Wire Sizing and Ampacity Practice

The ampacity table, the termination temperature trap, and the two factors that reduce what a conductor may actually carry. The largest single block of marks on most outlines.

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What it is, and why electricians need it

Conductor sizing is four separate checks, and candidates lose marks by treating it as one: the conductor must carry the load, the overcurrent device must protect the conductor, the terminations must tolerate the temperature, and on a long run the voltage drop must be acceptable. Satisfying one says nothing about the others.

Branch circuits and conductors is the largest block on the Texas journeyman knowledge portion, at 10 of 56 scored items, and it is where the most routine mistakes happen. The usual culprit is the termination temperature limitation. It is tempting to read the 90 degree Celsius column for THHN and be done, but unless the equipment is listed otherwise, a circuit rated 100 amperes or less must use the 60 degree Celsius ampacity and a circuit over 100 amperes the 75 degree Celsius ampacity. The higher-rated insulation is still useful — you work the correction and adjustment arithmetic in its column — but the answer is capped by what the terminal can stand, because the terminal is the hottest and most vulnerable point in the connection.

The two reduction factors are the other half of the topic. Ambient correction applies when the surroundings are hotter or colder than the table's 30 degree Celsius basis. Conductor-count adjustment applies when more than three current-carrying conductors share a raceway or cable, because bundled conductors trap each other's heat. Both are multipliers, and they multiply together against the table value in a single step.

Finally, the small-conductor rule overrides the table for the smallest sizes: 14 AWG copper is capped at a 15-ampere device, 12 AWG at 20 and 10 AWG at 30, regardless of what their insulation would otherwise allow. This is the origin of the familiar residential pairings, and it is why a 12 AWG conductor good for 30 amperes still gets a 20-ampere breaker.

Formulas and principles

  • Corrected ampacity = table ampacity × ambient factor × adjustment factor

    Apply both multipliers in one step against the value from the conductor's own temperature column.

    Then cap the result at the termination temperature rating.

  • Terminations: 60 °C at 100 A or less, 75 °C over 100 A

    The default unless the equipment is listed and marked for higher-temperature conductors.

    Also keyed to conductor size: 14 AWG through 1 AWG takes 60 °C, larger than 1 AWG takes 75 °C.

  • Adjustment: 4-6 → 80%, 7-9 → 70%, 10-20 → 50%, 21-30 → 45%, 31-40 → 40%, 41+ → 35%

    Current-carrying conductors in one raceway or cable, spares included.

    Equipment grounding conductors are not counted.

  • Small conductors: 14 AWG Cu → 15 A, 12 AWG Cu → 20 A, 10 AWG Cu → 30 A

    Maximum overcurrent device rating regardless of insulation rating.

    Aluminium is one step lower: 12 AWG → 15 A, 10 AWG → 25 A.

The ampacity values that come up most

Allowable ampacities for copper conductors, not more than three current-carrying conductors, 30 degree Celsius ambient. These are the rows worth knowing without looking them up.

Copper conductor allowable ampacity by temperature rating
Size60 °C (TW)75 °C (THWN)90 °C (THHN)
14 AWG152025
12 AWG202530
10 AWG303540
8 AWG405055
6 AWG556575
4 AWG708595
3 AWG85100115
2 AWG95115130
1 AWG110130145
1/0 AWG125150170
2/0 AWG145175195
3/0 AWG165200225
4/0 AWG195230260

Ambient correction factors for a 30 degree Celsius basis, by conductor temperature rating: 31–35 °C 0.91 / 0.94 / 0.96 · 36–40 °C 0.82 / 0.88 / 0.91 · 41–45 °C 0.71 / 0.82 / 0.87 · 46–50 °C 0.58 / 0.75 / 0.82, reading 60 °C / 75 °C / 90 °C.

Source: NEC 2017 Table 310.15(B)(16), Table 310.15(B)(2)(a) and Table 310.15(B)(3)(a), from Public.Resource.Org's publication of incorporated standards. Values are stated here as facts for working problems; no NEC table is reproduced.

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Worked examples

Scenario: Six current-carrying 10 AWG copper THHN conductors run through an area at 38 degrees Celsius. Find the corrected and adjusted ampacity of each.

  1. THHN is 90 degree Celsius insulation, so start in that column: 10 AWG is 40 amperes.
  2. Ambient 38 °C falls in the 36–40 band, giving 0.91 for a 90 °C conductor.
  3. Six current-carrying conductors fall in the 4–6 band, giving 0.80.
  4. Multiply in one step: 40 × 0.91 × 0.80 = 29.12 amperes.
  5. Finally, check terminations: if this is a circuit of 100 amperes or less, the 60 °C column also caps it at 30 amperes — so 29.12 A stands.
  6. And remember the small-conductor rule still limits a 10 AWG copper conductor to a 30-ampere device.

Answer: 29.12 amperes

Scenario: A 100-ampere feeder is to be run in 1 AWG copper THHN, landed on equipment with no special temperature marking. Which ampacity applies?

  1. The circuit is rated 100 amperes, which is "100 amperes or less", and 1 AWG is within the 14 AWG through 1 AWG range.
  2. So the termination provision defaults to the 60 degree Celsius column.
  3. 1 AWG copper at 60 °C is 110 amperes, which exceeds the 100-ampere requirement.
  4. The 90 °C value of 145 amperes may be used for correction and adjustment arithmetic, but the final usable ampacity is capped at 110 amperes.

Answer: 110 amperes, from the 60 °C column

Common mistakes

  • Using the 90 degree Celsius column for the final answer. Unless the equipment is listed and marked for 90 degree Celsius conductors, the answer is capped at 60 or 75 degrees Celsius depending on the circuit rating. The higher column is for the arithmetic, not the result.
  • Applying correction and adjustment one at a time with rounding between. Multiply the table value by both factors in a single step. Rounding in the middle introduces error that can change which conductor size the question wants.
  • Leaving spares out of the conductor count. The count includes every conductor in the raceway, spares included. Equipment grounding conductors, being non-current-carrying, are excluded.
  • Forgetting the ambient correction entirely. The tables assume 30 degrees Celsius. An attic, a rooftop raceway or a boiler room in summer is well above that, and skipping the correction is one of the most common real-world sizing errors.
  • Treating ampacity as the circuit rating for small conductors. 12 AWG copper is good for 30 amperes at 90 degrees Celsius but the small-conductor rule caps the device at 20 amperes. The device rating, not the ampacity, is the circuit rating.
  • Assuming copper values apply to aluminium. Aluminium has its own columns, roughly two sizes lower. Read whether the question specifies a material, and remember Texas tells candidates to assume copper unless told otherwise.

Related practice topics

Topics that come up alongside this one on the exam.

Questions about this topic

What is the difference between a conductor ampacity and the circuit rating?

They are separate tests that both have to pass. A 12 AWG copper conductor with 90 degree Celsius insulation has a table ampacity of 30 amperes, but the small-conductor rule caps its overcurrent device at 20 amperes — so the circuit is a 20-ampere circuit. Conversely, a long run may need a larger conductor for voltage drop than either test demands.

Why can I not just use the 90 degree Celsius column?

Because the terminations usually cannot tolerate it. Unless equipment is listed and marked otherwise, circuits rated 100 amperes or less use the 60 degree Celsius ampacity and circuits over 100 amperes use the 75 degree Celsius ampacity. You may use a 90 degree Celsius conductor and work the correction and adjustment arithmetic in that column, but the final ampacity is capped at what the terminal is rated for.

In what order do I apply correction and adjustment factors?

Multiply them together against the table value in one step: table ampacity times the ambient correction factor times the conductor-count adjustment factor. Do not apply one, round, then apply the other — that introduces error. A 10 AWG THHN at 38 degrees Celsius with six current-carrying conductors is 40 × 0.91 × 0.80 = 29.12 amperes.

Do spare conductors count toward the adjustment factor?

Yes. The footnote to the adjustment table states that the count is the total number of conductors in the raceway or cable, including spares. The reasoning is practical: a spare exists so it can be energized later, and nobody recalculates the raceway when it is. Equipment grounding conductors are not current-carrying and are not counted.

What ambient temperature are the ampacity tables based on?

30 degrees Celsius, or 86 degrees Fahrenheit. Anything hotter requires a correction factor, and that is easy to overlook in attics, boiler rooms and rooftop raceways, where summer temperatures run far above the table basis. A 75 degree Celsius conductor at 43 degrees Celsius ambient takes a 0.82 factor.

How much larger does aluminium need to be?

Roughly two AWG sizes for the same ampacity, because aluminium conducts about 61 percent as well as copper. 2/0 aluminium is 135 amperes at 75 degrees Celsius against 175 amperes for 2/0 copper. Note that Texas instructs candidates to assume copper unless a question states otherwise, so a question naming aluminium is testing the second column deliberately.

Which ampacity table rows are worth memorising?

The copper rows from 14 AWG to 4/0, in all three temperature columns. They come up constantly, and looking each one up costs time you need elsewhere. Particularly worth knowing: 12 AWG at 20/25/30, 10 AWG at 30/35/40, 8 AWG at 40/50/55, 6 AWG at 55/65/75, and 4/0 at 195/230/260.

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