Electrical Wire Size Calculator
Estimate a wire size by checking both ampacity and voltage drop for copper or aluminum conductors.
Before sizing electrical wire
Start with load current: Use the actual amps the circuit must carry, then mark whether the load is continuous so the calculator can apply a 125% planning factor.
Check length and voltage drop: Long cable runs often need a larger conductor than ampacity alone would suggest.
Confirm local code: This tool is for planning only. Final wire size, breaker size, conduit fill, grounding, and installation method must be checked by a qualified electrician or local authority.
Recommended Wire Size
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Design Current
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Estimated Voltage Drop
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Ampacity Check
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Sizing Drivers
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Important: This calculator is for planning and education only. It does not check breaker rules, conductor count details, conduit fill, ambient temperature correction, equipment terminals, grounding, local amendments, or special loads.
How to Use This Calculator
- Enter load current: Use the amps the circuit must supply. If the load runs for 3 hours or more, check continuous load.
- Enter one-way length: Measure from the panel or source to the load. The calculator accounts for the return path in the voltage-drop formula.
- Choose conductor details: Select copper or aluminum and the terminal temperature column that matches the equipment.
- Review both checks: The final wire size is the larger size required by ampacity and voltage drop.
Electrical Wire Sizing Rules of Thumb
An electrical wire size calculator estimates conductor size from current, circuit length, voltage, conductor material, and allowable voltage drop. The correct size must carry the load safely and keep voltage drop within a practical limit.
Calculate electrical wire size by matching amperage, voltage, wire length, material, and allowable voltage drop. Most 15-amp circuits use 14 AWG copper wire, 20-amp circuits use 12 AWG, and 30-amp circuits use 10 AWG. Longer runs require larger wire to reduce voltage drop.
- Ampacity comes first: The conductor must be rated to carry the load after any required continuous-load factor and derating.
- Long runs may need upsizing: Voltage drop rises as distance and current increase, so a 20 amp circuit may need larger wire on a long run.
- Copper and aluminum differ: Aluminum has higher resistance, so it often needs a larger size for the same voltage-drop target.
- Terminal rating matters: Do not assume the highest insulation temperature column can be used for the final ampacity.
- Local code controls: The NEC is widely adopted in the United States, but local rules and equipment instructions still matter.
Understanding the Wire Size Formula
This page combines two checks: ampacity and voltage drop. The ampacity check compares design amps with the selected conductor table value after derating. The voltage-drop check uses the circular-mil formula commonly used by wire calculators.
Single-Phase Voltage Drop = (2 x K x Amps x Distance) / Circular Mils
Three-Phase Voltage Drop = (1.732 x K x Amps x Distance) / Circular Mils
- K value: This calculator uses 12.9 for copper and 21.2 for aluminum.
- Distance: The input length is one-way distance in feet; metric entries are converted internally.
- Final recommendation: The calculator chooses the larger conductor required by the ampacity check or voltage-drop check.
Common Wire Ampacity Reference
| Wire Size | Copper 60C | Copper 75C | Aluminum 75C | Common Planning Use |
|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | -- | Lighting circuits |
| 12 AWG | 20 A | 25 A | 20 A | General receptacles |
| 10 AWG | 30 A | 35 A | 30 A | Appliances, small feeders |
| 8 AWG | 40 A | 50 A | 40 A | Ranges, air conditioners |
| 6 AWG | 55 A | 65 A | 50 A | Subpanels, larger loads |
| 4 AWG | 70 A | 85 A | 65 A | Large feeders |
| 2 AWG | 95 A | 115 A | 90 A | 100 amp class feeders |
| 1/0 AWG | 125 A | 150 A | 120 A | Larger feeders |
| 4/0 AWG | 195 A | 230 A | 180 A | Service-size planning |
Sources: Cerrowire ampacity charts, Southwire voltage drop calculator, and NFPA 70 National Electrical Code.
Interesting Fact
Electrical wire sizing is not just a math exercise; it is part of fire prevention. According to NFPA Research, U.S. fire departments responded to an estimated average of 32,620 home fires per year involving electrical distribution and lighting equipment in 2015-2019. NFPA also reported that wiring and related equipment accounted for 68% of those fires. That is why conductor gauge, breaker protection, voltage drop, conduit conditions, and code-compliant installation all matter when planning a circuit.
Wire Size by Breaker Amperage Quick Guide
These are common copper wire starting points for short residential branch circuits, before adjusting for distance, voltage drop, conduit fill, temperature, terminal rating, and local code. Always verify the final conductor size against the actual load, cable type, breaker, and appliance instructions.
15 Amp Circuit
14 AWG
Often used for lighting and light-duty outlets.
20 Amp Circuit
12 AWG
Common for kitchen, bath, garage, and general receptacle circuits.
30 Amp Circuit
10 AWG
Often used for dryers, water heaters, and smaller equipment loads.
40-50 Amp Circuit
8-6 AWG
Typical planning range for ranges, EV circuits, and larger appliances.
Voltage Drop Examples for Long Wire Runs
Long distance is where many wire size calculators become useful. For example, a 20 amp, 120 volt copper circuit at 100 feet one-way can exceed a 3% voltage-drop target if it stays at 12 AWG, while upsizing the conductor can reduce the loss and improve appliance or outlet performance.
50 ft, 20 A, 120 V
12 AWG copper lands near a 3.3% voltage drop, so it may be close to the usual planning target.
100 ft, 20 A, 120 V
12 AWG copper lands near a 6.6% voltage drop, which is why longer circuit runs often need upsizing.
100 ft, 20 A, 120 V
8 AWG copper lands near a 2.6% voltage drop, showing how larger wire can help long runs.
Example values are approximate and use the same circular-mil voltage-drop method as the calculator.
What Can Change the Final Wire Size?
A calculator can narrow the planning range, but the final installation can change once the full wiring method is known. Use this checklist before buying cable, pulling conductors through conduit, or connecting a new breaker in the panel.
Temperature and insulation: Terminal ratings, conductor insulation, attic heat, and ambient temperature can affect allowable ampacity.
Conduit and cable bundling: Multiple current-carrying conductors in the same raceway may require derating and a larger gauge.
Equipment instructions: Appliances, HVAC units, EV chargers, and motors may specify minimum circuit ampacity and breaker limits.
Permits and inspections: NEC rules, local amendments, utility requirements, and inspector interpretation can control the final installation.
Frequently Asked Questions
What electrical wire size or AWG gauge do I need for a circuit?
The required electrical wire size depends on amperage, voltage, conductor material, terminal rating, circuit distance, voltage drop target, and installation conditions. A short 20 amp copper branch circuit from a panel to an outlet may start at 12 AWG, but a longer cable run to an appliance may need 10 AWG or larger to keep voltage drop within range.
Why does voltage drop affect wire size over distance?
All conductors have resistance. As wire length, amperage, and distance increase, more voltage is lost before power reaches the load. Upsizing the conductor reduces resistance and helps motors, appliances, lights, and electronics receive voltage closer to the source circuit voltage.
Is copper conductor better than aluminum cable?
Copper has lower resistance and usually carries more current at the same wire size, but aluminum cable is commonly used for larger feeders and service conductors. Aluminum must be sized correctly, terminated on rated equipment, and installed with the practices required by NEC code, local rules, and the manufacturer.
What do the 60C and 75C temperature columns mean?
The temperature column is tied to conductor insulation and equipment terminal ratings. Many smaller residential circuits are limited by 60C terminals, while some larger equipment uses 75C terminals. The final selection must match the installed equipment and applicable electrical code, not only the wire insulation label.
Does this calculator choose the breaker size too?
No. Breaker sizing has its own code rules and depends on conductor type, load type, equipment instructions, continuous load, motor rules, and small-conductor limitations. Treat this calculator result as a conductor estimate, then have the breaker, panel connection, and overcurrent protection checked separately before installation.
Can I use this for underground cable or outdoor wire runs?
You can use it for early voltage-drop planning, but underground and outdoor runs add requirements for cable type, burial depth, conduit, wet-location ratings, physical protection, grounding, and local inspection. Those details must be verified before installation because NEC and local code requirements can change the approved wiring method.
When should I check the continuous load option for a circuit?
Use the continuous load option when the electrical load is expected to run for 3 hours or more. The calculator increases the design amperage by 125%, which can require a larger wire size and helps avoid sizing a conductor too close to its limit for lights, equipment, or an appliance that runs for long periods.
How far can I run electrical wire before upsizing?
There is no single distance that applies to every circuit because voltage, amperage, conductor material, and allowable voltage drop all matter. A lightly loaded 240 volt circuit may tolerate a longer run than a heavily loaded 120 volt circuit serving an outlet or appliance, so it is best to enter the actual length and compare the voltage-drop result before choosing the final wire size.
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Disclaimer: This electrical wire size calculator provides a simplified planning estimate only. Electrical work can cause fire, shock, property damage, injury, or death if designed or installed incorrectly. Always follow the National Electrical Code, local amendments, equipment instructions, permits, inspections, and qualified professional guidance.
Last updated: April 28, 2026