Wire size calculator
Enter the load, run length and installation conditions. The calculator works through each NEC step in order and shows why each smaller size was rejected.
Check before you buy wire: the breaker must not exceed the wire's allowable ampacity (except where 240.4(B) permits the next size up, or for motor and HVAC circuits). The equipment's installation instructions may require a specific size. Local amendments and the inspector take priority.
Step through the sizes the calculator tried
It starts at the smallest conductor and works up, applying each NEC rule in turn. Step back to see exactly which rule rejected each size.
- Table 310.16 base ampacity30 A
- × ambient 1 × bundling 130 A
- Capped at terminal column → allowable30 A
- 240.4(D) max overcurrent device30 A
- Voltage drop on this run1.24%
- Needs 24 APasses
How the wire size is chosen
- Load. Continuous loads (3 hours or more) are multiplied by 125% (NEC 210.19(A), 215.2(A)).
- Base ampacity. Read from NEC Table 310.16 in the column matching the insulation rating: 60, 75 or 90 °C.
- Corrections. Multiply by the ambient temperature factor, Table 310.15(B)(1), and by the adjustment for more than three current-carrying conductors, Table 310.15(C)(1).
- Terminal limit. Allowable ampacity can't exceed the table value in the terminal's temperature column (110.14(C)). This is why 90 °C THHN on a standard 60/75 °C breaker is still sized from the 60 or 75 °C column.
- Small conductors. 240.4(D) caps overcurrent protection: 14 AWG Cu 15 A, 12 AWG Cu 20 A, 10 AWG Cu 30 A; 12 AWG Al 15 A, 10 AWG Al 25 A.
- Voltage drop. If the run is long, step up until the drop is within your target.
Common sizes at a glance
Copper, 30 °C ambient, no more than three current-carrying conductors, 60 °C terminals up to 100 A and 75 °C above. Short runs only. For longer runs use the calculator.
Questions people ask
What size wire do I need for 30 amps?
10 AWG copper under standard conditions (30 °C ambient, no more than three current-carrying conductors). NEC 240.4(D) limits 10 AWG copper to 30 A overcurrent protection. Long runs may need 8 AWG to keep voltage drop down.
Which temperature column should I use?
NEC 110.14(C): the lowest temperature rating of the wire, terminals or device. For equipment rated 100 A or less, or marked for 14–1 AWG, use the 60 °C column unless the equipment is marked 75 °C. NM-B cable is limited to 60 °C ampacity (334.80). 90 °C values are only used as the starting point for derating.
What is a continuous load?
A load expected to run at maximum current for 3 hours or more, such as EV charging, water heaters or space heaters. Conductors and breakers for continuous loads are sized at 125% of the load.
Does voltage drop change the wire size?
It can. Ampacity keeps the wire from overheating; voltage drop keeps equipment working properly. On long runs the voltage-drop target often calls for a larger size than ampacity alone.