Skip to content
BuildWire Calc
All tools

Power

Wire & cable

Conduit & loads

Motors & gensets

Wire Ampacity Calculator

Pick a conductor and see its allowable amps after the temperature column, ambient and conductor-count adjustments.

Assumes NEC 2023 Table 310.16 (copper or aluminum, 60/75/90 °C insulation columns, conductors in a raceway, cable or earth) with the ambient, bundling and terminal rules you set. Free, runs in your browser. Verify with your local code and a licensed electrician before you install.

Open on BuildWire Calc

This embed was switched off for this website. Open on BuildWire Calc

Add this tool to your siteFree embed code, no sign-up. Keep the credit link.

Embed this tool

Copy this code into your website, blog or class page. The tool runs inside your page.

Optional settings, added to the address after embed=1: &accent=0f6b5c (colour, 3 or 6 hex digits). Other parameters are ignored.

Please keep the credit link below the tool.

Wire ampacity is the current a conductor may carry continuously without exceeding its temperature rating, and this wire ampacity calculator works it out for one size at a time with every NEC 2023 factor shown. It starts from the NEC 310.16 value in your insulation column, corrects it for ambient temperature under 310.15(B)(1), adjusts it for more than three current-carrying conductors under 310.15(C)(1), caps it at the terminal column of 110.14(C)(1) and checks the small-conductor breaker limit of 240.4(D). The Deciding limit row names the rule that set the answer. A second mode works backward from a load, with 125 % for continuous loads, to the smallest copper or aluminum size that passes every check, and reports the breaker it was sized for.
Wire ampacity calculator example

How to use the Wire Ampacity Calculator

  1. Under Calculate, choose “Ampacity of a wire size” to check a size you have, or “Smallest wire for a load” to find one.
  2. In the first mode, pick a Wire size from 14 AWG to 2000 kcmil.
  3. In the second mode, type the Load current (A), tick Continuous load (3 hours or more) if it applies, and optionally enter a Breaker rating (A, optional). Left empty, the tool uses the next standard rating above the design current.
  4. Choose the Conductor: Copper or Aluminum. Aluminum starts at 12 AWG.
  5. Set the Insulation temperature rating: 60 °C for NM-B or UF-B, 75 °C for THW, THWN or XHHW, 90 °C for THHN, THWN-2 or XHHW-2.
  6. Leave Terminal temperature rating on Auto, or pick 75 °C only when the breaker and equipment are listed for it.
  7. Enter the Ambient temperature and set the Ambient unit to °C or °F. 30 °C (86 °F) needs no correction.
  8. Enter the number of Current-carrying conductors in the raceway or cable. The ground is never counted.
  9. Read Allowed ampacity, Deciding limit and Largest standard breaker for this wire (a standard NEC 240.6(A) rating not above the ampacity), plus Next size up allowed by NEC 240.4(B) when the ampacity is not a standard rating, then the warnings.

The example buttons above the form load typical cases, such as “10 AWG THHN in a 50 °C attic” or “4/0 aluminum SER”. Copy result, Copy link and Print / save as PDF keep the answer with its steps, and Reset restores the defaults.

How do I calculate the ampacity of a wire?

Take the NEC 310.16 value for the size, metal and insulation column, multiply it by the ambient correction and the bundling adjustment, then keep the smaller of that and the terminal column value. Small sizes have one more check: NEC 240.4(D) limits the breaker on small conductors such as 14, 12 and 10 AWG copper.

The example “12 AWG copper, 90 °C insulation, 40 °C ambient, six conductors, 75 °C terminals” runs like this in the tool:

  1. NEC 310.16, 90 °C column: 30 A.
  2. Ambient correction at 40 °C: 0.91 (NEC 310.15(B)(1)).
  3. Six current-carrying conductors: 0.8 (NEC 310.15(C)(1)).
  4. Corrected: 30 A × 0.91 × 0.8 = 21.84 A; the 75 °C terminal value is 25 A, so 21.84 A is allowed.
  5. NEC 240.4(D): 12 AWG may be protected at no more than 20 A, so the largest standard breaker is 20 A.

When the allowed ampacity is not a standard rating, for example 55 A for 6 AWG copper in the 60 °C column, the largest standard breaker is the rating below it (50 A). NEC 240.4(B) lets you go to the next standard size up (60 A) only when the circuit is not a multioutlet branch circuit for cord-and-plug loads and the breaker is 800 A or less; the tool shows that value on its own row.

How many amps will #4 THHN wire carry?

THHN is a 90 °C insulation, so 4 AWG copper starts at 95 A in the 90 °C column. What you can use depends on the terminals. With Auto, a 4 AWG conductor is held to the 60 °C column, 70 A, at 30 °C with three conductors. With terminals listed for 75 °C it is 85 A.

Terminal temperature ratingAllowed ampacity, 4 AWG copper THHNDeciding limit
Auto (60 °C for 14 to 1 AWG)70 Aterminal temperature
75 °C (listed equipment)85 Aterminal temperature

The 90 °C value still matters: it is the starting point for derating, so a hot or crowded run of THHN loses less than a 75 °C wire would. The tool warns that the result cannot exceed the terminal value.

How much does a hot attic reduce wire ampacity?

Heat lowers ampacity through the ambient correction factor, and the drop is steeper for lower-rated insulation. At 50 °C, a 90 °C conductor keeps 82 % of its table value, a 75 °C conductor 75 % and a 60 °C conductor 58 %. Enter the real air temperature along the run, not the room below it.

Case at 50 °CTable valueFactorAllowed ampacity
10 AWG THHN, 75 °C terminals40 A0.8232.8 A (breaker 30 A, NEC 240.4(D))
8 AWG THHN, 75 °C terminals55 A0.8245.1 A
10 AWG, 75 °C insulation, Auto35 A0.7526.25 A
12 AWG NM-B (60 °C)20 A0.5811.6 A

Worked examples

InputResult
12 AWG copper 90 °C, 30 °C, 3 conductors20 A
12 AWG copper 90 °C, 40 °C, 6 conductors21.84 A
6 AWG copper 60 °C, 30 °C, 3 conductors55 A
4/0 AWG aluminum 75 °C, 30 °C, 3 conductors180 A
40 A continuous, copper 75 °C, 30 °C, 3 conductors8 AWG (50 A)

Calculated by the tool above; open it with the same values to see each step.

Questions, briefly answered

What size wire do I need for 100 amps?

For a 100 A non-continuous load with Auto terminals, the tool returns 1 AWG copper (110 A in the 60 °C column). With 75 °C insulation and terminals listed for 75 °C, it returns 3 AWG copper or 1 AWG aluminum, both 100 A.

What size wire do I need for a 60-amp circuit?

With 75 °C insulation and Auto terminals, the 60 °C column applies and the answer is 4 AWG copper (70 A). If the terminals are listed for 75 °C, 6 AWG copper at 65 A is enough.

How many amps can I run through a 12 gauge wire?

12 AWG copper is allowed 20 A at 30 °C with up to three conductors, the same as its 240.4(D) breaker limit. Heat or bundling reduce it: in NM-B at 50 °C it falls to 11.6 A.

Do I count the neutral as a current-carrying conductor?

A neutral that carries only the unbalanced current of the other conductors of its circuit is not counted, and a grounding conductor never is (NEC 310.15(E), (F)). A neutral of a two-wire circuit carries the full load current, so it counts.

When is aluminum acceptable instead of copper?

The NEC allows aluminum and it is common for feeders and services. It needs a larger size for the same current: 4/0 aluminum with 75 °C terminals is 180 A. Use connectors and devices marked for aluminum.

Can I use the 90 °C column because my wire is THHN?

Only as the starting point for derating. The final ampacity cannot exceed the terminal column value under NEC 110.14(C), which is 60 °C or 75 °C for most equipment.

How do I size wire for a 48 A EV charger?

Choose “Smallest wire for a load”, enter 48 A and tick the continuous box. The design current is 60 A, and with 75 °C insulation and terminals the tool returns 6 AWG copper at 65 A.