Wire Calculators Hub
Every wire sizing tool you need in one place — the NEC-based wire size calculator, the reverse ampacity calculator, copper and aluminum ampacity tables, voltage-drop worked examples, and standard breaker sizes — with task presets for branch circuits, EV chargers, subpanels, solar, and long-run feeders.
NEC-based tools, instant resultsPhoto: UnsplashThe Right Wire Calculator for Every Stage of Your Circuit
Wire sizing looks like a lookup problem, and that is exactly why charts fail. A single ampacity row ignores the two questions that change most answers: how long the run is, and what conditions the conductor lives in. The tools on this site treat wire sizing as the three-constraint problem it actually is — table ampacity, overcurrent protection rules, and voltage drop — and report which constraint decided your result.
The engine is built on the National Electrical Code: ampacities from Table 310.15(B)(16) for raceway, cable, and direct-burial installations and Table 310.15(B)(17) for free air, filtered through your 60°C / 75°C / 90°C insulation selection; the 240.4(D) small-conductor caps for 14, 12, and 10 AWG; voltage drop computed as Vd = k × L × R × I ÷ 1000 with k = 2 for single-phase and √3 for three-phase; and breaker recommendations drawn from the standard 240.6(A) sizes. Copper and aluminum carry separate ampacity and resistance data throughout.
This page is both an index and a tutorial. Below you will find the two calculators, task-based presets that show exactly which parameters to enter for the most common projects, a four-step sizing workflow, reference tables you can check results against, pro tips from working electricians, and a detailed FAQ. Everything is free, requires no sign-up, and runs entirely in your browser — your circuit parameters never leave your device.
The Two Core Calculators
Both tools live on the home page, share one engine, and update in real time as you type. Pick the direction that matches your question.
Wire Size Calculator
Forward sizing — load to gauge
Enter circuit voltage, load amperes, and one-way distance, then choose single- or three-phase power, a 60°C / 75°C / 90°C insulation rating, copper or aluminum conductors, an installation method (raceway, cable, direct burial, or free air), and a voltage-drop ceiling from 1% to 5%. The engine returns the smallest code-compliant AWG, its table ampacity, the voltage arriving at the load, the limiting factor behind the answer, and the notes you need to defend it.
Reverse Ampacity Calculator
Inverse sizing — gauge to amps
Start from the wire you already have. Enter the AWG or kcmil size, the run length, and the supply voltage with the same condition settings, and the engine computes the maximum continuous amperes the conductor can safely carry — plus the largest standard NEC 240.6(A) breaker that protects it. The result names the binding constraint: table ampacity, the 240.4(D) small-conductor rule, or your voltage-drop limit, with the precise unfloored limit shown for borderline circuits.
Specialty Wire Calculators
Dedicated tools for the systems that don’t follow AC premises-wiring rules: low-voltage DC power, NEC 690.8 PV circuits, ABYC E-11 boat cable, SAE automotive wiring, and distributed landscape-lighting loads. Each ships with its own interactive tool and standard-specific reference tables.
Wire Calculators by Task
The same engine answers every wire-sizing question — what changes per project is which parameters you enter and how careful you need to be about continuous loads and distance. These six task guides show the typical starting points, with common presets you can verify against the calculator in seconds.
🏠Residential Branch Circuits
The everyday workhorse questions: lighting circuits, receptacle circuits, kitchen and bathroom runs. A 15 A lighting circuit lands on 14 AWG copper, a 20 A kitchen or bath receptacle circuit needs 12 AWG, and a 30 A water heater or dryer circuit calls for 10 AWG — all before distance adjustments. Open the Wire Size Calculator, enter the breaker size as your load amperage and your actual cable route length, and confirm the gauge before the holes are drilled.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| 15 A lighting | 14 AWG copper | 15 A |
| 20 A receptacles | 12 AWG copper | 20 A |
| 30 A water heater | 10 AWG copper | 30 A |
⚡Service Entrances & Subpanels
Feeders and service conductors live or die by voltage drop because the distances are long and the loads are everything at once. A 100 A detached-garage subpanel typically lands on 2 AWG copper or 1/0 aluminum for short runs, but 150 feet of trench changes the math — often pushing to 1 AWG copper or 2/0 aluminum. Model the feeder in the calculator at your real route length, then verify the main-breaker sizing and grounding separately with your electrician.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| 60 A subpanel | 6 AWG Cu / 4 AWG Al | 60 A |
| 100 A subpanel | 2 AWG Cu / 1/0 Al | 100 A |
| 200 A service | 4/0 Cu / 250 kcmil Al | 200 A |
🔌EV Charging Circuits
EV supply equipment is the classic continuous load: a 48 A Level-2 charger must be wired at 125% — a 60 A circuit — and long garage or driveway runs add voltage drop on top. In conduit with 75°C copper, 6 AWG handles a 48 A EVSE comfortably at moderate distances, while a 150-foot run may justify 4 AWG to hold the 3% drop target. Enter the continuous amperage after applying the 125% multiplier, and let the calculator check whether distance forces an upsize.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| 32 A EVSE | 8 AWG Cu @ 40 A | 40 A |
| 48 A EVSE | 6 AWG Cu @ 60 A | 60 A |
| 80 A EVSE | 3 AWG Cu @ 100 A | 100 A |
❄️HVAC & Major Appliances
Air conditioners, heat pumps, ranges, and welders carry motor inrush and nameplate complexity: minimum circuit ampacity (MCA) and maximum overcurrent protection (MOP) come from the data plate, not from a rule of thumb. Use the MCA as your amperage input and the MOP as a sanity check on the breaker. A typical 40 A AC branch circuit lands on 8 AWG copper in 75°C wiring; a 50 A range circuit on 8 or 6 AWG depending on the run.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| AC condenser (MCA 40 A) | 8 AWG Cu | 50 A max |
| Electric range | 8 AWG Cu @ 50 A | 50 A |
| Shop welder | 10 AWG Cu @ 30 A | 30 A |
☀️Solar PV & Battery Systems
PV string runs, inverter output circuits, and battery interconnects combine modest currents with long distances — the exact condition where voltage drop quietly erases production. Rooftop conduit adds temperature derating that the calculator's baseline does not include, so treat its answer as a starting point and apply your derating factors. For battery interconnects and inverter feeders, size for the continuous inverter output at 125% and check drop at maximum charge current.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| PV string (12 A Isc) | 10 AWG PV wire | 20 A |
| 5 kW inverter output | 10 AWG Cu @ 30 A | 30 A |
| 48 V battery bank | 2/0 Cu interconnect | Per inverter |
🏗️Long-Run Feeders & Detached Structures
Workshops, barns, pumps, and gate operators share one trait: the wire route is much longer than the property line suggests. Every 50 feet of extra run steals capacity through resistance, and underground runs add burial-depth and conduit-fill considerations on top. Run the calculator at the true trench length, compare 3% versus 5% drop limits to see the cost of each decision, and remember that aluminum feeders are often the economical choice above 100 A.
| Typical Circuit | Common Wire Size | Typical Breaker |
|---|---|---|
| Well pump (15 A) | 12 AWG Cu @ 240 V | 15 A ×2 |
| Workshop feeder | 6 AWG Cu @ 60 A | 60 A |
| Gate operator | 12 AWG Cu @ 20 A | 20 A |
The 4-Step Wire Sizing Workflow
Professional estimators and electricians follow the same sequence on every circuit. Skipping a step is how wires end up warm to the touch and inspectors end up writing red tags.
Measure the route, not the roomPhoto: UnsplashStep 1 — Nail Down the Load
Read the nameplate, not the breaker. Appliance data plates state running amps, minimum circuit ampacity (MCA), and whether the load is continuous. Continuous loads (EV chargers, water heaters, space heating) must be multiplied by 1.25 before anything else — a 48 A EVSE is a 60 A circuit. For three-phase equipment, confirm the phase before entering the calculator, because the voltage-drop constant differs by a factor of √3.
Step 2 — Measure the Real Cable Route
Measure the path the wire will actually take — up through walls, across the attic, around obstructions, down to the equipment — not the straight-line room distance. Add 10–15% for routing waste. A run you call 60 feet is often 85 feet of cable. Distance is the input most often underestimated, and it is the one that drives voltage drop directly: doubling the run doubles the drop.
Step 3 — Set the Installation Conditions
Choose the insulation rating that matches the wire you will actually buy (THWN/THHN and XHHW are typically 75°C/90°C; NM-B cable is rated 60°C at the device), the conductor material, and the installation method — raceway, cable, direct burial, or free air. Set your voltage-drop ceiling deliberately: 3% is the common branch-circuit target, 5% is often acceptable for feeders, and sensitive loads may justify 2%.
Step 4 — Read the Limiting Factor, Then Verify
The result tells you which constraint governed: table ampacity, the small-conductor rule, or voltage drop. If voltage drop governed, a shorter route or a lower drop limit changes the answer. Then apply what the calculator deliberately does not: ambient-temperature correction, conductor bundling, terminal temperature limits, and your AHJ's interpretations. The tool gets you to the right conversation; the electrician and inspector finish it.
Copper Wire Ampacity Chart (NEC Table 310.15(B)(16))
Published ampacities for copper conductors in a raceway, cable, or direct burial — three current-carrying conductors at a 30°C (86°F) ambient. The 240.4(D) column shows the small-conductor cap that overrides these values for 14–10 AWG. This is the same table the Wire Size Calculator draws from, so any result you get can be verified row by row.
| Wire Size | 60°C (TW, UF) | 75°C (THWN, XHHW) | 90°C (THHN, XHHW-2) | 240.4(D) Cap | Typical Use |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A | 15 A (Cu) | Lighting circuits |
| 12 AWG | 20 A | 25 A | 30 A | 20 A (Cu) | Receptacle circuits |
| 10 AWG | 30 A | 35 A | 40 A | 30 A (Cu) | Water heaters, dryers |
| 8 AWG | 40 A | 50 A | 55 A | — | Ranges, AC units |
| 6 AWG | 55 A | 65 A | 75 A | — | EV chargers, feeders |
| 4 AWG | 70 A | 85 A | 95 A | — | Subpanels, long EV runs |
| 3 AWG | 85 A | 100 A | 115 A | — | 80–100 A loads |
| 2 AWG | 95 A | 115 A | 130 A | — | 100 A subpanels |
| 1 AWG | 110 A | 130 A | 145 A | — | Large feeders |
| 1/0 AWG | 125 A | 150 A | 170 A | — | 110–125 A feeders |
| 2/0 AWG | 145 A | 175 A | 195 A | — | 125–150 A services |
| 3/0 AWG | 165 A | 200 A | 225 A | — | 150–175 A services |
| 4/0 AWG | 195 A | 230 A | 260 A | — | 200 A services |
| 250 kcmil | 215 A | 255 A | 290 A | — | 200 A+ services |
| 350 kcmil | 260 A | 310 A | 350 A | — | Large commercial feeders |
| 500 kcmil | 320 A | 380 A | 430 A | — | Heavy feeders & services |
Aluminum Wire Ampacity Chart (75°C Column)
Aluminum needs roughly two gauge steps compared to copper for the same job, but costs far less per ampacity point — which is why services and feeders at 100 A and above are so often aluminum. The 240.4(D) caps bite harder here: 12 AWG aluminum is capped at 15 A and 10 AWG at 25 A.
| Wire Size | 75°C Ampacity | 240.4(D) Cap | Typical Use |
|---|---|---|---|
| 10 AWG | 30 A | 25 A (Al) | Small loads (capped) |
| 8 AWG | 40 A | — | 40 A circuits |
| 6 AWG | 50 A | — | 60 A breakers (derated) |
| 4 AWG | 65 A | — | 60 A subpanels |
| 2 AWG | 90 A | — | 90–100 A feeders |
| 1/0 AWG | 120 A | — | 100–110 A subpanels |
| 2/0 AWG | 135 A | — | 125 A feeders |
| 4/0 AWG | 180 A | — | 175–200 A services |
| 250 kcmil | 205 A | — | 200 A services |
| 350 kcmil | 250 A | — | 225–250 A services |
| 500 kcmil | 310 A | — | 300 A+ feeders |
Voltage Drop Worked Examples (Single-Phase, Copper)
Distance is the dimension every wall chart ignores. Each example below is computed with Vd = 2 × L × R × I ÷ 1000 using the calculator’s own resistance data — run the same inputs through the tool and you will get the same numbers, which is the point.
| Scenario | Wire | Load | One-Way Run | Voltage Drop | Verdict |
|---|---|---|---|---|---|
| 20 A receptacle circuit | 12 AWG Cu | 20 A @ 120 V | 50 ft | 3.9 V (3.2%) | Borderline — keep runs short or upsize |
| 30 A dryer feed | 10 AWG Cu | 30 A @ 240 V | 100 ft | 7.3 V (3.0%) | Meets the 3% target |
| 48 A EV charger | 6 AWG Cu | 48 A @ 240 V | 150 ft | 7.1 V (2.9%) | Meets 3% — good EV run sizing |
| 40 A workshop feeder | 8 AWG Cu | 40 A @ 240 V | 200 ft | 12.2 V (5.1%) | Fails 3% and 5% — upsize to 6 AWG (3.3%) |
| 100 A garage subpanel | 2 AWG Cu | 100 A @ 240 V | 150 ft | 5.8 V (2.4%) | Comfortable margin at 3% |
| 200 A service | 4/0 Al | 200 A @ 240 V | 50 ft | 2.0 V (0.8%) | Minimal drop — standard service sizing |
Standard Breaker Sizes (NEC 240.6(A))
Overcurrent devices come in standard sizes, and the reverse calculator recommends the largest one that does not exceed your conductor’s effective ampacity. These are the ratings it draws from.
| Application Range | Standard Sizes (A) |
|---|---|
| Small branch circuits | 15, 20, 25, 30, 35, 40, 45, 50 A |
| Feeders & subpanels | 60, 70, 80, 90, 100, 110, 125 A |
| Services & large feeders | 150, 175, 200, 225, 250 A |
| Commercial & industrial | 300, 350, 400, 500, 600, 800, 1000, 1200, 1600, 2000 A |
Pro Tips for Accurate Wire Sizing
Eight habits that separate a clean inspection from a callback — collected from electricians and installers who size wire every working day.
The 80% Rule Is Really the 125% Rule
You will hear 'breakers should run at 80% of their rating' — the code expresses the same idea from the other side: conductors and overcurrent devices must be rated at 125% of a continuous load. 20 A × 0.8 = 16 A usable; 16 A × 1.25 = 20 A. Size continuous loads with the 125% multiplier in the calculator and the 80% thinking takes care of itself.
Insulation Rating Is Not a Performance Upgrade
A 90°C THHN conductor is not 'better' than 75°C wire in most circuits, because the equipment terminals — breaker lugs, disconnects, device straps — are typically rated 75°C or 60°C, and NEC 110.14(C) caps the ampacity you may use at the lowest terminal rating in the chain. The 90°C column exists mainly as a starting point for derating calculations.
Copper vs Aluminum Is an Economics Question
Aluminum conducts about 61% as well as copper but weighs far less and costs substantially less per ampacity point, which is why large services and feeders are routinely aluminum. The tradeoffs: larger gauge for the same ampacity, AL/CU-rated terminations, inhibitor paste, and torque discipline. The calculator handles both materials — compare them on your actual run before deciding.
Free Air Is Not a Real-World Setting for Most Jobs
Table 310.15(B)(17) free-air ampacities are tempting because they are 20–30% higher, but conductors bundled in panels, cable trays, or conduit runs almost never qualify. Choose free air only for genuinely isolated conductors — otherwise you are sizing wire with ampacity your installation cannot legally or thermally use.
Voltage Drop Costs Money Twice
An undersized long run wastes energy as heat every hour the load runs, and sensitive equipment (motors, compressors, EV chargers) works harder and ages faster at low voltage. One upsize in gauge on a long feeder often pays for itself in efficiency and nuisance-trip avoidance within a few years, especially for high-duty-cycle loads.
Double-Check Units Before You Trust Any Answer
The calculator expects one-way distance in feet and load in amperes at the voltage you select. Mixing up one-way and round-trip distance, entering kilowatts as amperes, or plugging a 208 V three-phase load into a 240 V single-phase model are the three most common ways a correct engine produces a wrong answer. Garbage in, code violation out.
Know Which NEC Edition Your AHJ Enforces
Table numbering and some rules shift between the 2014, 2017, 2020, and 2023 NEC cycles, and many jurisdictions amend what they adopt. Our engine follows the table structure of recent editions, but the edition your inspector enforces is the one that matters. A five-minute call to the permit office prevents an expensive re-inspection.
Use Reverse Mode Before You Reuse Old Cable
Found a spool of 10 AWG in the garage and think it will feed the new shed? Reverse mode answers precisely: enter the gauge, length, and conditions, and it returns the maximum safe amperes and the largest standard breaker. Reusing oversized leftover wire is thrifty; reusing undersized leftovers is how attics catch fire.
Wire Calculator FAQ
How do I know what size wire I need?▼
Three inputs decide it: the load in amperes, the one-way distance, and the installation conditions (conductor material, insulation rating, and method). Enter them in the Wire Size Calculator and it applies the NEC ampacity tables, the 240.4(D) small-conductor rule, and your voltage-drop limit to return the smallest compliant gauge with its reasoning. For continuous loads like EV chargers, multiply the nameplate amps by 1.25 first.
What is the difference between the forward and reverse calculators?▼
The forward calculator answers 'what wire does this circuit need?' from load, voltage, and distance. The reverse calculator answers 'what can this wire safely carry?' from an AWG size, length, and conditions. Both share the same engine and tables, so a 6 AWG answer from the forward tool and a 55-amp maximum from the reverse tool will always agree with each other.
Why does 12 AWG show 25 A ampacity but only work on a 20 A breaker?▼
NEC 240.4(D) — the small-conductor rule — caps overcurrent protection for small gauges regardless of the table value: 15 A for 14 AWG copper, 20 A for 12 AWG copper, and 30 A for 10 AWG copper. The 75°C table ampacity of 25 A exists for calculation purposes and for certain exceptions, but your breaker selection is capped at 20 A. The calculator shows both numbers and applies the cap automatically.
What voltage drop percentage should I use?▼
The widely used targets are 3% for a branch circuit and 5% total for feeder plus branch (from informational notes to NEC 210.19(A) and 215.2(A)(3) — recommendations, not mandates). Use 3% for most circuits, tighten to 2% for sensitive electronics or long motor runs, and 5% for feeders where an upsize would be disproportionate. The calculator enforces whatever ceiling you set from 1% to 5%.
Does the calculator account for wire in a hot attic or bundled with other circuits?▼
No — and this is deliberate. The baseline assumes three current-carrying conductors at 30°C (86°F) ambient. Hot attics, rooftop conduit, and bundles of more than three conductors require correction and adjustment factors under NEC 310.15(B)(2) and 310.15(C)(1) that depend on your specific conditions. The result notes flag when these rules apply so your electrician can apply the right multipliers.
Can I use aluminum wire instead of copper?▼
For most feeder and service sizes, yes — utilities and electricians use aluminum routinely, and it is significantly cheaper per ampacity point at large gauges. Select Aluminum in the calculator to switch to aluminum ampacity and resistance data. The caveats are real, though: larger gauge for the same capacity, AL/CU-rated terminations, antioxidant compound, and careful torque. Small-gauge aluminum branch circuits (15–20 A) in older homes are a known hazard and should be evaluated by an electrician.
What size wire do I need for a 60 amp subpanel?▼
For a 60 A subpanel feeder with 75°C copper in conduit, 6 AWG is the standard short-run answer (65 A table ampacity). In aluminum, 4 AWG is the usual equivalent. Distances beyond roughly 100 feet may push you to 4 AWG copper or 2 AWG aluminum to hold a 3% drop — run the exact length through the calculator rather than trusting a chart, because feeders are where distance quietly changes the answer.
What size wire for an EV charger on a 150-foot run?▼
A 48 A Level-2 EVSE is a continuous load, so the circuit must be rated 60 A (48 × 1.25). At 75°C that points to 6 AWG copper, and at 150 feet the voltage drop works out to about 2.9% — right at the 3% target, so 6 AWG survives the distance check in the example above. Longer runs, aluminum, or a 90°C-insulated-but-75°C-terminated comparison all change the answer; the calculator gives you the definitive result for your exact inputs.
Is this calculator a substitute for an electrician?▼
No. It is a planning and education tool built on published NEC table data. It does not apply temperature or bundling derating, it does not evaluate terminal temperature limits or equipment instructions, and it cannot know which code edition or amendments your jurisdiction enforces. Use it to plan, budget, and understand — then have a licensed electrician and your AHJ verify anything that gets wired.
Putting the Tools to Work
Start with the task presets above to frame your question, then open the Wire Size Calculator and enter your actual numbers — the answer updates in real time, names its limiting factor, and shows the ampacity and voltage-drop figures behind the recommendation. If you already have wire in hand, the Reverse Ampacity Calculator tells you what it can safely carry and which breaker protects it.
The tables on this page are the same data the engine uses, so you can always verify a result by hand — that transparency is the whole point. What the tables cannot do is apply your site-specific derating, your equipment’s terminal ratings, or your inspector’s interpretations; those remain the province of a licensed electrician, as our disclaimer explains in detail.
When you are ready to go deeper — the reasoning behind voltage-drop targets, insulation types, aluminum handling, and code topics — the guides library covers each subject in plain language with worked examples.