How to Calculate Voltage Drop Percentage: The 3% Rule
Voltage drop percentage is the volts lost in the conductor divided by the source voltage, times 100. Find the drop in volts with the National Electrical Code (NEC) K-factor formula, then divide by the supply voltage. The NEC recommends holding a branch circuit to 3% and the combined feeder plus branch to 5%. Both are advisory notes, not mandatory rules, but they are the numbers most specs and inspectors expect.
Skip the hand math with the Voltage Drop Calculator instead. It applies these same NEC 2020 constants and flags 3% and 5% automatically. The steps below show exactly what it computes.
The two-step method
Step one, find the drop in volts. Step two, express it as a percentage of the source. For conductors below #2 AWG the K-factor form is standard:
Single-phase: VD = 2 × K × I × L / CM
Three-phase: VD = √3 × K × I × L / CM
VD% = VD / source voltage × 100
K is 12.9 for copper and 21.2 for aluminum at 75°C. I is the load current in amps, L is the one-way run length in feet, and CM is the circular-mil area from NEC Chapter 9 Table 8. The 2× on single-phase accounts for the round trip; three-phase uses √3 (about 1.732) for the line-to-line relationship, which works out to roughly 86.6% of the single-phase drop for the same conductor.
A useful shortcut for the percentage side: 3% of 120V is 3.6V, 3% of 240V is 7.2V, and 3% of 208V is 6.24V. If your drop in volts is under that number, you are inside the 3% target.
Worked example: a circuit that fails
20-amp load, #12 copper, 100 feet one way. Circular mils for #12 from Table 8 is 6,530.
VD = 2 × 12.9 × 20 × 100 / 6,530
VD = 51,600 / 6,530 = 7.90 V
On 120V: 7.90 / 120 = 6.59% (fails, target 3.6V)
On 240V: 7.90 / 240 = 3.29% (still over 3%)
The same conductor, same drop in volts, reads very differently against each source. On a 120V branch it is more than double the 3% target. This is why the voltage you divide by matters as much as the conductor you pick.
Worked example: a circuit that passes
Keep 20 amps and 100 feet, move to 240V, and step up to #10 copper (10,380 circular mils).
VD = 2 × 12.9 × 20 × 100 / 10,380
VD = 51,600 / 10,380 = 4.97 V
On 240V: 4.97 / 240 = 2.07% (within the 3% target)
One AWG size up and a doubled supply voltage take the same load from failing to comfortably compliant. When a run is long, those are the two levers you reach for first.
Three-phase example
40-amp load, #8 copper, 150 feet, on a 208V three-phase system. Circular mils for #8 is 16,510. Swap the 2× for √3.
VD = 1.732 × 12.9 × 40 × 150 / 16,510
VD = 134,060 / 16,510 = 8.12 V
On 208V: 8.12 / 208 = 3.90% (fails the 3% target)
Step to #6 copper (26,240 circular mils) and the numerator is unchanged: 134,060 / 26,240 = 5.11 V, or 2.46% at 208V, which passes. Note the same pattern as the long-run branch case worked out in wire size for a 200-foot, 30-amp circuit: the ampacity-legal conductor clears heat but not drop, and one size up fixes it.
What the 3% and 5% rules actually say
The 3% branch and 5% combined figures live in Informational Notes to NEC 210.19(A)(1) and 215.2(A)(1). Informational Notes are explanatory and are not enforceable text under NEC 90.5(C), so the base code does not require a voltage drop calculation. In practice the numbers still govern most work: engineering specifications call them out, some jurisdictions adopt them as local amendments, and equipment simply performs better with the conductor holding at least 95% of nominal at the load. The 5% figure is a combined budget, so if a feeder already spends 2%, the branch it serves has 3% left before the total reaches 5%.
A note on the K-factor and larger conductors
The K-factor tracks DC resistance and stays accurate through the small and mid AWG sizes. On #2 AWG and larger, AC skin and proximity effects pull the true resistance away from the DC value, so the more precise method switches to the AC resistance published in NEC Chapter 9 Table 9. The site calculator makes that switch automatically at #2 AWG, which is why its large-conductor results differ slightly from a pure K-factor spreadsheet.
NEC reference
NEC 2020. K-factor constants and circular mils derive from Chapter 9 Table 8; AC resistance for #2 AWG and up is Chapter 9 Table 9. The 3% and 5% recommendations are the Informational Notes to 210.19(A)(1) and 215.2(A)(1); their advisory status follows from 90.5(C).
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Get the full guide
NEC Code Quickstart
This guide covers one calculation. NEC Code Quickstart walks all twelve exam calculations start to finish, each with a worked example and the exact code reference: ampacity and derating, breaker sizing, voltage drop, box and conduit fill, grounding, dwelling load, range and dryer demand, motor circuits, and transformer current. Written for the 2023 NEC with notes for the 2026 edition.
Related
- Voltage Drop Calculator: enter your circuit and read the percentage and 3% / 5% status.
- Wire Size for a 200-Foot, 30-Amp Circuit: the method applied to a long branch run.
- Wire Size Calculator: size for ampacity and voltage drop in one step.