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What ampacity does a wire carry? An NEC calculator app and its limits

Six 12 AWG copper wires at 90°C in a 40°C ambient come out at 21.8 A in PipeMaster's Ampacity calculator (Pro). It multiplies the NEC Table 310.16 value by a temperature factor and a conductor-count factor. NEC and AWG only, no metric cable.

3 min read · Updated 2026-09-10

Download on the App Store

Take 12 AWG sharing a conduit in a hot ceiling with six current-carrying wires: the table says 30 A at 90°C, but the Ampacity screen says 21.8 A for that situation, and it shows the three numbers it multiplied so you can check them against the code book.

The three factors

Three steps, all NEC. The base value from Table 310.16 for the wire size and temperature column, the ambient correction from 310.15(B)(1) with 30°C as the base, and the adjustment for more than three current-carrying conductors from 310.15(C)(1). Up to three wires the adjustment is 100%, then 80% for 4 to 6, 70% for 7 to 9, 50% for 10 to 20, 45% for 21 to 30, 40% for 31 to 40 and 35% above that.

How to run it

  1. Open Ampacity under Electrical (Pro) on the main list.
  2. Under Conductor choose Material: copper (14 to 4/0) or aluminum (12 to 4/0).
  3. Pick Wire Size (AWG) and Temp Rating: 60°C, 75°C or 90°C.
  4. Set Current-carrying with the stepper, 1 to 60, and Ambient Temp from 10 to 55°C.
  5. Read Base Ampacity, Ambient Correction, Count Adjustment and Adjusted Ampacity under Result.
Results worked out with the same tables and rounding as the app
WireRatingCurrent-carryingAmbientBase AmpacityAmbient CorrectionCount AdjustmentAdjusted Ampacity
12 AWG Cu90°C640°C30 A0.9180%21.8 A
10 AWG Cu90°C945°C40 A0.8770%24.4 A
10 AWG Cu75°C635°C35 A0.9480%26.3 A
4 AWG Al75°C340°C65 A0.88100%57.2 A

The ambient bands are upper-inclusive: 40°C uses the 36 to 40°C factor, not the one above it.

Where the numbers need a second check

The 90°C column makes 12 AWG look like a 30 A wire, and the screen reports 24.0 A for six of them at 30°C without a blink. On a real job a 12 AWG circuit is still held down by the breaker rules and the terminal temperature rating, and the app applies neither.

It gives the derated wire value. It does not tell you what you may protect the wire with.

Not for metric cable or other national codes

Sizes are AWG and the tables are the US NEC. The app has no mm² cable sizes and no IEC or other national tables, so do not use its figures for a job under another country's rules.

What it costs

Ampacity is Pro, along with the other three electrical calculators. The free side of PipeMaster is bending: Offset, 90° / Stub-Up and Bend Reference, with ads. Pro, either a subscription or a one-time purchase, removes the ads. It is worth paying for mainly if you do the electrical sums on your phone often.

  • Table 310.16 only, copper and aluminum, 60/75/90°C, wire sizes 14 or 12 AWG to 4/0.
  • No small-conductor overcurrent limit, no termination temperature limit, no breaker sizing.
  • No edition of the NEC is stated, and nothing from other national codes.
  • iPhone only, works offline.

Questions and answers

How do you calculate ampacity with derating?
Take the Table 310.16 value for your wire size and temperature column, multiply by the ambient temperature correction factor, then by the factor for the number of current-carrying conductors. For 10 AWG copper at 90°C with nine wires at 45°C that is 40 × 0.87 × 0.70 = 24.4 A.
Does the calculator include the 3 conductor rule?
Yes. One to three conductors count as 100%, and each larger group drops the factor, down to 35% above 40.
Does it work for metric cable sizes or other national codes?
No. It follows the US NEC with AWG sizes. It has no mm² cable sizes and no tables from other codes, so the numbers cannot be reused there.
Is the ampacity calculator free?
No, it is part of PipeMaster Pro. The app itself is free to download and its offset and 90° bend calculators are free.

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