Voltage Drop for Long Wire Runs: When 3% Matters and How to Calculate It
On short runs, voltage drop is an afterthought. A 20A circuit on 12 AWG to a receptacle 30 feet away loses less than 1% — nobody notices, no inspector cares. But stretch that same circuit to 150 feet, and you’re at 4.8% drop on 120V. Lights dim visibly. Motors overheat. The inspector red-tags your work.
Long wire runs are where voltage drop moves from academic to job-critical. Warehouses, parking lot lighting, farm buildings, outbuildings, well pumps, EV chargers in detached garages — any run over 100 feet demands a voltage drop calculation before you pull a single conductor. This is the one calculation where ampacity won’t save you: the wire may be sized perfectly for the load current, but the electrons still lose energy fighting through 200 feet of copper.
What the NEC Actually Says About Voltage Drop
NEC 210.19(A) Informational Note No. 4 and 215.2(A)(4) Informational Note No. 2 recommend:
- 3% maximum voltage drop on branch circuits
- 5% maximum total voltage drop (feeder + branch circuit combined)
The word “recommend” matters. These are informational notes, not enforceable code requirements. However, most AHJs (Authorities Having Jurisdiction) treat them as de facto requirements during inspections. Some jurisdictions have adopted them as enforceable local amendments. Plan to meet the 3%/5% targets unless your AHJ has explicitly told you otherwise.
The 2026 NEC reorganized some article numbers (Article 220 moved to Article 120), but the voltage drop recommendations remain substantively the same in their new locations.
The Voltage Drop Formula
There are two common forms of the voltage drop formula. Both give the same answer — they just use different conductor property references.
Circular Mils Method (NEC Chapter 9, Table 8)
Single-phase:
VD = (2 × K × I × L) / CM
Three-phase:
VD = (1.732 × K × I × L) / CM
Where:
- VD = voltage drop in volts
- K = resistivity constant: 12.9 for copper, 21.2 for aluminum (at 75°C)
- I = load current in amperes
- L = one-way conductor length in feet (not round-trip)
- CM = circular mil area of the conductor (from NEC Chapter 9, Table 8)
Resistance Method (NEC Chapter 9, Table 9)
Single-phase:
VD = 2 × L × I × R / 1000
Three-phase:
VD = 1.732 × L × I × R / 1000
Where R = conductor resistance in ohms per 1,000 feet (from NEC Chapter 9, Table 9, which includes both resistance and reactance values for conductors in various raceway types).
To convert the voltage drop to a percentage: VD% = (VD / V_source) × 100
Circular Mil Reference Table
You need the circular mil area to use the K-factor formula. Here are the values from NEC Chapter 9, Table 8 for the most common conductor sizes:
| Conductor Size | Circular Mils (CM) |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
| 250 kcmil | 250,000 |
| 350 kcmil | 350,000 |
| 500 kcmil | 500,000 |
Worked Example 1: Parking Lot Lighting (Single-Phase, 120V)
You’re running a 120V, 20A lighting circuit to pole lights at the far end of a parking lot. One-way distance: 250 feet. Copper conductors.
Try 12 AWG (6,530 CM):
VD = (2 × 12.9 × 20 × 250) / 6,530 = 19.8V
VD% = 19.8 / 120 × 100 = 16.5% — far over the 3% limit.
Try 6 AWG (26,240 CM):
VD = (2 × 12.9 × 20 × 250) / 26,240 = 4.9V
VD% = 4.9 / 120 × 100 = 4.1% — still over 3%.
Try 4 AWG (41,740 CM):
VD = (2 × 12.9 × 20 × 250) / 41,740 = 3.1V
VD% = 3.1 / 120 × 100 = 2.6% — passes.
A 20A circuit that only needs 12 AWG for ampacity requires 4 AWG at 250 feet to meet the 3% recommendation. That is a jump of four wire sizes — and a substantial cost increase in copper. This is why voltage drop controls long runs.
Worked Example 2: Workshop Sub-Panel Feeder (Single-Phase, 240V)
You’re feeding a 100A sub-panel in a detached workshop, 175 feet from the main panel. 240V single-phase, copper conductors.
Since this is a feeder, the combined feeder + branch circuit drop should stay under 5%. If you allocate 3% to the feeder and 2% to the branch circuits, the feeder budget is 3% of 240V = 7.2V.
Try 3 AWG (52,620 CM):
VD = (2 × 12.9 × 100 × 175) / 52,620 = 8.6V
VD% = 8.6 / 240 × 100 = 3.6% — over 3% feeder budget.
Try 2 AWG (66,360 CM):
VD = (2 × 12.9 × 100 × 175) / 66,360 = 6.8V
VD% = 6.8 / 240 × 100 = 2.8% — passes the 3% feeder target.
Note that 3 AWG copper has 100A ampacity at 75°C (NEC Table 310.16) — perfect for the load. But voltage drop forces you up to 2 AWG. If you only checked ampacity, you’d install 3 AWG and the sub-panel would see only 231V under full load.
Worked Example 3: Three-Phase Motor Circuit (480V)
A 25 HP, 480V three-phase motor in a warehouse, 300 feet from the panel. Per NEC Table 430.250, the full-load current for a 25 HP, 480V motor is 34A. Copper conductors.
Try 8 AWG (16,510 CM):
VD = (1.732 × 12.9 × 34 × 300) / 16,510 = 13.8V
VD% = 13.8 / 480 × 100 = 2.9% — under 3%, passes.
At 480V, the higher source voltage works in your favor. The same 300-foot run on a 208V system would produce 6.6% drop — requiring a larger conductor. Higher voltage systems inherently tolerate longer runs, which is one reason commercial and industrial facilities use 480V distribution.
When Voltage Drop Controls vs. When Ampacity Controls
A practical rule of thumb for when to expect voltage drop problems:
| System Voltage | Run Length Where VD Usually Controls |
|---|---|
| 120V | Over 75–100 feet |
| 208V | Over 125–175 feet |
| 240V | Over 150–200 feet |
| 277V | Over 175–225 feet |
| 480V | Over 300–400 feet |
These are rough thresholds. The actual crossover depends on the load current and the conductor size selected for ampacity. The point is: at 120V, even moderate distances can be problematic, while 480V runs rarely have voltage drop issues unless you are running feeders across very large facilities.
Voltage Drop Strategies for Long Runs
When a voltage drop calculation shows you need to upsize significantly, consider these alternatives before jumping to expensive larger conductors:
1. Increase the System Voltage
A 240V circuit has half the voltage drop percentage of a 120V circuit carrying the same power, because the current is halved. For lighting loads over 100 feet, consider 277V circuits (common in commercial) or 240V circuits with appropriate transformers.
2. Relocate the Panel
If you’re feeding multiple loads at the far end of a building, installing a sub-panel near the loads shortens every branch circuit run. One larger feeder to the sub-panel is often cheaper than multiple oversized branch circuits.
3. Use Aluminum Conductors
Aluminum costs significantly less per foot than copper for equivalent ampacity. The K factor for aluminum is 21.2 vs. 12.9 for copper, so you need larger wire — but the price per ampere-foot is still lower. For long feeder runs, aluminum is standard practice. Just size appropriately (aluminum requires larger wire for the same ampacity) and use AL-rated lugs.
4. Split the Load Across Multiple Circuits
Two 10A circuits have half the voltage drop of one 20A circuit on the same wire size. For distributed loads like parking lot lights, multiple circuits with closer home runs may be more economical than one oversized circuit.
Accounting for Voltage Drop in the Feeder + Branch Budget
The 5% total budget requires coordinating feeder and branch circuit sizing. A common allocation:
- Feeder: 2–3% voltage drop budget
- Branch circuit: 2–3% voltage drop budget
- Total: ≤ 5%
If your feeder uses 4% of the budget, every branch circuit at the sub-panel is limited to 1% — which may force oversized branch conductors. Plan the feeder and branch together, not independently.
The Aluminum vs. Copper Decision for Long Feeders
For feeder runs over 100 feet, aluminum is worth evaluating. Here’s a comparison for a 100A, 240V feeder at 200 feet:
| Conductor | Size for Ampacity | VD at 200 ft | Size After VD Check | Relative Cost |
|---|---|---|---|---|
| Copper | 3 AWG | 3.3% | 2 AWG | Higher |
| Aluminum | 1 AWG | 3.6% | 1/0 AWG | Lower |
Aluminum requires a larger physical conductor (1/0 vs. 2 AWG) but typically costs 40–60% less for the same run. The conduit may need to be one size larger to accommodate the bigger aluminum conductors — factor that into the total cost comparison.
Common Voltage Drop Calculation Mistakes
- Using round-trip distance instead of one-way — The formula already accounts for the return path with the “2” multiplier. L is the one-way distance only. Doubling it means you’re calculating four times the actual conductor length.
- Calculating at 125% current for continuous loads — Voltage drop is calculated at the actual load current, not the 125% sizing current. The 125% multiplier applies to conductor ampacity sizing and OCPD sizing, not to voltage drop. A 40A continuous load drops voltage based on 40A flowing through the conductor, not 50A.
- Ignoring feeder drop when calculating branch circuits — If the feeder already drops 3%, the branch circuit starts 3% low. The branch circuit’s 3% drop is calculated from the reduced voltage at the sub-panel, and the total exceeds 5%.
- Using the wrong K factor for conductor temperature — The standard K values (12.9 copper, 21.2 aluminum) are based on 75°C operating temperature. At lower temperatures, resistance is lower and actual voltage drop is slightly less. The 75°C values are conservative and appropriate for code compliance calculations.
- Forgetting power factor for inductive loads — The circular mils formula assumes resistive loads. For motors and other inductive loads, conductor reactance also contributes to voltage drop. The NEC Chapter 9, Table 9 resistance method with the effective Z column accounts for power factor. For large motor loads on long runs, use the Table 9 method.
Quick Voltage Drop Reference Table
Maximum one-way distance in feet for 3% voltage drop on 120V single-phase copper circuits (based on actual load current):
| Wire Size | 10A Load | 15A Load | 20A Load |
|---|---|---|---|
| 14 AWG | 57 ft | 38 ft | 29 ft |
| 12 AWG | 91 ft | 61 ft | 46 ft |
| 10 AWG | 145 ft | 96 ft | 72 ft |
| 8 AWG | 230 ft | 154 ft | 115 ft |
| 6 AWG | 366 ft | 244 ft | 183 ft |
| 4 AWG | 582 ft | 388 ft | 291 ft |
For 240V circuits, double these distances. For 208V three-phase, multiply by 1.5. These are conservative values suitable for inspection.
Use our voltage drop calculator to run these numbers for your specific installation — enter the load, distance, and system voltage to see the exact conductor size needed.
The Bottom Line
Voltage drop is the silent constraint that ampacity tables alone do not capture. For any run over 100 feet on a 120V circuit — or over 200 feet at higher voltages — calculate the drop before selecting your conductor. The formula is straightforward (VD = 2KIL/CM), but the consequences of skipping it are not: dimming lights, overheating motors, nuisance tripping, and red tags from the inspector. When voltage drop controls the wire size, consider your options — higher voltage, relocated panels, aluminum conductors, or split circuits — before accepting the copper bill for four sizes up.
If this is your first time sizing conductors for a circuit, start with our complete wire sizing guide, which covers the full NEC process including ampacity, derating, and how voltage drop fits into the bigger picture.
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