Free calculator · 2026 and 2023 NEC

Voltage Drop Calculator

Enter the circuit and get the voltage drop in volts and percent, the voltage at the load, and the smallest wire that stays within your limit, with every step and the NEC table it comes from. Single-phase, three-phase and dc; copper or aluminum; 2026 or 2023 NEC.

11.16 V voltage drop, 4.65% of 240 V

Over the 3% you set. 228.84 V reaches the load.

The smallest copper size that stays at or under 3% is 8 AWG (7 V, 2.92%).

How it was worked out

Multiplier: 2 for a single-phase circuit (the current goes out and back, so the run is counted twice).
Chapter 9, Table 8 gives 10 AWG copper (stranded, uncoated) a dc resistance of 1.24 Ω per 1,000 ft at 75°C.
VD = 2 × R × I × L ÷ 1,000 = 2 × 1.24 × 30 × 150 ÷ 1,000 = 11.16 V.
Percent drop = 11.16 ÷ 240 × 100 = 4.65%. Voltage at the load = 240 − 11.16 = 228.84 V.
The exam's K-factor formula (K = 12.9) gives 11.18 V for the same circuit, close to the Table 8 answer.

The 3% branch-circuit and 5% feeder-plus-branch figures are recommendations in informational notes, not requirements.

Ampacity is a separate check: 10 AWG copper is 35 A at 75°C in Table 310.16. Size the wire for ampacity and derating.

Code sections (2026 NEC): Chapter 9, Table 8; 210.19(A), informational note; 215.4(A), informational note.

Practice this on exam-style questions

Voltage drop and conductor sizing questions are on the calculations portion of the journeyman exam. Work them against the clock in a free practice test for your state.

Your state

Take the free practice test

AmpPrep courses teach voltage drop for each state's journeyman exam, with lessons, review and timed practice exams. Find your state's exam.

What this calculator does not account for

Conductor temperatures other than 75°C (both tables are at 75°C) and power factors other than 0.85 in the Table 9 method.
Coated copper, compact-stranded or solid conductors other than the stranded values used here, and sizes above 750 kcmil.
Parallel conductors, motor starting current, and unbalanced loads on multiwire circuits.
The ampacity of the conductor: a size that passes voltage drop may still be too small for the current. Use the wire size calculator for that.

The installed design must follow the code edition your jurisdiction has adopted, its local amendments, and the authority having jurisdiction. Check every value in your own code book.

How to calculate voltage drop

Voltage drop is the voltage a conductor uses up carrying current. For a single-phase circuit, VD = 2 × K × I × L ÷ CM; for a three-phase circuit, replace the 2 with 1.732. K is 12.9 for copper, I is the load current, L is the one-way length in feet and CM is the conductor area in circular mils from Chapter 9, Table 8. To size a conductor for a voltage drop limit, solve the same formula for CM and round up to the next conductor size.

Voltage drop with the K-factor formula

Long runs lose voltage in the conductors. The K-factor formula estimates that loss from the conductor's circular-mil area.

Single-phase (2 conductors in the loop): VD = 2 × K × I × L ÷ CM
Three-phase: VD = 1.732 × K × I × L ÷ CM

K is the resistance of a 1-circular-mil, 1-foot conductor: use 12.9 for copper unless told otherwise. I is load current, L is the one-way length in feet, and CM is the conductor's area in circular mils.

Where to look
Chapter 9, Table 8 (Conductor Properties), "Area, circular mils" column. For kcmil sizes the number is in the name (250 kcmil = 250,000 cmil).

Procedure

Find the load current (single-phase or three-phase, as you have already learned).
Look up CM in Chapter 9, Table 8.
Plug into the formula. Percent drop = VD ÷ source voltage × 100.

Worked example
120 V, 2-wire, 16 A load, 100 ft one way, 12 AWG copper. Table 8 gives 12 AWG as 6,530 cmil.

VD = 2 × 12.9 × 16 × 100 ÷ 6,530 = 41,280 ÷ 6,530 = 6.32 V.
Percent = 6.32 ÷ 120 × 100 = 5.3%.

Most common mistake
Doubling twice: the 2 in the formula already accounts for the return conductor, so L is the one-way distance. Also, never use the 2 for a three-phase circuit; it uses 1.732.

Sizing a conductor for voltage drop

Voltage drop is a recommendation, not a requirement: the informational note under 210.19 suggests at most 3% on a branch circuit and 5% for feeder plus branch circuit. Exam questions give the percentage to use.

Where to look

210.19, informational note under the title (3% / 5%).
Chapter 9, Table 8: circular mils (and dc resistance per 1,000 ft).
Table 310.16 for the ampacity check.

Procedure

Allowed drop in volts = percentage × source voltage (3% of 240 V = 7.2 V).
Rearrange the formula for CM: single-phase CM = 2 × K × I × L ÷ VD; three-phase CM = 1.732 × K × I × L ÷ VD.
In Table 8, pick the smallest size whose area is at least that many circular mils. Always round UP.
Check ampacity in Table 310.16; the larger size wins.

Worked example
240 V single-phase, 50 A, 200 ft one way, copper, 3% limit, 75°C terminations.

Allowed VD = 0.03 × 240 = 7.2 V.
CM = 2 × 12.9 × 50 × 200 ÷ 7.2 = 258,000 ÷ 7.2 = 35,833 cmil.
Table 8: 6 AWG is 26,240 (too small); 4 AWG is 41,740, so 4 AWG.
4 AWG copper at 75°C easily carries 50 A. Answer: 4 AWG.

Most common mistake
Choosing the size closest to the calculated area when it is smaller. A conductor below the required circular mils will exceed the drop limit.

Sections to look up

All in the 2026 NEC, with the 2023 number where it is different. Open your own book to each one; this page explains the rules in its own words and does not reproduce the code.

Chapter 9, Table 8
210.19
Table 310.16

2023 NEC and 2026 NEC

The rules and values this page teaches are the same in the 2023 and 2026 editions.

Every exam-relevant change from the 2023 to the 2026 NEC

Practice questions

Work each one with your code book open, then check the answer.

A 240 V single-phase load draws 40 A at the end of a 150 ft (one-way) run of 8 AWG copper. Using K = 12.9 and the circular-mil area from Chapter 9, Table 8, what is the voltage drop? Round to 0.01 V.
- A. 4.69 V
- B. 9.38 V
- C. 8.12 V
- D. 18.75 V
Show the answer to question 1
Answer: B. 9.38 V
Chapter 9, Table 8 gives 8 AWG as 16,510 cmil. VD = 2 × 12.9 × 40 × 150 ÷ 16,510 = 154,800 ÷ 16,510 = 9.38 V.
Reference: Chapter 9, Table 8 (circular-mil area); K = 12.9 is a trade constant, not a code value (2026 NEC)
A 240 V single-phase, 40 A load is 250 ft (one-way) from the panel. Using K = 12.9, Chapter 9, Table 8, and a 3% voltage-drop limit, what is the smallest copper conductor that meets the limit? (THWN, 75°C terminations.)
- A. 6 AWG
- B. 4 AWG
- C. 8 AWG
- D. 1 AWG
Show the answer to question 2
Answer: B. 4 AWG
Allowed VD = 0.03 × 240 = 7.2 V. CM = 2 × 12.9 × 40 × 250 ÷ 7.2 = 258,000 ÷ 7.2 = 35,833 cmil. Table 8: 6 AWG = 26,240 (too small); 4 AWG = 41,740 (enough). Ampacity: 4 AWG copper at 75°C is well above 40 A. Answer: 4 AWG.
Reference: 210.19 Informational Note; Chapter 9, Table 8; Table 310.16 (2026 NEC)
A 208 V three-phase load draws 50 A at the end of a 200 ft (one-way) run of 6 AWG copper. Using K = 12.9 and Chapter 9, Table 8, what is the voltage drop? Use 1.732 for √3. Round to 0.01 V.
- A. 9.83 V
- B. 4.92 V
- C. 17.03 V
- D. 8.51 V
Show the answer to question 3
Answer: D. 8.51 V
Table 8 gives 6 AWG as 26,240 cmil. VD = 1.732 × 12.9 × 50 × 200 ÷ 26,240 = 223,428 ÷ 26,240 = 8.51 V.
Reference: Chapter 9, Table 8 (circular-mil area); K = 12.9 is a trade constant, not a code value (2026 NEC)
A 480 V three-phase, 80 A load is 400 ft (one-way) from its source. Using K = 12.9, Chapter 9, Table 8, and a 3% voltage-drop limit, what is the smallest copper conductor that meets the limit? (THWN, 75°C terminations.)
- A. 4 AWG
- B. 2 AWG
- C. 1/0 AWG
- D. 3 AWG
Show the answer to question 4
Answer: D. 3 AWG
Allowed VD = 0.03 × 480 = 14.4 V. CM = 1.732 × 12.9 × 80 × 400 ÷ 14.4 = 714,970 ÷ 14.4 = 49,651 cmil. Table 8: 4 AWG = 41,740 (too small); 3 AWG = 52,620 (enough). Ampacity: Table 310.16 gives 3 AWG copper at 75°C as 100 A, above 80 A. Answer: 3 AWG.
Reference: 210.19 Informational Note; Chapter 9, Table 8; Table 310.16 (2026 NEC)

Common questions

What is the maximum voltage drop allowed by the NEC?

For ordinary branch circuits and feeders the NEC sets no mandatory limit. Informational notes recommend no more than 3 percent on a branch circuit and 5 percent for the feeder and branch circuit together, and engineers and inspectors commonly design to those figures. A few articles for special equipment set their own limits, so check the article for the equipment you are wiring.

What is the voltage drop formula?

For a single-phase or dc circuit, VD = 2 × R × I × L ÷ 1,000, where R is the conductor's resistance in ohms per 1,000 feet from Chapter 9, Table 8, I is the current and L is the one-way length in feet. For three-phase, use 1.732 in place of 2. The K-factor version, VD = 2 × K × I × L ÷ CM with K = 12.9 for copper, gives nearly the same answer and is the one exam questions usually use.

Do I use the one-way or the round-trip length?

The one-way length, from the source to the load. The 2 in the single-phase formula already counts the conductor going out and the one coming back. Doubling the length as well is the most common mistake.

How much voltage drop is 3 percent?

3 percent is 3.6 V on a 120 V circuit, 7.2 V on 240 V, 6.24 V on 208 V and 14.4 V on 480 V. Multiply the system voltage by 0.03.

How do I size a wire for voltage drop?

Find the allowed drop in volts (for example 3 percent of the voltage), solve the formula for the circular mils needed, CM = 2 × K × I × L ÷ VD, and pick the first size in Chapter 9, Table 8 at or above that area. Then check that the size also has enough ampacity. The calculator does this for you and shows the smallest size that stays within the limit you set.

What is the voltage drop on a 100-foot 12 AWG circuit at 16 amps?

On 120 V single-phase with copper, Chapter 9, Table 8 gives 6.34 V, which is 5.28 percent: over the 3 percent recommendation. 10 AWG brings it under.

Next step

Test yourself across the whole exam, or see how the course teaches a skill from the start. The course teaches this topic with the rest of the exam: a starting assessment finds what you need, each skill starts with a lesson, and review and timed practice exams check that it stays with you.

Take the free practice test Try a free lesson Texas exam guide

Voltage Drop Calculator