Wire Size Calculator: NEC Ampacity and Breaker Size

This wire size calculator finds the smallest copper or aluminum conductor that can carry a load under the National Electrical Code. It checks each size three ways: its ampacity in the terminal temperature column of Table 310.16 against 125% of a continuous load, its ampacity after the ambient temperature and bundling factors of 310.15 against the load, and whether a breaker or fuse large enough for the load may still protect it. For a 48 A EV charger on copper THHN with 75 °C terminals, the answer is 6 AWG on a 60 A breaker.

How to use the calculator

Enter the load current and tick “Continuous load” if it runs for 3 hours or more, as EV chargers, water heaters and store lighting do. HVAC equipment is sized from the minimum circuit ampacity on its nameplate instead, which already includes the 125% (Article 440). Pick copper or aluminum, the insulation on the wire and the temperature rating marked on the breaker and equipment terminals. Then enter the hottest ambient temperature along the run and how many current-carrying conductors share the raceway or cable. The result shows the size, both ampacity checks, the breaker or fuse range and a link that opens the voltage drop calculator with this wire.

How to size a wire for a load

1. Find the design current. Use the load in amperes. If it is continuous, multiply by 1.25 (NEC 210.19(A) for branch circuits, 215.2(A) for feeders).
2. Check the terminals. In Table 310.16, pick a size whose ampacity in the column of the lowest terminal rating, usually 75 °C, is at least the design current (110.14(C)).
3. Apply the conditions of use. Multiply the ampacity in the insulation’s column by the ambient correction factor (Table 310.15(B)(1)(1)) and the adjustment factor for more than three current-carrying conductors (Table 310.15(C)(1)). The result must be at least the load.
4. Check the overcurrent protection. The breaker or fuse must be at least the design current, rounded up to a standard rating (240.6(A)), and the conductor must be allowed on it: up to the next standard rating above its ampacity, to 800 A (240.4(B)), but not on a branch circuit with more than one receptacle for cord-and-plug-connected portable loads (240.4(B)(1)); above 800 A, no more than its ampacity (240.4(C)); and no more than 15, 20 and 30 A for 14, 12 and 10 AWG copper (240.4(D)).
5. Check voltage drop. A long run can need a larger conductor than ampacity alone.

Worked example

A 16 A continuous load is fed with copper THHN through a hot attic at 50 °C (122 °F), in a conduit with six current-carrying conductors. The breaker terminals are rated 75 °C.

• Design current: 16 × 1.25 = 20 A.
• 14 AWG has 20 A in the 75 °C column, enough at the terminals. Under the conditions it has 25 A (90 °C column) × 0.82 × 0.80 = 16.4 A, enough for 16 A. But the breaker must be at least 20 A, and 240.4(D) limits 14 AWG copper to 15 A, so 14 AWG fails.
• 12 AWG has 25 A at the terminals and 30 × 0.82 × 0.80 = 19.7 A under the conditions, and a 20 A breaker is allowed on it. 12 AWG on a 20 A breaker works.

Pick the “Hot attic, bundled” quick start in the calculator above to see each step.

Rules that change the answer

• Terminal ratings. Equipment for circuits of 100 A or less, or marked for 14 to 1 AWG, uses the 60 °C column unless it is listed and marked for 75 °C (110.14(C)(1)). Most breakers and panels sold today are marked 60/75 °C.
• The 90 °C column is for derating. THHN and XHHW-2 may start from their 90 °C ampacity when factors are applied, but the final ampacity may not exceed the terminal column.
• NM cable. Type NM (Romex) is held to the 60 °C column, although its 90 °C rating may be used for the derating (334.80). It is made only in 14 to 2 AWG copper and 12 to 2 AWG aluminum (334.104), so a larger load needs another wiring method.
• Rooftops. Raceways or cables in sunlight less than 7/8 in. (23 mm) above a roof add 33 °C (60 °F) to the ambient (310.15(B)(2)). Type XHHW-2 is exempt.
• Neutrals and grounds. A neutral that carries only the unbalanced current is not counted; it counts on a three-wire circuit from a wye system and with nonlinear loads (310.15(E)). Equipment grounding conductors are never counted (310.15(F)).
• Parallel sets. Conductors in parallel must be 1/0 AWG or larger, with every set the same length, material, size and insulation (310.10(G)).

Method and sources

The calculator uses the ampacities of NEC Table 310.16 for copper from 14 AWG to 1000 kcmil and aluminum from 8 AWG to 1000 kcmil, the correction factors of Table 310.15(B)(1)(1) and the adjustment factors of Table 310.15(C)(1), as printed in the 2020 and 2023 editions. The full ampacity table, the formulas and the code sections are under the calculator. Motor circuits follow Article 430 instead: use the motor circuit calculator for them.

Frequently asked questions

What size wire for a 60 A breaker?

With 75 °C terminals and no derating, 6 AWG copper (65 A) or 4 AWG aluminum (65 A). With 60 °C terminals, copper needs 4 AWG (70 A). Heat, bundling and continuous loads can push the size up.

Can I use the 90 °C column?

Only as the starting point for the ambient and bundling factors. The final ampacity may not exceed the column of the lowest-rated termination, which is 60 °C or 75 °C for most equipment.

What size wire for a 100 A subpanel?

With 75 °C terminals and 100 A of load that is not continuous, 3 AWG copper or 1 AWG aluminum, both 100 A. The main feeder of a single-family dwelling may instead be sized under 310.12, which allows smaller conductors.

Does the calculator check voltage drop?

No. Ampacity sets the smallest size. The result links to the voltage drop calculator with the chosen wire already entered; add the voltage and the length of the run there.

When to bring in an engineer

Ampacity is one line in a feeder schedule. Fault current, voltage drop, conduit fill and coordination with the breakers upstream all have to agree. Dreiym’s licensed Professional Engineers design branch circuits, feeders and services, PE-stamped where required. Related tools: the voltage drop calculator, the conduit fill calculator and the motor circuit calculator. See our electrical design services, or contact us for a free quote. We typically respond within one business day.