Wire Size Calculator
Calculate the correct wire gauge for electrical runs. Enter amps, one-way distance, and voltage to get the recommended AWG size with voltage drop analysis.
Quick Answer
Wire size depends on current (amps), distance, and acceptable voltage drop. NEC recommends no more than 3% voltage drop for branch circuits and 5% total. Longer runs need thicker wire to compensate for resistance.
Circuit Parameters
Results
AWG 8
Recommended Wire Size
3.11 V
Voltage Drop (2.59%)
62.2 W
Power Loss in Wire
40 A
Wire Ampacity (75°C)
AWG 12
Min. by Ampacity Only
About This Tool
The Wire Size Calculator determines the correct American Wire Gauge (AWG) for electrical runs based on current draw, distance, voltage, and acceptable voltage drop. It considers both the NEC ampacity rating and voltage drop to recommend the appropriate wire gauge, since long runs often require thicker wire than ampacity alone would suggest.
Why Wire Size Matters
Undersized wire causes voltage drop, energy waste, and heat buildup. Excessive heat can melt insulation and start fires. The National Electrical Code (NEC) sets minimum wire sizes based on the current each gauge can safely carry (ampacity). But the NEC also recommends limiting voltage drop to ensure equipment operates properly.
Voltage Drop Explained
All wire has resistance. As current flows through this resistance, some voltage is lost as heat. The voltage drop depends on wire gauge, length, and current. NEC recommends no more than 3% drop for branch circuits and 5% total (feeder plus branch). At 120V, a 3% drop means only 116.4V reaches the outlet, which is usually acceptable for most equipment.
Copper vs Aluminum
Copper has about 61% the resistance of aluminum for the same gauge, making it more efficient but more expensive. Aluminum wire is common for large feeders and service entrances where cost savings are significant. When using aluminum, you typically need to go up one or two wire sizes compared to copper. Aluminum also requires special connectors rated for aluminum to prevent corrosion at connections.
NEC Ampacity Ratings
NEC Table 310.16 lists the maximum current for each wire gauge based on insulation temperature rating and installation method. The 75 degrees C column is most commonly used for residential and commercial wiring. Continuous loads (running 3+ hours) must be derated to 80% of the ampacity. Wires in conduit with many other conductors must be further derated per NEC 310.15.
Frequently Asked Questions
What wire size do I need for a 20-amp circuit?
What is the maximum voltage drop allowed by NEC?
When should I use aluminum wire instead of copper?
How does distance affect wire size?
What is the 80% rule for circuit breakers?
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