What is a wire size calculator?
A wire size calculator determines the correct wire gauge for an electrical circuit based on load current, run length, voltage, and installation conditions. It evaluates two critical criteria simultaneously — ampacity (the maximum current a wire can safely carry) and allowable voltage drop — then recommends the smallest AWG or mm² size that satisfies both.
Electricians use it to properly size branch circuits and feeders. Engineers rely on it for system design across AC and DC applications. DIYers use it to install home circuits correctly and stay within code. Whatever the application, the right wire size is a critical component of any safe, efficient electrical system.
Why wire size matters
Safety and protection
Undersized wire builds up heat under load, creating a fire risk. Correct sizing keeps conductor temperature within the insulation's rating and ensures proper overcurrent protection.
Performance and efficiency
Excessive voltage drop causes equipment to run poorly or fail early. Keeping drop within allowable limits — typically 3% for branch circuits — maintains reliable operation.
Cost control
Oversizing wastes material and increases cable cost. The calculator finds the minimum appropriate size, balancing upfront cost against long-term energy efficiency.
What the calculator needs
Accurate results depend on accurate inputs. Enter these values before you run any calculation:
The continuous current the circuit will carry. For continuous loads, the NEC requires sizing at 125% of the actual current — a consideration the calculator applies automatically.
Enter the one-way distance from source to load. The calculator uses round-trip distance internally for voltage drop — one of the most common input mistakes when sizing manually.
Specify single-phase or three-phase and the system voltage. Single-phase and three-phase circuits use different voltage drop formulas, so this input directly affects the result.
Copper and aluminum have different resistance values and ampacity ratings. Insulation temperature rating (60°C, 75°C, or 90°C) affects the maximum allowable current the wire can carry.
Set your target limit — typically 3% for branch circuits and 5% total for feeder plus branch. The calculator will size up if the standard gauge exceeds this threshold.
High ambient temperature or multiple conductors bundled in conduit reduce effective ampacity. Enter installation conditions so the calculator can apply the correct derating factors.
How the wire size calculator works
The calculator uses your inputs to run two parallel checks. First, it looks up or calculates the minimum wire size needed to handle the load current within the conductor's thermal rating — applying derating factors for ambient temperature and conduit fill where relevant. Second, it runs a voltage drop calculator pass using resistance per unit length for your chosen conductor material, source voltage, and circuit distance to confirm the drop stays within your allowable voltage drop limit.
The output is the recommended AWG or mm² size — whichever of the two checks demands the larger gauge — along with estimated voltage drop, drop as a percentage, and any notes on derating or installation conditions. That output is your starting point, not a substitute for code compliance review on complex jobs.
How to use this wire size calculator
- 1
Enter load current
Input the circuit's operating current in amperes. For continuous loads, use 125% of the actual draw.
- 2
Set source voltage and phase
Choose AC or DC, enter the system voltage, and select single-phase or three-phase. Each combination uses a different voltage drop formula.
- 3
Enter run length
Measure the one-way distance from the panel or source to the load. The calculator handles round-trip internally.
- 4
Choose conductor material and insulation rating
Select copper or aluminum and the insulation temperature rating. THHN at 90°C is the most common choice for conduit installations.
- 5
Set maximum allowable voltage drop
Enter your target — 3% is the standard recommendation for branch circuits. Adjust for feeders or sensitive equipment.
- 6
Review the recommended wire size
Check the output AWG or mm², the estimated voltage drop percentage, and any derating notes before ordering cable.
Worked examples
Single receptacle circuit — detailed
The ampacity check clears on 12 AWG copper. The voltage drop check at 60 ft round-trip (120 ft total) with 12 AWG resistance returns a drop of approximately 2.8% — within the 3% allowable voltage drop. The calculator recommends 12 AWG copper and confirms compliance. Had the run been 80 ft, drop would exceed 3% and the tool would size up to 10 AWG automatically.
Three-phase motor circuit
For long-run motor circuits, starting current and continuous load rating push wire size beyond the basic ampacity table. Enter the full-load ampere rating and a tighter allowable voltage drop limit — the calculator determines whether the voltage drop or thermal limit governs the final gauge.
Solar PV and DC circuits
DC solar runs often cover long distances at relatively low voltage, making voltage drop the dominant sizing factor. Use the DC mode, enter the maximum system current and the full string run length, and set a 1–2% drop limit to determine the correct cable size for your solar installation.
Wire size chart — common AWG ratings
Use this size chart as a quick reference for standard household and light commercial circuits. Values assume copper conductor, 75°C insulation, and a single circuit in conduit per NEC standard ampacity table.
| AWG | mm² | Ampacity (75°C) | Typical circuit |
|---|---|---|---|
| 14 AWG | 2.1 mm² | 15 A | General lighting, 15 A receptacles |
| 12 AWG | 3.3 mm² | 20 A | Kitchen outlets, 20 A branch circuits |
| 10 AWG | 5.3 mm² | 30 A | Dryers, water heaters, AC units |
| 8 AWG | 8.4 mm² | 50 A | Large appliances, EV chargers |
| 6 AWG | 13.3 mm² | 65 A | Sub-panels, heavy equipment |
| 4 AWG | 21.2 mm² | 85 A | Large sub-panels, service entrance |
Always increase wire size for long runs, high ambient temperature, or multiple conductors bundled in conduit. Use the calculator to verify — do not rely on the table alone for critical installations.
Common mistakes to avoid
Using one-way length for voltage drop
Voltage drop acts over the full round-trip path. Entering only the one-way distance halves the calculated drop and leads to undersized wire on longer runs.
Ignoring temperature and conduit fill
High ambient temperature or bundled conductors reduce the wire's effective current capacity. Skipping derating inputs can result in a wire that overheats under real installation conditions.
Forgetting the continuous load rule
Circuits running at full current for three hours or more are classified as continuous. Properly sizing these requires multiplying the load by 125% before entering the current value.
Mismatching insulation and temperature rating
Using a 60°C rating where 75°C or 90°C insulation is installed underestimates capacity. Always match the insulation type to the correct temperature column when checking the data.
Size your wire correctly — start here
Use the wire size calculator above to determine the correct electrical wire gauge for any circuit. Enter your load, source voltage, run length, and allowable drop limit to get a properly sized recommendation with voltage drop percentage included. For complex or safety-critical installations, always consult a licensed electrician or electrical engineer to verify the final design.
Frequently asked questions
What inputs do I need for a wire size calculator?
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Load current in amps, circuit voltage and phase, one-way run length, conductor material, insulation temperature rating, and maximum allowable voltage drop. Ambient temperature and conduit fill are optional but improve accuracy for real installation conditions.
What is the allowable voltage drop for a circuit?
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The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% combined for feeder plus branch. Sensitive equipment such as motors and solar inverters may require tighter limits — check the manufacturer's requirement before setting your target.
Can I use aluminum instead of copper?
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Yes, but aluminum has higher resistance than copper and aluminum wire, so a larger gauge is needed to carry the same current. Aluminum is commonly used for service entrance and large feeder cables where the cost saving justifies the larger size. Always verify termination compatibility before installing aluminum conductors.
How do I account for temperature derating?
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Enter the ambient temperature and the number of conductors in conduit. The calculator applies standard derating factors to reduce the wire's rated ampacity accordingly. For high-temperature environments or densely filled conduit, this can mean sizing up one or two gauges beyond the basic table value.