ToolGrit
Explore Tools
Electrical Free

Wire Sizing Calculator - NEC Voltage Drop & Ampacity Tables

Find the Right AWG Wire Gauge for Any Electrical Run

A guided, step-by-step wire sizing calculator that walks you through load type, voltage, distance, ambient temperature, and conduit fill - then recommends the correct AWG gauge for both copper and aluminum with real-world NEC derating applied.

Unlike simple voltage drop calculators, this tool forces you to think through the factors that actually matter: Is this a continuous load? How hot is that attic? How many conductors are in the conduit? These questions determine whether your wire passes inspection - and whether your equipment works reliably for decades.

See side-by-side copper vs aluminum recommendations with voltage at the load end, total material costs including conduit, and a visual circuit diagram showing exactly what voltage your equipment will receive. Built for electricians, contractors, and DIYers who want to get it right the first time.

Pro Tip: For runs over 100 feet, voltage drop usually matters more than ampacity. Always check both. A wire that passes ampacity at 150 feet may still deliver unacceptable voltage drop to your load.

Check if your circuit can handle a specific load

Load Checker →

Size a generator for your power needs

Generator Sizing →
Start Using This Tool See How It Works
Wire Sizing Calculator
Pop Out

How It Works

  1. Select Your Load

    Pick from common presets (EV charger, welder, subpanel, HVAC) or enter custom amps and voltage. The calculator automatically applies NEC load type rules - continuous loads, motors, and EV chargers are sized at 125% per code.

  2. Set Load Type & Distance

    Tell the calculator what kind of load you are feeding. Motor loads, continuous loads, and EV chargers require 125% wire sizing per NEC. Enter the one-way distance from your panel to the equipment.

  3. Configure Environment

    Set your ambient temperature and conduit fill count. Hot attics, rooftops, and bundled conductors require ampacity derating per NEC 310.15. Choose your installation method (conduit, direct burial, or aerial) and conduit material.

  4. Review Derating Impact

    See exactly how temperature and conduit fill derating affect your wire capacity. The calculator shows the combined derating factor so you understand why a wire rated for 100A might only carry 73A in your conditions.

  5. Compare Copper vs Aluminum

    View side-by-side recommendations for both materials with wire size, voltage drop percentage, voltage at the load end, conduit sizing, and total material cost including conduit. See the savings at a glance.

  6. Check the Circuit Diagram

    A visual circuit diagram shows your panel voltage, wire run, and the voltage actually arriving at your equipment. Color-coded voltage drop indicators make it obvious if your run is marginal.

Built For

  • Licensed electricians sizing branch circuits and feeders for residential and commercial projects
  • Homeowners planning garage, workshop, or detached shop wiring with long runs
  • Solar installers sizing DC and AC runs from panels to inverters and main panels
  • EV charger installations requiring dedicated 40A or 50A circuits at various distances
  • Well pump circuits running 200+ feet from the house panel to a well house or pump
  • Long-distance runs to barns, outbuildings, pole buildings, and detached garages
  • Sub-panel feeder sizing for additions, finished basements, or accessory dwelling units

Features & Capabilities

NEC 310.16 Ampacity Tables

Built-in ampacity data from the NEC 310.16 table (75°C column) for copper and aluminum conductors. Includes 14 AWG through 4/0 AWG with automatic derating for temperature and conduit fill.

Load Type Intelligence

Automatically applies NEC rules for different load types. Continuous loads, motors, EV chargers, and lighting circuits are sized at 125% per NEC requirements. Welders get special voltage drop warnings.

Temperature & Fill Derating

Full NEC 310.15(B)(2)(a) ambient temperature correction and NEC 310.15(C)(1) conduit fill derating. See the combined impact on your wire capacity with 11 temperature presets from 50°F to 140°F.

Parallel Run Detection

When a single wire run cannot handle your load, the calculator automatically recommends parallel runs (up to 4) and adjusts the cost estimate accordingly. Includes warnings about parallel termination requirements.

Circuit Diagram Visualization

Interactive SVG diagram shows your panel, wire run, and load with color-coded voltage drop. See exactly what voltage arrives at your equipment and whether your run is in the green, yellow, or red zone.

Total Material Cost Estimate

Calculates wire and conduit cost for both copper and aluminum. Choose between EMT, PVC Schedule 40, or rigid metallic conduit materials. See the total savings from aluminum at a glance.

Comparison

AWG Gauge Copper Ampacity (75°C) Aluminum Ampacity (75°C) Typical Use
14 AWG 20A N/A 15A lighting circuits
12 AWG 25A 20A 20A general-purpose receptacles
10 AWG 35A 30A 30A dryers, water heaters
8 AWG 50A 40A 40-50A ranges, sub-panels
6 AWG 65A 50A 60A sub-panels, large appliances
4 AWG 85A 65A 70-80A feeders
2 AWG 115A 90A 100A sub-panels
1/0 AWG 150A 120A 150A feeders
2/0 AWG 175A 135A 150-175A feeders
4/0 AWG 230A 180A 200A main service entrance

Frequently Asked Questions

The NEC recommends no more than 3% voltage drop on branch circuits and 5% total (feeder plus branch circuit combined). While this is a recommendation rather than a hard code requirement, exceeding these values can cause equipment to malfunction, lights to dim, and motors to overheat. Most electricians treat 3% as the target for any individual run.
Aluminum is a cost-effective choice for large feeders (typically 4 AWG and larger), such as 100A or 200A sub-panel feeds. You must upsize aluminum by approximately two gauges compared to copper and use anti-oxidant compound on all connections. Aluminum is not recommended for 15A and 20A branch circuits. Modern AA-8000 series aluminum is far more reliable than the older AA-1350 alloy that caused problems in the 1960s and 1970s.
NEC Chapter 9, Table 1 limits conduit fill to 40% when you have three or more conductors. Two conductors allow 31% fill, and a single conductor allows 53%. Exceeding fill limits makes it difficult to pull wire and can cause heat buildup. This calculator provides conduit size recommendations based on your wire gauge and conductor count.
When ambient temperature exceeds 86°F (30°C) or when more than three current-carrying conductors share a conduit, you must reduce the allowable ampacity using NEC correction factors. For example, four to six conductors in a conduit require an 80% derating factor. In a hot attic at 120°F, 90°C-rated wire must be derated to roughly 82% of its table value. This calculator handles these adjustments automatically.
The NEC requires wire to be sized at 125% of the load for continuous loads (anything running 3+ hours), motor circuits (NEC 430), and EV chargers. A 40A EV charger actually requires wire rated for 50A. The calculator applies this automatically based on your load type selection so you do not accidentally undersize.
Most jurisdictions require a licensed electrician for new circuit installations, service upgrades, and commercial work. Some areas allow homeowners to pull permits for their own primary residence. Always check with your local building department before starting work. Even if permitted to do your own wiring, an inspection is typically required before energizing new circuits.
For a 100A feeder at 200 feet on a 240V circuit, you typically need 1/0 AWG copper or 3/0 AWG aluminum to keep voltage drop under 3%. The exact size depends on whether this is a continuous load (which requires 125% sizing to 125A), your ambient temperature, and conduit fill. Enter your specific parameters into this calculator to get the precise recommendation with derating factors applied. Aluminum is commonly used for long feeder runs like this because it costs significantly less than copper at these larger sizes.
A 50A EV charger circuit requires 6 AWG copper or 4 AWG aluminum wire at minimum. However, EV chargers are continuous loads under NEC, so the wire must be sized at 125% - meaning 62.5A capacity is required. For runs over 50 feet, voltage drop becomes a factor and you may need to upsize to 4 AWG copper or 2 AWG aluminum. Enter your exact distance, voltage, and installation conditions into this calculator for a precise recommendation.
On a 20A/120V circuit with 12 AWG copper, you can run approximately 70-80 feet before voltage drop exceeds the NEC-recommended 3%. At 240V, the same wire can go about 140-160 feet. These distances decrease if the circuit carries its full rated load continuously. For longer runs, you need to upsize the wire. This calculator shows the exact maximum distance for any wire gauge, load, and voltage combination.
Disclaimer: This calculator provides estimates for educational and planning purposes only. Wire sizing depends on local codes, installation conditions, and specific equipment requirements. Always verify results with a licensed electrician and your local electrical inspector before purchasing materials or beginning installation.

Related Tools

Electrical Live

Can I Run This On That?

Check if your circuit breaker and wiring can handle a specific appliance. Enter breaker size, wire gauge, and load wattage for a pass/fail verdict based on NEC standards.

Launch Tool →
Electrical Live

Generator Sizing Calculator

What size generator do you need? Add your appliances and loads to calculate total running watts and starting surge. Get a recommended generator size with built-in headroom.

Launch Tool →
Electrical Live

Panel Load Study

Do you actually need a panel upgrade? Walk your breaker panel with NEC Article 220 demand factors. See connected load vs. calculated demand and test whether an EV charger, heat pump, or hot tub fits.

Launch Tool →