Wire Size Calculator
Determine the correct wire gauge for your circuit based on amperage, distance, voltage, and wire material. Meets NEC ampacity and voltage drop requirements.
8 AWG copper meets the 20A ampacity requirement and keeps voltage drop at 2.59% — within the NEC 3% recommendation for a 100 ft run at 120V.
How It Works
This calculator finds the smallest wire gauge that meets both NEC ampacity requirements and the 3% voltage drop recommendation for branch circuits.
How it works: Starting from the smallest gauge (14 AWG), the calculator checks if the wire's ampacity is sufficient for your load. If so, it then verifies that voltage drop stays at or below 3%. The first gauge that passes both checks is recommended.
Ampacity values are based on NEC Table 310.16 for 60°C rated conductors. Voltage drop is calculated using standard NEC resistance values for each gauge and material.
If no standard gauge meets both requirements, the calculator recommends 1/0 AWG or larger and suggests consulting an engineer for the specific application.
Frequently Asked Questions
How do I determine the right wire size?
Wire size depends on three factors: the amperage of the circuit, the distance from the panel to the load, and the acceptable voltage drop (typically 3% per NEC recommendations). This calculator finds the smallest wire gauge that satisfies both the ampacity requirement and the 3% voltage drop limit.
What is the difference between wire gauge and ampacity?
Wire gauge (AWG) is the physical size of the wire — lower numbers mean thicker wire. Ampacity is the maximum current a wire can safely carry without overheating. For example, 12 AWG copper has an ampacity of 20 amps, while 10 AWG copper can carry 30 amps.
When should I use copper vs aluminum wire?
Copper is the standard for branch circuits in residential and commercial buildings. It has better conductivity, is easier to work with, and requires smaller conduit. Aluminum is used for larger feeder circuits and service entrances where cost savings are significant, but it requires anti-oxidant compound and properly rated connectors.
Does distance affect what wire size I need?
Yes. Longer wire runs experience more voltage drop, which may require you to upsize the wire beyond what ampacity alone requires. A 20-amp circuit might need 12 AWG for a 50-foot run but 10 AWG for a 150-foot run to keep voltage drop under 3%.
What happens if I use wire that is too small?
Undersized wire overheats, which can melt insulation, trip breakers, damage equipment, and create a fire hazard. It also causes excessive voltage drop, leading to dim lights, slow motors, and wasted energy. Always size wire per NEC ampacity tables and voltage drop recommendations.