Wire Size (AWG) Calculator
To size a wire, find the smallest conductor in NEC Table 310.16 whose ampacity in your temperature column is at least the circuit current, then apply the small-conductor limits of 240.4(D): copper 14 AWG is limited to 15 A, 12 AWG to 20 A and 10 AWG to 30 A of overcurrent protection. The answer is a minimum from the table. Minimum per table; derating for bundling and ambient temperature may require larger; follow local code and a licensed electrician.
Wire size calculator
Enter the circuit current and choose the conductor and temperature column. Results update as you type and are saved in the page address. Nothing leaves your browser.
Minimum per table; derating for bundling and ambient temperature may require larger; follow local code and a licensed electrician.
How the wire size calculator works
The calculator walks down NEC Table 310.16 from the smallest size. It skips a size when its ampacity in your column is below the required current, when the table has no value for it, or when 240.4(D) caps the overcurrent protection below the required current. The first size left is the minimum.
required current = circuit current (continuous load: × 1.25) a size passes when: table ampacity (material, temperature column) ≥ required current and, for the small conductors, required current ≤ the 240.4(D) limit: copper 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A aluminum 12 AWG 15 A, 10 AWG 25 A minimum size = the smallest size that passes optional: move up one size at a time until VD = 2 × K × I × L ÷ CM is at most 3% of the voltage
- ampacity
- NEC Table 310.16, not more than three current-carrying conductors, 30 °C ambient
- K, CM
- see the voltage drop calculator: K = 12.9 (copper), 21.2 (aluminum)
Worked example
Given: a 25 A circuit; copper; 75 °C column; no continuous load.
- 14 AWG: table ampacity 20 A:
below 25 A, skipped - 12 AWG: table ampacity 25 A:
25 A is enough, but 240.4(D) limits 12 AWG copper to 20 A, skipped - 10 AWG: table ampacity 35 A and 240.4(D) limit 30 A:
passes
Answer: 10 AWG copper is the minimum from the table. Minimum per table; derating for bundling and ambient temperature may require larger; follow local code and a licensed electrician.
Table 310.16 values used (copper)
| Size | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 14 AWG | 15 A | 20 A | 25 A |
| 12 AWG | 20 A | 25 A | 30 A |
| 10 AWG | 30 A | 35 A | 40 A |
| 8 AWG | 40 A | 50 A | 55 A |
| 6 AWG | 55 A | 65 A | 75 A |
| 4 AWG | 70 A | 85 A | 95 A |
| 3 AWG | 85 A | 100 A | 115 A |
| 2 AWG | 95 A | 115 A | 130 A |
| 1 AWG | 110 A | 130 A | 145 A |
| 1/0 AWG | 125 A | 150 A | 170 A |
| 2/0 AWG | 145 A | 175 A | 195 A |
| 3/0 AWG | 165 A | 200 A | 225 A |
| 4/0 AWG | 195 A | 230 A | 260 A |
Source 1. 14, 12 and 10 AWG are additionally limited by 240.4(D). The calculator holds sizes through 500 kcmil.
Table 310.16 values used (aluminum and copper-clad aluminum)
| Size | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 12 AWG | 15 A | 20 A | 25 A |
| 10 AWG | 25 A | 30 A | 35 A |
| 8 AWG | 35 A | 40 A | 45 A |
| 6 AWG | 40 A | 50 A | 55 A |
| 4 AWG | 55 A | 65 A | 75 A |
| 3 AWG | 65 A | 75 A | 85 A |
| 2 AWG | 75 A | 90 A | 100 A |
| 1 AWG | 85 A | 100 A | 115 A |
| 1/0 AWG | 100 A | 120 A | 135 A |
| 2/0 AWG | 115 A | 135 A | 150 A |
| 3/0 AWG | 130 A | 155 A | 175 A |
| 4/0 AWG | 150 A | 180 A | 205 A |
Source 1. The table lists no aluminum value for 14 AWG.
Assumptions and limits
- Table 310.16 applies to not more than three current-carrying conductors in a raceway, cable or earth at a 30 °C (86 °F) ambient. The calculator does not apply temperature correction or bundling adjustment factors.
- It does not choose a breaker, check terminal ratings, motor circuits, EV or air-conditioning rules, service or feeder rules, or the box and device ratings.
- The input current is treated as both the required ampacity and the overcurrent protection for the 240.4(D) check. Standard breaker sizes are not selected for you.
- Voltage drop (optional) is a recommendation check at 3%, not a requirement. See the volts-lost estimator for the formula.
- NM-B and other cable types have their own listing and use rules. Aluminum wiring in an older home is a reason to call a licensed electrician, not to size it yourself.
Frequently asked questions
What wire size do I need for a 20 amp circuit?
12 AWG copper is the minimum for a 20 A circuit under the small-conductor limits of 240.4(D): 14 AWG copper is limited to 15 A and 12 AWG copper to 20 A. Long runs, bundled cables and hot locations can require a larger wire, and the voltage drop check below shows when a longer run needs it. Your local code and a licensed electrician decide the final size.
What wire size do I need for a 30 amp circuit?
10 AWG copper is the minimum for a 30 A circuit (240.4(D) allows 30 A on 10 AWG copper, and Table 310.16 gives it 30 A in the 60 °C column). For aluminum the table points to 8 AWG in the 60 °C column because 10 AWG aluminum is limited to 25 A.
Which temperature column should I use?
Use the lowest temperature rating among the wire insulation and the terminals or devices it connects to. Many home devices and breakers are rated 60 °C or 75 °C. If you do not know the ratings, choosing the 60 °C column is the conservative option (that is our guidance, not a code rule). A 90 °C wire is still limited by the lower-rated terminals and by 240.4(D).
Why is the result a minimum?
Table 310.16 applies to not more than three current-carrying conductors at a 30 °C ambient. More conductors in the same cable or conduit, hot attics, sun exposure, long runs, motor loads and terminal ratings can each require a larger wire. The calculator does not apply those factors.
What is the 125% continuous load option?
Where a branch circuit supplies a continuous load (running three hours or more), 210.19(A)(1)(a) requires conductor ampacity of at least the noncontinuous load plus 125% of the continuous load. Tick the option and the calculator multiplies your current by 1.25 before using the table.
Does the calculator check voltage drop?
Optionally. Enter the one-way length and system voltage and it applies the voltage drop formula to the minimum size, moving up to a larger size if the drop is above 3%. That 3% is an NEC informational-note recommendation, not a requirement.
Sources
- NFPA 70 (NEC) 2020 text as reproduced in the 2022 California Electrical Code: Table 310.16 ampacities, p. 70-187 (law.resource.org) Accessed 2026-11-14. Same ampacity values appear in the 2011 NEC reprint below; the 2023 NEC edition is unconfirmed here.
- Same document: 240.4(D) Small Conductors, p. 70-106, and 210.19(A)(1)(a) continuous loads, p. 70-69 (law.resource.org) Accessed 2026-11-14.
- NEC 2011 Table 310.15(B)(16) (formerly 310.16) reprint, IBEW/NECA local: cross-check of the same copper and aluminum ampacities (necaibewelectricians.com) Accessed 2026-11-14.
- IAEI Magazine, "Voltage Drop Formulas": K values and the voltage drop formula used by the optional check (iaeimagazine.org) Accessed 2026-11-14.
- ESFI, Do-It-Yourself (DIY) Electrical Safety (esfi.org) Accessed 2026-11-14.
- CPSC, Plug into Safety! (NSN 18-07) (cpsc.gov) Accessed 2026-11-14.