Voltage drop calculator for single- and three-phase cable
Enter the load, the one-way length, and the wire size. You get the volts lost, the percent drop, and the formula with your numbers in it.
Built on the 2026 NEC · Free, no sign-up · Updated October 2026
Result
4.76 V (3.97%)
How it was calculated
- single-phase VD = 2 x K x I x D / circular mils
- VD = 2.000 x 12.95 x 16 x 75 / 6530
Assumptions
- copper conductor
- single-phase circuit
- 75 ft one-way length
- Voltage drop uses calculator K and circular-mil constants; the NEC citation is informational guidance, not the source of the numeric constants.
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
How to use it
- Pick single-phase for 120 V and 240 V circuits in a house. Pick three-phase for 208 V, 480 V, and other three-phase circuits.
- Enter the one-way length: the distance from the panel or source to the load, not the round trip. The formula doubles it for you on single-phase.
- Enter the steady-state current the load draws, not the breaker size.
- Compare the percent to your target. A common design target is 3% on a branch circuit and 5% total from the service to the farthest outlet.
The formula
Single-phase: VD = 2 × K × I × D ÷ CM. Three-phase: VD = 1.732 × K × I × D ÷ CM.
K is the resistance of a circular-mil-foot of conductor: 12.95 for copper and 21.2 for aluminum in this calculator. I is the current in amps, D is the one-way length in feet, and CM is the conductor's area in circular mils from Chapter 9, Table 8. The 2 on single-phase covers the trip out and back. Three-phase uses the square root of 3 instead.
This is the quick method most electricians and exam books use. It ignores reactance, which matters more on large conductors and long feeders. For those, an engineer will use the Chapter 9, Table 9 impedance method.
What the NEC says about voltage drop
For most circuits, voltage drop is a recommendation, not a requirement. The informational note to 210.19 (branch circuits) and Informational Note No. 2 in 215.4(A) (feeders) say that sizing conductors so drop stays under 3% on the branch circuit or feeder, and under 5% overall, gives reasonable efficiency. Informational notes are not enforceable rules.
A few places do make voltage drop mandatory, such as some fire pump and sensitive electronic equipment circuits. Your local energy code or your specs can also set a limit. Check what applies to your job.
Common mistakes
- Entering the round-trip length. Enter the one-way distance. The formula already doubles it.
- Using the breaker rating as the load. Use what the load actually draws.
- Upsizing the hots for voltage drop and forgetting the ground. When you increase the ungrounded conductors for voltage drop, 250.122(D) requires you to increase a wire-type EGC in proportion.
2023 to 2026: our list has 3 changes in Article 215. See the changes
Worked example
A 16 A load at the end of a 75 ft run of 12 AWG copper on a 120 V circuit:
- Phase: Single-phase
- Conductor material: Copper
- Conductor size (AWG): 12
- Circuit voltage (volts): 120
- Load (amps): 16
- One-way length (ft): 75
Result
4.76 V (3.97%)
How it was calculated
- single-phase VD = 2 x K x I x D / circular mils
- VD = 2.000 x 12.95 x 16 x 75 / 6530
Assumptions
- copper conductor
- single-phase circuit
- 75 ft one-way length
- Voltage drop uses calculator K and circular-mil constants; the NEC citation is informational guidance, not the source of the numeric constants.
Sections used
These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.
Questions electricians ask
What does a voltage drop calculator under NEC 2026 figure out?
It sets the volts lost and the percent drop on a run from the load current, the one-way length, the wire size, and copper or aluminum. You compare that percent with the 3% and 5% targets in the informational notes to 210.19 and 215.4(A)(2). The NEC 2026 does not require those targets for most circuits.
Is there a voltage drop chart in the NEC?
No. The NEC 2026 has no chart that gives voltage drop for a wire size and length. Chapter 9, Table 8 lists the area of each conductor in circular mils. Table 9 lists ac resistance, reactance, and effective impedance per 1000 feet, and Note 2 says current times effective impedance approximates the drop. You still work out the drop with a formula, which this calculator does for you.
Where is the voltage drop formula in the NEC?
The NEC gives no formula for sizing a conductor to a voltage drop target. It gives the 3% and 5% targets in informational notes (210.19 and 215.4(A)(2)). Chapter 9, Table 9, Note 2 gives one way to approximate the drop: current times effective impedance. The formula on this page is the common circular-mil method, using Chapter 9, Table 8.
How do I calculate voltage drop on three-phase cable?
Pick three-phase, enter the load current, the one-way cable length, and the conductor size and material. The calculator multiplies by 1.732 instead of 2, then shows the volts lost and the percent of your supply voltage. The cable length is the run from the source to the load.
Is the 3% voltage drop rule required by the NEC?
Not for general circuits. It comes from informational notes in 210.19 and 215.4(A), and informational notes are not enforceable. Some specs, energy codes, and specific articles do set limits, so check the job.
What length do I enter, one-way or round trip?
One-way. Measure from the source to the load. The single-phase formula multiplies by 2 to account for the return path.
Which wire sizes does this calculator handle?
14 through 6 AWG, copper or aluminum. For larger feeders, use the same formula with the circular-mil area from Chapter 9, Table 8, or the impedance method from Table 9.
What voltage drop is too much for a motor or a long run to a shed?
Many electricians design to 3% on the branch circuit so lights don't dim and motors start cleanly. On a long run, try the next wire size up in the calculator and compare.
Practice it hands-on
Stretch a run across the yard and watch the lamp dim, then size the wire to keep the drop in spec.
Teach it in one class
Free slides, quiz, and instructor note: Voltage drop (NEC 2026 informational notes and Chapter 9, Table 8).
How this calculator was built
The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.
How we test the calculators. See something wrong? Tell us and we will check it against the code.
Related calculators
- Ampacity deratingApply ambient temperature and bundling adjustments to a conductor.
- Conduit fillSmallest EMT, IMC, RMC, or PVC that fits your conductors.
- Ground wire (EGC) sizeMinimum equipment grounding conductor from the breaker or fuse size.
Have a code question the calculator doesn’t cover?
Ask Shorty in plain English. You get a straight answer with the NEC 2026 section it came from.