Precision Utility
Wire Size
Calculator
Standard
NEC Code
System
120V/240V / 60Hz
Find the correct wire gauge for any US electrical installation. Enter your circuit current, wire run length, voltage and material — the calculator sizes the wire per NEC Table 310.16 ampacity ratings and checks voltage drop stays within the recommended 3% limit. Works for residential, commercial and industrial circuits using AWG sizing.
Circuit Parameters
Recommended Wire Size
12 AWG
Wire Gauge
12 AWG
Ampacity
20 A
Voltage Drop
2.37 V
Voltage Drop %
1.98%
Voltage drop is within the NEC 3% recommendation
How the wire size calculator works
The calculator uses two NEC criteria to determine the correct wire gauge for your circuit. First, it checks NEC Table 310.16 to find the smallest wire with enough ampacity to carry your specified current safely. Second, it calculates voltage drop using the circular mil area method to make sure the wire can deliver power over your run length without excessive loss.
Enter the circuit current in amps, the one-way wire length in feet, your system voltage, and the wire material. The calculator tests each AWG size starting from 14 gauge and selects the smallest wire that satisfies both the ampacity requirement and the NEC-recommended 3% maximum voltage drop for branch circuits.
Voltage drop is calculated using the formula VD = (2 x K x I x L) / CM, where K is a material resistivity constant (12.9 for copper, 21.2 for aluminum), I is current, L is one-way length, and CM is the wire's circular mil area. The factor of 2 accounts for the full circuit length — out and back.
The result card shows your recommended AWG size, its rated ampacity, the actual voltage drop in volts, and the drop as a percentage of system voltage. A status indicator tells you instantly whether the installation meets NEC voltage drop guidelines.
NEC wire sizing rules you need to know
The National Electrical Code (NEC) governs wire sizing in the United States. Two key tables drive most residential and commercial wire sizing decisions:
- NEC 310.16 — lists the maximum allowable ampacity for each wire size based on insulation temperature rating and conductor material
- NEC 210.19(A)(1) Informational Note 4 — recommends that branch circuit conductors be sized so voltage drop does not exceed 3%
- NEC 215.2(A)(4) Informational Note 2 — recommends the total voltage drop for feeder plus branch circuit not exceed 5%
- Aluminum wire requires a larger gauge than copper for the same ampacity — typically one or two sizes up
- Conduit fill, ambient temperature, and bundling can all require further derating beyond the base table values
This calculator uses the 60 degree C column from NEC 310.16 for copper conductors, which is the most conservative and widely applicable rating for standard residential wiring. Always verify final sizing with a licensed electrician and your local authority having jurisdiction (AHJ).
Frequently asked questions
What wire size do I need for a 20 amp circuit?
For a 20 amp circuit, you need a minimum of 12 AWG copper wire per NEC Table 310.16. However, if your wire run is long, you may need to upsize to 10 AWG to keep voltage drop below 3%. Use the calculator above to check your specific run length.
How do I calculate voltage drop for a wire run?
Voltage drop is calculated using the formula VD = (2 x K x I x L) / CM, where K is a resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is one-way length in feet, and CM is the circular mil area of the wire. The NEC recommends keeping voltage drop at or below 3% for branch circuits.
What is the maximum recommended voltage drop per NEC?
The NEC recommends a maximum 3% voltage drop for branch circuits and 5% total voltage drop for the combined feeder and branch circuit. These are recommendations, not hard code requirements, but exceeding them can cause equipment to underperform or overheat.
Should I use copper or aluminum wire?
Copper wire has better conductivity and requires a smaller gauge for the same ampacity. Aluminum is lighter and cheaper but needs a larger wire size and special connectors rated for aluminum. Most residential branch circuits use copper. Aluminum is common for larger feeder and service entrance cables.
What AWG wire size do I need for a 100 amp sub-panel?
A 100 amp sub-panel typically requires 1 AWG copper or 1/0 AWG aluminum wire. For longer runs you may need to upsize to limit voltage drop. Enter your specific current, distance, and wire material into the calculator to get the correct wire size for your installation.
How does wire length affect the wire size I need?
Longer wire runs increase voltage drop. Even if a wire meets ampacity requirements, the voltage at the end of a long run can be too low for equipment to operate properly. The NEC 3% voltage drop guideline means that a 100-foot run may need a larger gauge than a 25-foot run carrying the same current.