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.
Input Values
NEC recommends 3% max for branch circuits
Results
Enter your parameters and click Calculate to see results.
Need help selecting the right wire or panel?
Voltage Drop Equations
Single-phase / DC: VD = 2 × I × R × PF × (L ÷ 1000)
Three-phase: VD = 1.732 × I × R × PF × (L ÷ 1000)
R = DC resistance in Ω/1000 ft at 75°C from NEC Ch. 9, Table 8. I = load current in amperes. L = one-way circuit length in feet. PF = power factor.
AWG Wire Size Reference Chart
Ampacity values from NEC Table 310.16 at 75°C (THWN/THWN-2 insulation), based on not more than three current-carrying conductors in raceway at 30°C ambient. Derate for higher temperatures or more than three bundled conductors.
| 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, ≤3 CCC, 30°C ambient. Use the calculator above to apply derating factors for your specific installation.
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 from NEC Table 310.16 whose derated ampacity meets the required load. Continuous loads (≥3 hours) are sized at 125% per NEC 210.19(A)(1). Ampacity is derated for ambient temperature above 30°C per Table 310.15(B)(1) and for more than 3 conductors in a raceway per Table 310.15(C)(1). Terminal temperature ratings limit the usable ampacity column per NEC 110.14(C).
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)
R = Ω/1000 ft · I = amperes · L = one-way feet · PF = power factor
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 (dryer, HVAC) 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 (EV chargers running 3+ hours) 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 (not mandates) a maximum 3% voltage drop on branch circuits and 5% total on the combined feeder and branch circuit. Exceeding these limits leads to dimming lights, motor overheating, and nuisance tripping. Enter your desired limit in the calculator's Allowable Voltage Drop field.
When should I use aluminum wire instead of copper?
Aluminum wire is significantly cheaper and lighter than copper, making it the standard choice for service entrances, feeders, and runs longer than 100 feet where material cost matters. Aluminum requires connectors rated for aluminum (AL/CU or AL9CU), anti-oxidant compound, and is generally one wire size larger than equivalent copper. Aluminum is not recommended below 12 AWG.
What does AWG mean in wire sizing?
AWG stands for American Wire Gauge — a standardized system where lower numbers indicate larger wire. 4/0 AWG (0000) is larger than 4 AWG. Above 4/0 AWG, conductors are measured in kcmil (thousands of circular mils): 250 kcmil, 500 kcmil, 1000 kcmil.
Does wire ampacity change based on ambient temperature?
Yes. NEC Table 310.16 ampacity values are based on a 30°C (86°F) ambient. Installations in hotter environments (attics, engine rooms, outdoor conduit in direct sun) require derating using NEC Table 310.15(B)(1). Use the Advanced Options toggle in the calculator to apply temperature correction factors.
How does this wire size calculator determine the conductor size?
The calculator first 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)) factors. It then checks whether that conductor satisfies 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 (National Electrical Code). Results should be verified by a licensed electrician before installation. Local amendments and AHJ requirements may differ from the NEC. Always consult applicable codes and a qualified professional for safety-critical electrical work.