Wire sizing (NEC 2026 310.16, 240.4, 110.14(C), 210.19(A))

1 of 6

In 50 minutes, your students learn to size a branch-circuit wire and its breaker: read the ampacity table, respect the termination limit, add 125 percent for a continuous load, and apply the small-wire breaker cap.

At a glance

Level: First-year apprentices
Time: 50 minutes, with a 90-minute variant
Sections: Table 310.16, 310.16, 310.15(A), 310.15(C)(1), 110.14(C), 210.19(A), 240.4, 240.4(B), 240.4(D)
Students need: Their own 2026 code book and a calculator
Edition: 2026 NEC
Last checked: October 2026

Class plan by minutes

0 to 5 minWarm-upWrite a 16 A continuous load on the board and ask students to name a wire and a breaker. Collect two or three answers and leave them up.
5 to 20 minVisualsPlay Breaker and wire. At Predict, have students commit to an answer before you pick one. Have students size a new load on paper, then set it in Size it to check. Finish with Crowded conduit.
20 to 35 minSimulationPairs play Wire Size jobs 1, 2, 3, and 5. Each pair says the load, the column, and the cap out loud before they pick a spool.
35 to 45 minQuizRun the Quiz scene. Students write an answer alone before you choose Show the answer. They can use the code book and a calculator.
45 to 50 minWrap-upIn Size it, set 16 A continuous and 0 A other, and check the warm-up answers against it. Go over the question most students missed.

For a 90-minute block, give the simulation 30 minutes so pairs finish all six jobs, including Rooftop pipe and Shop subpanel, and add 10 minutes to go back over the three questions with the code book open.

What students learn

By the end of the period, each student can size the wire and the largest allowed breaker for a branch circuit with a continuous load, using the ampacity table, the termination limit, and the small-wire cap.

Table 310.16: Gives each wire size an ampacity in the 60°C, 75°C, and 90°C columns.
310.16: The table holds at a 30°C ambient with no more than three current-carrying conductors.
310.15(A): Correction and adjustment can start from a higher column, but the result cannot go above the termination rating.
310.15(C)(1): Cut the ampacity when more than three current-carrying conductors run together, using the table of factors.
110.14(C): Use the column for the lowest-rated part: the wire or the terminal it lands on.
210.19(A): The wire's ampacity must be at least the larger of two values: the continuous load counted at 125 percent plus the rest of the load, or the full load after correction and adjustment.
240.4: A conductor gets overcurrent protection matched to its ampacity, unless a subsection says otherwise.
240.4(B): You may round up to the next standard breaker size when a list of conditions is met.
240.4(D): Small copper wire has breaker caps: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG.

Section numbers only. Read the full text in your code book.

2023 to 2026: our list has 4 changes in Article 310. See the changes

Quiz questions

1. A branch circuit has 16 amperes of continuous load and 4 amperes of other load. What is the minimum conductor ampacity?
Answer. 24 amperes, which is the 4 amperes of other load plus the 16 ampere continuous load counted at 125 percent. (210.19(A))
2. Table 310.16 shows 20 amperes for 14 AWG copper in the 75 degree column. What is the most overcurrent protection the small-conductor rule allows for 14 AWG copper?
Answer. 15 amperes. The small-conductor cap applies even though the table value is higher. (240.4(D))
3. Six current-carrying 12 AWG copper conductors share one conduit. What adjustment factor applies, and what is the adjusted ampacity from the 90 degree column value of 30 amperes?
Answer. The factor is 80 percent, so 30 amperes becomes 24 amperes. (Table 310.15(C)(1))

Teaching notes

Before class

Students need their own 2026 code book with tabs at Table 310.16, 110.14(C), 210.19(A), and 240.4, and a calculator. Open this lesson on your teaching computer before class and project it. Step through the visuals, the Wire Size simulation, and the quiz with the arrow keys. Students can play the simulation on a phone or a laptop. If they have neither, you can project the simulation and have the class pick each wire and breaker.

How to use the simulation in class

Run it after Crowded conduit, so students already know the order: load, wire, termination, cap.
Pairs take jobs 1, 2, 3, and 5 (Toaster circuit, EV charger, Water heater, Shared pipe). Each pair says the load, the column, and the breaker cap out loud before they pick a spool.
Early finishers play jobs 4 and 6 (Rooftop pipe and Shop subpanel). Rooftop pipe adds a heat correction that this lesson does not work the math for, so have them find it in their code book.
When the breaker trips or the inspector tags a pick, ask the pair which limit they missed: the table, the terminal, the continuous load, or the cap.

Two discussion questions

1. A customer says the table shows 20 amps for 14 AWG, so a 20 amp breaker is fine. What do you tell them?
Look for: The table gives ampacity, but 240.4(D) caps the breaker for 14 AWG copper at 15 amps. Students should name both sections and say which one wins.
2. You pull four 12 AWG THHN conductors in one conduit to a circuit that ends on 60 degree terminals. Walk through the ampacity you can use.
Look for: Start from the 90 degree column, 30 A, and apply the 80 percent factor from Table 310.15(C)(1) to get 24 A. The result cannot go above the 60 degree termination value of 20 A (310.15(A), 110.14(C)). Students should state the 20 A result and its source.

Mistakes to watch for

Reading the 90 degree column as the final answer when the terminals are rated lower.
Skipping the 125 percent on a continuous load.
Setting a 20 A breaker on 14 AWG copper because the table value looks high enough.
Counting conductors in the raceway wrong, or forgetting to derate past three current-carrying conductors.
Using the table when the 30 degree ambient and three-conductor conditions are not met.

If a student’s book is a different edition

This lesson follows the 2026 NEC. Table 310.16 and the wiring rules in this lesson can differ in an older edition, so tell students with an older book to compare the table and sections in their own book and to check what their state has adopted. Hold the final answer to the code your authority having jurisdiction enforces.

Questions people ask

What size wire do you need for a continuous load?

Count the continuous load at 125 percent, add all the other load, and size the wire for that (210.19(A)). A continuous load is one that runs at its maximum current for 3 hours or more (Article 100). Then check the termination limit (110.14(C)) and the breaker cap (240.4).

Why can't 14 AWG wire go on a 20 amp breaker?

Table 310.16 shows 14 AWG copper above 15 amps in the 75 degree and 90 degree columns, but 240.4(D) caps its breaker at 15 amps. The same section caps 12 AWG copper at 20 amps and 10 AWG copper at 30 amps.

Which column of Table 310.16 should I use?

Use the column that matches the lowest-rated part in the circuit, usually the terminal (110.14(C)). For circuits of 100 amps or less, that is usually the 60 degree column unless the equipment's listing allows more.

When do you derate for more than three wires in a raceway?

When more than three current-carrying conductors run together, multiply by the factor in Table 310.15(C)(1): 80 percent for 4 to 6, 70 percent for 7 to 9, and 50 percent for 10 to 20 (310.15(C)(1)).

Ask Shorty is independent and is not affiliated with NFPA. Check every answer against your adopted code and your authority having jurisdiction. This lesson follows the 2026 NEC and was last checked October 2026. See something wrong? Tell us and we will check it against the code.

The visuals on their own

Open any of these full size, outside the lesson.