Voltage drop (NEC 2026 informational notes and Chapter 9, Table 8)

In 50 minutes, your students learn to work voltage drop by hand with Chapter 9, Table 8, and they learn the point that trips up most apprentices: the 3% and 5% figures are advice in informational notes, not a requirement. They also meet the few places where drop is a rule.

At a glance

Level

Second-year apprentices

Time

50 minutes, with a 90-minute variant

Sections

210.19, 215.4(A)(2), 90.5(C), 647.5(D), 695.8(A), 695.7(B)(2), Chapter 9, Table 8, 240.4(D), Table 310.16, 250.122(D)

Students need

Their own 2026 code book, open to Chapter 9, and a calculator

Edition

2026 NEC

Last checked

October 2026

Class plan by minutes

1. 0 to 5 minWarm-upAsk: a lamp at the end of a long extension cord looks dim. Why? List their answers on the board and leave them up.
2. 5 to 20 minVisualPlay the Why voltage drops visual. Then put the formula on the board and work one example. Have students work 10 AWG on the same run before you show the answer.
3. 20 to 35 minSimulationPairs play Voltage Drop jobs 1 to 3 (Lamp post circuit, Shed circuit, Kiln circuit). Each pair predicts whether the run will pass before they turn the power on.
4. 35 to 45 minQuestionsAsk the three quiz questions below, one at a time. Students write an answer alone, then you give the answer and its section. They can use the code book and a calculator.
5. 45 to 50 minWrap-upCheck the warm-up list against the three ways to cut the drop. Ask which of the three answers came from a rule and which from advice.

For a 90-minute block, give the simulation 30 minutes so pairs finish jobs 1 to 5 including the feeder and branch split, and use the voltage drop calculator to check one pair's answer on the projector.

What students learn

By the end of the period, each student can find a wire's resistance in Chapter 9, Table 8, work volts lost and percent drop for a run, say which limits are advice and which are rules, and name three ways to cut the drop.

210.19

The informational note advises about 3% drop on a branch circuit and 5% in total with the feeder. It is advice.

215.4(A)(2)

The informational note gives the same 3% and 5% advice for feeders.

90.5(C)

Informational notes explain the code. An inspector cannot fail a job on them.

647.5(D)

Sensitive electronic equipment systems have real drop limits. For fixed equipment: 1.5% on a branch circuit and 2.5% for feeder and branch together. For receptacle circuits: 1% and 2.0%.

695.8(A)

A fire pump controller has a limit on how far the voltage may sag during motor starting.

695.7(B)(2)

Fire pump motor conductors send you to 695.8 for the drop check.

Chapter 9, Table 8

Ohms per 1,000 feet for each wire size and metal. Read the stranded row for building wire.

240.4(D)

Small copper wire has breaker limits: 15 A for 14 AWG, 20 A for 12 AWG, 30 A for 10 AWG.

Table 310.16

After you upsize, read the ampacity of the new size from the table.

250.122(D)

Make the hots bigger for voltage drop, and a wire-type ground grows by the same ratio.

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

2023 to 2026: our list has 3 changes in Article 215. See the changes

Quiz questions

1. 1. Does the NEC require you to keep branch-circuit voltage drop under 3 percent?

   Answer. No. The 3 percent figure is in an informational note, and those notes explain the code without setting rules. (90.5(C))

2. 2. A sensitive electronic equipment system has a branch circuit that feeds fixed equipment, wired with Chapter 3 wiring methods. What is the most drop the code allows on it?

   Answer. 1.5 percent. This one is a requirement. (647.5(D))

3. 3. You upsize the hot conductors to cut voltage drop. What happens to a wire-type equipment grounding conductor?

   Answer. It grows by the same ratio of circular mils as the hots. (250.122(D))

Teaching notes

Before class

Students need their own 2026 code book open to Chapter 9, Table 8, and a calculator. Open this lesson and the voltage drop calculator on your teaching computer before class, and project the lesson. Step through the visual and the Voltage Drop simulation 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 choose each wire. Tell students to use the stranded rows of Table 8 and to read ohms per 1,000 feet.

How to use the simulation in class

• Run it after the board example, so students can read Table 8 and know the formula.
• Pairs take jobs 1 to 3 (Lamp post circuit, Shed circuit, Kiln circuit). Each pair says whether the run will meet the spec, and why, before they turn the power on.
• Early finishers play jobs 4 to 6 (Garage feeder, Pole barn feeder and branch, Cabin feeder and branch) to compare copper and aluminum and to split the drop between a feeder and a branch.
• After a job, have one pair enter the same run in the voltage drop calculator and compare the numbers. When they differ, find out why. The calculator uses its own constant for the metal, so small differences from the Table 8 method are normal.

Two discussion questions

1. 1. An inspector fails your job because the far outlet drops 4%. What do you say?

   Look for: The 3% and 5% figures sit in informational notes (210.19, 215.4(A)(2)), and those notes do not set rules (90.5(C)). Students should also say the owner, the engineer, or a local rule can still set a limit, so they check the spec.

2. 2. Name a circuit where the drop limit is a rule, and what the limit is.

   Look for: Sensitive electronic equipment systems: for fixed equipment, 1.5% on a branch circuit and 2.5% for feeder and branch together, and for receptacle circuits, 1% and 2.0% (647.5(D)). Fire pump controllers have a motor-starting limit (695.8(A)). Look for students who can tell these apart from the advice.

Mistakes to watch for

• Treating the 3% and 5% advice as a code requirement.
• Forgetting to count the length twice, out and back.
• Using feet in the formula but reading ohms per kilometer from Table 8.
• Averaging the feeder and branch percents instead of adding them.
• Upsizing the hots and forgetting the wire-type ground.
• Reading a solid-wire row of Table 8 for stranded building wire.

If a student’s book is a different edition

This lesson follows the 2026 NEC. The feeder rules in Article 215 were renumbered for 2026, so the feeder note has a different number in older books. Article 647 sections also sit one number lower in a 2023 book, so this voltage drop rule is in 647.4 there. If a student has an older book, tell them to find the 3% and 5% notes by topic, and to check which edition your authority having jurisdiction enforces.

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.