NEC Voltage Drop Calculator
Calculate voltage drop per NEC 210.19(A) and 215.2(A) recommendations. Free, no account required.
Circuit Details
6.60% voltage drop
Exceeds NEC recommended 3% maximum. Consider upsizing the conductor.
NEC Voltage Drop Recommendations
NEC 210.19(A) Informational Note No. 4: Branch circuits should not exceed 3% voltage drop. The total voltage drop for feeder and branch circuit combined should not exceed 5%.
NEC 215.2(A) Informational Note No. 2: Feeders should be sized so that the voltage drop is not more than 3% at the farthest outlet of power, heating, and lighting loads.
Note: These are recommendations (informational notes), not mandatory code requirements. However, they are frequently tested on electrician licensing exams and considered best practice.
Common Voltage Drop Scenarios
| Circuit | Wire | Max Distance (3%) |
|---|---|---|
| 20A, 120V | 12 AWG Cu | 45 ft |
| 20A, 240V | 12 AWG Cu | 90 ft |
| 30A, 240V | 10 AWG Cu | 96 ft |
| 40A, 240V | 8 AWG Cu | 115 ft |
| 50A, 240V | 6 AWG Cu | 146 ft |
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This calculator uses DC resistance values from NEC Chapter 9 Table 8 at 75°C. AC circuits may have slightly different effective impedance due to reactance. Always verify with your local AHJ and the current NEC edition adopted in your jurisdiction.
How the voltage drop calculation works
Voltage drop is the voltage lost to conductor resistance between the source and the load. The NEC does not make 3 percent a hard requirement. It appears in informational notes at 210.19(A) for branch circuits and 215.2(A) for feeders, which recommend no more than 3 percent on either one and no more than 5 percent for feeder and branch circuit combined. Informational notes are not enforceable, but exam writers test them constantly, and most AHJs and specs treat them as the design target.
The single-phase formula is VD = 2 x K x I x L / CM, where K is the resistivity constant, I is the load in amps, L is the one-way length in feet, and CM is the circular mils of the conductor. The 2 accounts for the current traveling out on the ungrounded conductor and back on the grounded conductor. For three-phase, the multiplier is 1.732 instead of 2, because the return path is shared across phases.
This calculator uses the DC resistance values published in NEC Chapter 9 Table 8 at 75 degrees C rather than a generic K constant, which is the more accurate approach and the one the code book itself supports. Table 8 gives ohms per 1,000 feet for each size in uncoated copper and in aluminum, so the drop works out to VD = 2 x R x I x L / 1000 for single-phase.
Step by step
- 1
Identify the load
Use the actual circuit current in amps, not the breaker size. On a continuous load the exam may want 125 percent applied first per 210.19(A)(1).
- 2
Measure the one-way run
Length is source to load, one direction. The formula doubles it for you on single-phase.
- 3
Look up the resistance
NEC Chapter 9 Table 8, ohms per 1,000 ft at 75 C, for the material and size you are checking.
- 4
Apply the formula
Single-phase VD = 2 x R x I x L / 1000. Three-phase VD = 1.732 x R x I x L / 1000.
- 5
Convert to percent
Percent drop = VD / source voltage x 100. Compare against 3 percent branch, 3 percent feeder, 5 percent combined.
- 6
Upsize if it fails
Step up one conductor size and rerun. Ampacity still has to be satisfied independently per Table 310.16.
Worked example
A 120 V, 20 A single-phase branch circuit runs 150 feet one way in 12 AWG copper. Does it meet the 3 percent recommendation?
- Chapter 9 Table 8: 12 AWG uncoated copper is 1.98 ohms per 1,000 ft at 75 C.
- VD = 2 x 1.98 x 20 x 150 / 1000 = 11.88 V
- Percent = 11.88 / 120 x 100 = 9.9 percent
- Voltage at the load = 120 - 11.88 = 108.1 V
Answer: No. At 9.9 percent it is more than triple the 3 percent recommendation. Holding 12 AWG, the maximum run at 20 A and 120 V is about 45 feet. Getting 150 feet under 3 percent takes 6 AWG copper.
How this is tested on the exam
Voltage drop appears on nearly every journeyman and master exam, usually as a two-step problem: compute the drop, then decide whether it passes. The trap is using round-trip length instead of one-way, or forgetting that the 3 and 5 percent figures are recommendations in informational notes rather than mandatory rules. Read the question wording carefully, because some ask which conductor size is the minimum that satisfies the recommendation, which means solving the formula for CM instead of VD.
Frequently asked questions
Is 3 percent voltage drop an NEC requirement?
No. It appears in informational notes, 210.19(A) Informational Note No. 4 for branch circuits and 215.2(A) Informational Note No. 2 for feeders, which are advisory rather than enforceable. That said, exams test it as though it were a rule, and most job specifications and many local amendments make it one.
Do I use one-way or round-trip distance?
Enter the one-way distance. The single-phase formula multiplies by 2 to account for current going out and coming back. If you enter round-trip length you will double your answer.
Why is the three-phase multiplier 1.732 instead of 2?
1.732 is the square root of 3. In a balanced three-phase system the conductors share the return path, so the effective length of the current loop is shorter than the out-and-back path of a single-phase circuit.
Does fixing voltage drop change my ampacity calculation?
They are separate checks and both must pass. Ampacity per Table 310.16 sets the minimum size for heating. Voltage drop may force you larger than that minimum on a long run. Upsizing for voltage drop can also trigger 250.122(B), which requires the equipment grounding conductor to be increased proportionally.
Does this calculator account for AC reactance?
No. It uses DC resistance from Chapter 9 Table 8 at 75 C, which is the method used on licensing exams. Real AC circuits, particularly large conductors in steel raceway, have additional impedance from reactance. Chapter 9 Table 9 covers that case for design work.