputs stored anywhere?▼
No. Voltage, amperage, distance, phase, insulation, conductor, installation method, and voltage-drop settings are processed locally in your browser using client-side JavaScript. Nothing is transmitted to or persisted on our servers.
Why did the calculator give me a bigger wire than the ampacity table suggests?▼
Two reasons come up most often. First, the small-conductor rule in NEC 240.4(D) caps 14, 12, and 10 AWG at 15 A, 20 A, and 30 A respectively, no matter the insulation rating. Second, voltage drop on a long run frequently forces an upsize — table ampacity alone ignores distance entirely. The result panel names the binding constraint so you know which one applied.
Can I use this instead of hiring an electrician?▼
No. The calculator is a planning and educational aid. It does not model ambient-temperature correction factors, more-than-three-conductor bundling adjustments, terminal temperature ratings, continuous-load multipliers, or the many local amendments your jurisdiction may enforce. A licensed electrician and your local inspector remain responsible for the final design and approval.
Does it work for aluminum wiring?▼
Yes. Select Aluminum as the conductor material and the engine switches to aluminum ampacity and resistance data, including the lower 240.4(D) caps for 12 and 10 AWG aluminum (15 A and 25 A). Note that aluminum 14 AWG is not rated for building wiring in the tables, and the tool will exclude it.
Ready to Size Your Circuit?
Open the calculator, enter your load and distance, and get a code-aware answer with its reasoning attached — free, private, and instant. Then browse the guides to understand the “why” behind every number.