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Wire Size Calculator
Enter your circuit parameters to find the minimum NEC-compliant wire size for copper and aluminum conductors, accounting for ampacity, temperature derating, bundling, terminal ratings, and voltage drop per NFPA 70-2026.
Results
Set your inputs, then click Calculate Wire Size.
Calculation details
Copper
Aluminum
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Request a QuoteVoltage drop equations
Single-phase / DC: VD = 2 × I × R × PF × (L ÷ 1000)
Three-phase: VD = 1.732 × I × R × PF × (L ÷ 1000)
R is DC resistance in Ω/1000 ft at 75°C from NEC Chapter 9, Table 8. I is amperes, L is one-way feet, and PF is power factor.
AWG wire size reference chart
Ampacity values from NEC Table 310.16 at 75°C (THWN/THWN-2), based on not more than three current-carrying conductors in raceway at 30°C ambient.
| Wire size | Copper (A) | Aluminum (A) | Circular mils | Common applications |
|---|---|---|---|---|
| 14 AWG | 20 | — | 4,110 | 15 A branch circuits, lighting |
| 12 AWG | 25 | 20 | 6,530 | 20 A branch circuits, outlets |
| 10 AWG | 35 | 30 | 10,380 | Dryers, A/C units, EV Level 1 |
| 8 AWG | 50 | 40 | 16,510 | Ranges, large A/C, hot tubs |
| 6 AWG | 65 | 50 | 26,240 | Subpanels, EV Level 2, 50 A circuits |
| 4 AWG | 85 | 65 | 41,740 | Large subpanels, 60–80 A feeders |
| 2 AWG | 115 | 90 | 66,360 | 100 A service, feeders |
| 1/0 AWG | 150 | 120 | 105,600 | 100–150 A service entrances |
| 2/0 AWG | 175 | 135 | 133,100 | 150–175 A service entrances |
| 4/0 AWG | 230 | 180 | 211,600 | 200 A service entrances |
| 350 kcmil | 310 | 250 | 350,000 | 300–350 A services, large feeders |
| 500 kcmil | 380 | 310 | 500,000 | 400 A services, industrial feeders |
Source: NFPA 70-2026, Table 310.16. Values are for 75°C conductors in conduit, three or fewer current-carrying conductors, 30°C ambient. Use Advanced Options to apply derating.
How the calculator works
All sizing follows NFPA 70-2026 (National Electrical Code), Articles 210, 215, 240, 250, 310, and Chapter 9.
Ampacity sizing (NEC Article 310)
The calculator selects the smallest conductor in NEC Table 310.16 whose derated ampacity meets the required load current, applying temperature correction (NEC Table 310.15(B)(1)) and bundling derating (NEC Table 310.15(C)(1)) when Advanced Options are on. With Advanced Options off, it uses 75°C insulation, 75°C terminals, three current-carrying conductors, 30°C ambient, a non-continuous load, and a power factor of 1.0.
Voltage drop (NEC Chapter 9)
After selecting for ampacity, the conductor is upsized if voltage drop exceeds the specified limit. Resistance values come from NEC Chapter 9, Table 8 (DC resistance at 75°C).
Single-phase: VD = 2 × I × R × PF × (L ÷ 1000)
Three-phase: VD = 1.732 × I × R × PF × (L ÷ 1000)
Overcurrent protection (NEC Article 240)
OCPD ratings are selected from NEC Table 240.6(A). The next-size-up rule (NEC 240.4(B)) is applied where allowed. Small conductor limits per NEC 240.4(D) cap protection at 15 A (14 AWG Cu), 20 A (12 AWG Cu), and 30 A (10 AWG Cu).
Equipment grounding conductor (NEC Article 250)
EGC sizes come from NEC Table 250.122, keyed to the OCPD rating. If the phase conductor was upsized for voltage drop, the EGC is proportionally increased per NEC 250.122(B) and capped at the phase conductor size.
Frequently asked questions
What wire size do I need for a 20-amp circuit?
For a 20-amp branch circuit you need a minimum of 12 AWG copper (or 10 AWG aluminum) at 75°C insulation rating per NEC Table 310.16. For a continuous load, size to 125% (25 A required), which still selects 12 AWG copper. Always confirm with your local AHJ.
What gauge wire do I need for a 30-amp, 240-volt circuit?
A 30-amp, 240-volt non-continuous circuit requires 10 AWG copper or 8 AWG aluminum at 75°C. For a continuous load, size to 125% = 37.5 A required, which selects 8 AWG copper or 6 AWG aluminum.
What wire size is needed for a 50-amp circuit?
A 50-amp non-continuous circuit requires 6 AWG copper or 4 AWG aluminum at 75°C insulation per NEC Table 310.16. Continuous loads size to 125% = 62.5 A, which selects 4 AWG copper or 2 AWG aluminum.
What is the maximum voltage drop allowed by NEC?
The NEC recommends a maximum 3% voltage drop on branch circuits and 5% total on the combined feeder and branch circuit. Enter your desired limit in Allowable Voltage Drop.
When should I use aluminum wire instead of copper?
Aluminum wire is cheaper and lighter than copper, which is why it is common for service entrances, feeders, and runs longer than 100 feet. It requires connectors rated for aluminum and is generally one wire size larger than equivalent copper. Aluminum is not used below 12 AWG in this calculator.
What does AWG mean in wire sizing?
AWG stands for American Wire Gauge — a standardized system where lower numbers indicate larger wire. Above 4/0 AWG, conductors are measured in kcmil (thousands of circular mils).
Does wire ampacity change based on ambient temperature?
Yes. NEC Table 310.16 ampacity values are based on a 30°C (86°F) ambient. Hotter environments require derating using NEC Table 310.15(B)(1). Open Advanced Options to apply temperature correction.
How does this wire size calculator determine the conductor size?
It first selects the smallest conductor whose derated ampacity meets the required load, then checks the voltage drop limit and upsizes if needed. Equipment grounding conductors are sized per NEC Table 250.122.
Disclaimer: This calculator is provided for informational purposes and references NFPA 70-2026. Results should be verified by a licensed electrician before installation. Local amendments and AHJ requirements may differ.