Short answer: Voltage drop is the current times the resistance of the wire. For a single-phase circuit, VD = 2 × L × R × I ÷ 1,000, where L is the one-way length in feet and R is the conductor resistance in ohms per 1,000 feet from Chapter 9, Table 8. For three-phase, replace the 2 with 1.732. The NEC’s informational notes recommend about 3% on a branch circuit and 5% total for feeder plus branch circuit. Those notes are recommendations, not requirements, except where a specific article sets a limit.
PSI does not list voltage drop by name. Its Calculations outline includes Feeders (3 scored questions) and Branch Circuit Calculations and Conductors (4 scored questions), both areas where conductor calculations come up. These counts cover the full subject areas, not voltage drop alone. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Why voltage drop matters on the job
Every conductor has resistance. When current flows, some voltage is used up in the wire before it reaches the load. On a short run the loss is small. On a long run to a detached building, a well pump or a parking lot light, it can be large enough that motors run hot, lights dim and electronics misbehave.
Ampacity tells you how much current a conductor can carry without overheating. Voltage drop tells you how much voltage is left at the far end. A conductor can pass the ampacity check and still be too small for a long run.
What the NEC says, and what it does not
- Branch circuits. An informational note recommends sizing branch circuit conductors so the voltage drop at the farthest outlet is no more than about 3% (NEC 210.19).
- Feeders plus branch circuits. Informational notes also recommend no more than about 5% total from the start of the feeder to the farthest outlet, and the feeder rules carry a similar note.
- Informational notes are not requirements. They explain and recommend. An inspector cannot fail a general branch circuit for voltage drop based on a note alone, but the job’s design documents, the equipment instructions or local rules can still require it.
- A few articles do set limits. Some special systems, such as fire pumps, have mandatory voltage limits in their own articles. Check the article for the equipment you are wiring.
The formula and where R comes from
For a single-phase two-wire circuit, or the two ungrounded conductors of a 240 V load:
VD = 2 × L × R × I ÷ 1,000
- 2 counts the trip out and back.
- L is the one-way length in feet.
- R is the resistance of one conductor in ohms per 1,000 feet, from Chapter 9, Table 8. Table 8 lists DC resistance at 75°C for copper and aluminum, coated and uncoated, solid and stranded.
- I is the load current in amperes.
For a balanced three-phase circuit, the same resistance-only estimate uses 1.732 instead of 2. It does not account for reactance or power factor. For larger conductors on AC, Chapter 9, Table 9 gives AC resistance and reactance. Table 8 gives a resistance-only estimate that works well for small conductors on most branch circuits. On an exam, use the method and table the question names.
Many books use a second form, VD = 2 × K × I × L ÷ CM, where K is about 12.9 for copper and CM is the circular mil area from Table 8. It gives nearly the same answer. Use whichever form the question gives you.
Uncoated stranded copper values used below
| Size | Circular mils | Ohms per 1,000 ft (Table 8, stranded, uncoated copper) |
|---|---|---|
| 12 AWG | 6,530 | 1.98 |
| 10 AWG | 10,380 | 1.24 |
| 8 AWG | 16,510 | 0.778 |
| 6 AWG | 26,240 | 0.491 |
Confirm these values in your own 2026 NEC before you rely on them.
Worked example: 16 A at 120 V, 150 feet away
A 120 V single-phase load draws 16 A. The outlet is 150 feet from the panel, one way. The circuit is copper. Find the smallest size that keeps the drop at or under 3%.
- Target. 3% of 120 V = 3.6 V.
- 12 AWG. 2 × 150 × 1.98 × 16 ÷ 1,000 = 9.50 V, which is 7.9%. Too much.
- 10 AWG. 2 × 150 × 1.24 × 16 ÷ 1,000 = 5.95 V, which is 5.0%. Still too much.
- 8 AWG. 2 × 150 × 0.778 × 16 ÷ 1,000 = 3.73 V, which is 3.1%. Just over.
- 6 AWG. 2 × 150 × 0.491 × 16 ÷ 1,000 = 2.36 V, which is 2.0%. Meets the 3% target.
- Check. 2.36 V is less than 3.6 V. The voltage at the outlet is about 120 − 2.36 = 117.6 V.
12 AWG passes the ampacity check for 16 A in this example, but it would leave about 110.5 V at the outlet. That is the difference between ampacity and voltage drop. Also check that the larger conductor fits the terminals and the device, and that the equipment grounding conductor is increased in proportion where the code requires it.
To try other sizes quickly, use our voltage drop calculator.
Check yourself
- In VD = 2 × L × R × I ÷ 1,000, is L the one-way or round-trip length? One-way. The 2 accounts for the return path.
- What multiplier replaces the 2 for a balanced three-phase circuit? 1.732.
- Is the 3% branch circuit figure a code requirement for a general branch circuit? No. It is an informational note recommendation.
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Practice calculations with code lookup paths
Texas Journeyman Electrician Exam Prep 2026 NEC (paperback, 327 pages). 486 practice questions: 316 chapter questions across 20 chapters plus two full mock exams of 85 questions each, 59 NEC Knowledge in 130 minutes and 26 Calculations in 110 minutes. Includes calculation lessons and code lookup paths, and NEC questions come with article, section or table references.
FAQ
What is the maximum voltage drop allowed by the NEC?
For general branch circuits and feeders, the NEC does not set a maximum. Informational notes recommend about 3% on a branch circuit and 5% total for feeder plus branch circuit. Some specific articles, such as the one for fire pumps, do set limits.
Where do I find wire resistance for voltage drop?
Chapter 9, Table 8 lists the DC resistance at 75°C in ohms per 1,000 feet for copper and aluminum conductors. Chapter 9, Table 9 gives AC resistance and reactance for larger conductors.
How do I calculate voltage drop for three-phase?
A resistance-only estimate is VD = 1.732 × L × R × I ÷ 1,000, with the one-way length, the resistance per 1,000 feet from Table 8 and the line current. It leaves out reactance and power factor.
Is voltage drop on the Texas Journeyman exam?
PSI does not list voltage drop by name. The Calculations outline includes Feeders (3 scored questions) and Branch Circuit Calculations and Conductors (4 scored questions). These counts cover the full subject areas, not voltage drop alone.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Section 210.19 informational note, Chapter 9 Tables 8 and 9
