Find the right AWG wire size, check ampacity, and calculate voltage drop for any circuit.
Select your calculation mode using the tabs above. In Find Wire Gauge, enter your circuit's current load, voltage, wire length, and insulation type — the tool will recommend the minimum safe AWG and flag any voltage drop concerns. In Check Existing Wire, enter your current wire gauge and actual load to verify it's rated safely. In Voltage Drop, use the sliders to instantly see how wire gauge and length affect voltage loss.
Undersized wire is one of the leading causes of electrical fires in homes and commercial buildings. When a wire carries more current than it's rated for, it heats up — and heat means resistance, more heat, and eventually insulation damage or worse. The NEC (National Electrical Code) establishes ampacity ratings for good reason.
Here's a real-world example: a homeowner adds a 20-amp kitchen circuit, but uses leftover AWG 14 wire rated for 15 amps at 75°C. The breaker won't trip at 18 amps — but the wire is silently degrading. Over time, that's a fire hazard. The fix? AWG 12, which handles 20 amps comfortably.
Voltage drop matters too. Run a 20-amp, 120V circuit 150 feet with AWG 12 and you'll lose about 5.6 volts — nearly 5%. Lights may flicker and motors run hot. Upsizing to AWG 10 drops that loss to 3.5 volts, keeping everything inside NEC's recommended 3% guideline.
Electricians, DIYers, solar installers, RV builders, and marine electricians all depend on accurate wire sizing. This calculator uses NEC Table 310.15 ampacity values and the standard voltage drop formula for instant, reliable results.
Ampacity is looked up from NEC Table 310.15(B)(16) based on wire gauge (AWG) and insulation temperature rating. The calculator selects the smallest gauge whose ampacity exceeds your load, then applies a 125% continuous load factor where applicable.
Voltage Drop Formula (single-phase):
VD = (2 × K × I × L) / CM
Where: K = resistivity constant (12.9 for copper, 21.2 for aluminum), I = current in amps, L = one-way length in feet, CM = circular mils of the wire. For three-phase: multiply by √3 / 2 (factor ≈ 0.866).
Voltage drop percentage = (VD / Source Voltage) × 100. NEC recommends staying under 3% for branch circuits.
AWG 12 copper at 75°C is the standard choice for a 20-amp branch circuit in residential wiring. It's rated for 20 amps in conduit and up to 25 amps free-air. Always use a 20-amp breaker with AWG 12 — never a 15-amp breaker, which would be undersized for the wire's capacity.
AWG (American Wire Gauge) is an inverse scale — a lower number means a thicker wire. AWG 4 is much thicker than AWG 14. Each 6-gauge decrease roughly doubles the cross-sectional area. AWG 0 (commonly written 1/0) is followed by 2/0, 3/0, and 4/0 for very large conductors.
NEC recommends a maximum of 3% voltage drop on branch circuits and 5% total (including feeder) for optimal efficiency. For sensitive electronics or long motor runs, keep it under 2%. Excessive drop causes equipment to run hot, reduces efficiency, and can shorten appliance lifespan.
Yes, aluminum is commonly used for service entrance conductors and large feeders where the cost savings are significant. However, aluminum requires larger gauge wire (typically two sizes up from copper), anti-oxidant compound on connections, and AL-rated connectors. AWG 2 aluminum has roughly the same ampacity as AWG 4 copper at 75°C.