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Conductors and overcurrentJourneyworkerMasterNEC 2026

Conductor sizing from the load (NEC 310.16 and 110.14(C))

Short answer

To size a conductor, find the smallest size in Table 310.16 whose ampacity covers the load, reading the column your terminations allow under 110.14(C): 60 °C for circuits of 100 A or less unless the equipment is marked 75 °C, and 75 °C above 100 A. Then check 240.4(D) for 14, 12 and 10 AWG.

Updated Original problems, new numbers every time

Live practice

Practice: conductor sizing

Each problem uses new numbers, so you practice the method rather than memorizing an answer.

Formula

Required ampacity (A) = Load (A), from the termination temperature column
Column: 60 °C if the circuit is 100 A or less and terminals are not marked 75 °C; 75 °C if over 100 A
SymbolMeaningUnit
ILoad current the conductor must carryA
AmpacityAllowable ampacity from Table 310.16 in the termination columnA
OCPDOvercurrent device rating, a standard size from 240.6(A)A

Worked example

A branch circuit supplies a noncontinuous load of 59 A. The conductors are copper THHN/THWN-2, which has 90 °C insulation. The terminations have no temperature marking. Using Table 310.16 and 110.14(C)(1), what is the minimum conductor size?

  1. The circuit is 100 A or less, so 110.14(C)(1)(a) applies: use the 60 °C column unless the terminations are marked 75 °C. Here they are not marked, so use the 60 °C column. The 90 °C insulation does not change that.
  2. Look up Table 310.16, 60 °C copper column: the smallest size rated at least 59 A is 4 AWG, at 70 A.
  3. This size is larger than 10 AWG, so the 240.4(D) small conductor limits do not apply.
  4. Minimum conductor size: 4 AWG.

Answer: 4 AWG

Conductor sizing, explained

Table 310.16 lists three ampacities for each size: 60 °C, 75 °C and 90 °C. The insulation on the wire sets the highest column the wire itself can use, but the weakest part of the circuit is usually the terminal on the breaker or the equipment. 110.14(C) says the conductor ampacity you rely on may not be higher than the terminal temperature rating allows.

For circuits rated 100 A or less, or for conductors 14 through 1 AWG, use the 60 °C column unless the equipment is listed and marked for 75 °C. For circuits over 100 A, use the 75 °C column. A 90 °C wire such as THHN still sizes from the 60 °C or 75 °C column; its 90 °C rating is the starting point for derating, covered in the ampacity correction topic.

Small conductors have one more check. 240.4(D) caps the breaker on 14, 12 and 10 AWG conductors no matter what the table says. If the breaker the load needs is larger than that cap, go up a size. With 60 °C terminals the cap and the 60 °C ampacity match, so it only changes the answer when you use the 75 °C or 90 °C column for a small conductor.

Read the question for the conductor material and the termination rating. If it gives neither, the usual reading is copper with 75 °C terminations, but always use what the question states.

How to solve it on the exam

  1. Find the load in amperes. If part of it is continuous, multiply that part by 125% first (see the continuous loads topic).
  2. Decide the termination column: 100 A or less uses 60 °C unless the terminals are marked 75 °C; over 100 A uses 75 °C.
  3. Pick copper or aluminum and go to that column in Table 310.16. Read down until you reach the first ampacity equal to or larger than the load.
  4. For 14, 12 or 10 AWG, check that the breaker the load needs is not larger than the 240.4(D) limit for that size. If it is, move up a size.
  5. Answer with the size, and double check you did not read the 90 °C column just because the wire is THHN.

Common mistakes

  • Sizing from the 90 °C column because the wire is THHN or XHHW-2.
  • Using the 75 °C column on a 100 A or smaller circuit when the terminals are not marked 75 °C.
  • Reading the copper column for an aluminum question, or the reverse.
  • Forgetting 240.4(D) and putting 12 AWG copper on a 25 A breaker because its 75 °C ampacity is 25 A.
  • Adding 125% to a load that is not continuous.

Exam tips

  • Table 310.16 is the page you will open most. In an untabbed book, find Article 310 from the table of contents and remember how far into the book it sits. Lay a straight edge under the row you read.
  • Circle the words copper, aluminum, marked and continuous in the question before you open the book.
Calculator

Conductor sizing calculator

Result–

    Questions people ask

    Which column of Table 310.16 do I use to size a conductor?

    Use the column that matches the lowest temperature rating in the circuit, which is usually the terminals. Under 110.14(C), circuits of 100 A or less use 60 °C unless the equipment is marked 75 °C, and circuits over 100 A use 75 °C.

    Can I use the 90 °C column for THHN wire?

    Only as the starting point for derating. The final ampacity you rely on still cannot be more than the 60 °C or 75 °C termination column allows.

    Why can't 12 AWG copper go on a 25 A breaker if it is rated 25 A at 75 °C?

    Because 240.4(D) caps the overcurrent device on small conductors. For 12 AWG copper the cap is 20 A, so a load that needs a 25 A breaker needs 10 AWG copper.

    Should I assume copper or aluminum?

    Use what the question states. Most exam questions name the conductor material; when one does not, copper is the common assumption, but read the question twice before you pick a column.

    Sources

    About the tables: DLI gives you an untabbed 2026 NEC at the exam. These problems cite the section and table so you practice finding them in the book. We never print NEC text or tables.