How this voltage drop calculator works
Wire is a resistor, and long runs spend voltage on it. The classic estimating formula prices that loss from four numbers:
% drop = VD ÷ system volts · NEC note: ≤ 3% branch, ≤ 5% total
At the defaults — 15 A over 100 ft of 12 AWG copper — the drop is 5.93 V, 4.9% of 120 V, over the 3% note; the calculator flags it and names the gauge that passes. The line-by-line math is in the drop math section below.
What each term in VD = 2KIL÷CM means
Five symbols, and each one tells you a lever you can pull. Understanding them is the difference between reading the number and fixing it:
The 2 (two-wire)
Current makes a round trip — out on the hot, back on the neutral (or the other hot on a 240 V circuit) — so the wire length that resists it is double the one-way run. That factor of 2 is why a “100 ft” circuit computes on 200 ft of conductor.
K — the conductor constant
The resistivity of the metal in ohm-circular-mils per foot: 12.9 for copper, 21.2 for aluminum. It’s the whole reason aluminum runs fatter — 64% more area for the same drop.
I — the current
Amps of load. Drop scales straight with it, so a circuit near its breaker rating drops far more than a lightly loaded one over the same wire.
L — the one-way length
Feet from panel to load, one direction — the 2 handles the return. Length is usually the term you can’t change and the reason the gauge grows.
CM — circular mils
The conductor’s cross-sectional area (NEC chapter 9, table 8). It’s in the denominator, so fatter wire drops less — and every two AWG sizes roughly doubles the copper, halving the drop.
What counts as acceptable voltage drop?
The NEC doesn’t mandate a voltage-drop limit for most branch circuits — it recommends one in informational notes, and the industry treats it as a design rule:
- 3% on a branch circuit (panel to load) — the target this calculator checks against.
- 5% total across feeder plus branch — the combined ceiling for the whole path from service to device.
- Motors and A/C: honor the 3% strictly — low voltage means high current, heat and short life.
- Sensitive electronics and long LED runs: tighter is better; flicker and driver stress show up before the wire complains.
- Low-voltage (12/24 V): percentages explode — landscape and RV wiring often needs a gauge or two beyond what the length suggests.
The one hard rule: voltage drop is a performance check, layered on top of the binding ampacity requirement. The wire must first be big enough for the breaker; drop only ever makes it bigger, never smaller.
What wire size for the run? — 3% chart
Table 1 is the minimum copper gauge to stay inside 3% at 120 V and 15 A by run length; Table 2 is the circular-mil area behind the math.
| One-way run | Gauge |
|---|---|
| 25 ft | 14 AWG |
| 50 ft | 12 AWG |
| 75 ft | 10 AWG |
| 100 ft | 8 AWG |
| 150 ft | 8 AWG |
| 200 ft | 6 AWG |
| Gauge | Circular mils |
|---|---|
| 14 AWG | 4,107 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
How to size a run for voltage drop
Size for the breaker first, then check the drop and grow the wire if the run is long. The calculator does the second half.
- Size for ampacity first. Pick the gauge the NEC tables require for the breaker and load — that’s the binding minimum, non-negotiable.
- Enter the system volts — 120, 240, or a low-voltage number — and the load in amps.
- Enter the one-way run length from panel to load; the formula doubles it for the return.
- Pick the gauge and metal you’re considering — copper (K 12.9) or aluminum (21.2).
- Read the drop and the verdict. Over 3%? Step up to the recommended gauge the calculator names and re-check.
- Take the larger of the two. The final wire is whichever is bigger — the ampacity minimum or the voltage-drop size.
Worked example: voltage drop on a 100 ft run
The default, worked line by line — 15 A over 100 ft of 12 AWG copper (6,530 CM) on a 120 V circuit:
= 38,700 ÷ 6,530 = 5.93 V
% drop = 5.93 ÷ 120 = 4.9% over the 3% note
end voltage = 120 − 5.93 = 114.07 V
needed CM for 3% = 38,700 ÷ (0.03 × 120) = 10,750
first gauge ≥ 10,750 CM = 8 AWG (16,510 CM)
Here’s the trap: 14 AWG is legal for 15 A on ampacity alone, and 12 AWG is a common upgrade — yet at 100 ft even the 12 AWG drops 4.9%, and it takes 8 AWG, two sizes fatter, to hold 3%. On long runs the distance sizes the wire, not the breaker, and that’s exactly the case people under-build.
What this voltage drop calculator doesn’t do
It’s planning math for the drop, not a code sign-off. It does not:
- Check ampacity. The binding requirement — conductor sized to the breaker per NEC 310, with derating — is separate and comes first.
- Apply temperature or bundling derates. Conduit fill and ambient heat further cut ampacity.
- Handle 3-phase exactly. The 2-wire formula is for single-phase; 3-phase uses a √3 factor.
- Account for power factor or inductive reactance. Fine for resistive and small runs; big motor feeders want the fuller AC model.
- Read termination ratings. 60/75/90°C column limits at devices and breakers.
- Replace an electrician. Local amendments and inspection govern — confirm before you pull wire.
Voltage drop FAQ
How do I calculate voltage drop?
The standard 2-wire estimating formula: VD = 2 × K × I × L ÷ CM, with K = 12.9 for copper, I the amps, L the one-way feet, and CM the wire’s circular mils. The default 15 A over 100 ft of 12 AWG drops 5.93 V — 4.9% of a 120 V circuit.
What is an acceptable voltage drop?
The NEC’s informational note recommends no more than 3% on a branch circuit and 5% total with the feeder — a recommendation for performance, not a code mandate in most cases. Motors, compressors and electronics are why you honor it anyway.
What size wire do I need for a 100 ft run?
For 15 A at 120 V, 8 AWG copper is the first listed gauge inside 3% at 100 ft — two sizes fatter than the 14 AWG that ampacity alone would allow. Distance, not just load, sizes long runs; the chart below shows the crossover lengths.
What is the K constant in the voltage drop formula?
K is the resistivity of the conductor in ohm-circular-mils per foot — how much a mil-thin strand of the metal resists current. 12.9 for copper, 21.2 for aluminum at normal operating temperature. It is the one material constant in VD = 2KIL÷CM, and it is why aluminum needs a fatter wire for the same drop.
Does voltage drop matter less at 240 V?
Yes — the same volts lost are half the percentage at 240 V, which is why long feeders to shops and barns usually run 240. Low-voltage systems invert that: at 12 V even short runs drop brutal percentages, as landscape-lighting installers learn fast.
How does voltage drop affect motors and lights?
Low voltage at the load makes motors run hot and lose torque — a pump or compressor draws more current to make the same power, which heats windings and shortens life. Incandescent and halogen lamps dim and shift warm; LED drivers mostly shrug it off until the drop is severe. It is the equipment at the end of the run, not the wire, that pays for a skimped gauge.
Is aluminum wire worse for voltage drop?
Aluminum’s K is 21.2 against copper’s 12.9, so it needs about 64% more circular mils for the same drop — typically two AWG sizes up. It still wins on cost for big feeders, which is why service conductors are commonly aluminum.
Does this tell me the wire is code-legal?
No — it checks voltage drop only. The conductor must first be sized to the breaker per the NEC ampacity tables, with derating, termination ratings and local amendments on top. Treat the result as planning math and have a licensed electrician confirm.
Sources and standards
- NEC 210.19 & 215.2 informational notes: the 3% branch / 5% total voltage-drop recommendation.
- NEC Chapter 9, Table 8: the conductor circular-mil areas the formula divides by.
- NEC Article 310: the ampacity tables that set the binding minimum wire size — the check that comes first.
- Standard K values: 12.9 copper / 21.2 aluminum ohm-cmil/ft at operating temperature — the estimating constants used here.
Embed this voltage drop calculator on your site
Run the voltage drop calculator on your own site, free. Paste the snippet where you want it; the visible credit link is the only condition.