# Voltage Drop Calculator

Single-phase voltage drop is VD = 2 × K × I × L ÷ CM: K is 12.9 for copper or 21.2 for aluminum, I is amps, L is one-way feet and CM is circular mils. A 40 A, 240 V load 150 ft away on 8 AWG copper drops 9.4 V (3.9%); 6 AWG brings it under 3%.

## Key takeaways

- Single phase: VD = 2 × K × I × L ÷ CM. Three phase: VD = 1.732 × K × I × L ÷ CM. L is the one-way run.
- K ≈ 12.9 for copper and 21.2 for aluminum (ohm-circular-mil per foot at 75 °C). Circular mils come from NEC Chapter 9, Table 8.
- NEC Informational Notes suggest keeping branch-circuit drop to 3% and feeder plus branch to 5% total; Informational Notes are explanatory, not enforceable requirements (NEC 90.5(C)).
- Voltage drop never shrinks a conductor. Size for ampacity and terminations first, then upsize for distance.
- This calculator ignores reactance and power factor, which matter for large conductors and long AC runs.

> **Estimate only**
>
> This tool uses the standard DC-resistance approximation with K values for copper and aluminum. It does not check ampacity, overcurrent protection, conduit fill, temperature correction, terminal ratings or any code requirement, and it is not an engineering calculation. Conductor size is set by the NEC and the authority having jurisdiction; voltage drop can only make the wire bigger than that minimum.

## How do you calculate voltage drop?

Multiply the current by the resistance of the full circuit path. For a single-phase circuit the current goes out and back, so the length is doubled; for three-phase the factor is √3 (1.732). Resistance comes from the conductor's cross-section in circular mils and the K constant for its metal. Divide the drop by the source voltage for a percentage.

**Formula**

```
VD = 2 × K × I × L ÷ CM  (single phase)   VD = 1.732 × K × I × L ÷ CM  (three phase)
```

Where:
- VD = voltage drop in volts
- K = 12.9 for copper, 21.2 for aluminum: resistance in ohms of a conductor 1 ft long and 1 circular mil in area at 75 °C
- I = load current in amps
- L = one-way length from source to load, in feet
- CM = conductor area in circular mils (NEC Chapter 9, Table 8)

To find the minimum size for a target instead, rearrange it: CM = 2 × K × I × L ÷ (allowed volts), then pick the next larger conductor in the table. The calculator above does both.

1. **Size for ampacity first.** Pick the conductor the load and the NEC require (Article 310 ampacity tables, terminal ratings, continuous-load factors). That's the floor.
2. **Measure the one-way run.** Panel to the load along the actual wire route, not as the crow flies. Don't double it; the formula does that.
3. **Look up circular mils.** From NEC Chapter 9, Table 8 or the table on this page. 12 AWG is 6,530 CM; 6 AWG is 26,240.
4. **Run the formula.** VD = 2 × K × I × L ÷ CM. Divide by the source voltage for percent.
5. **Upsize if needed.** If the drop is above your target (commonly 3% for a branch circuit), step up until it isn't. Check that the larger conductor still fits the terminals and raceway.

> **Worked example: a feed to a detached shop**
>
> A 40 A, 240 V single-phase feed runs 150 ft to a detached garage on 8 AWG copper (16,510 circular mils).
>
> VD = 2 × 12.9 × 40 × 150 ÷ 16,510 = **9.38 V**, or 3.91% of 240 V. The shop sees about 230.6 V under full load.
>
> For 3% (7.2 V): CM = 2 × 12.9 × 40 × 150 ÷ 7.2 = 21,500 circular mils, so the next size up is **6 AWG** at 2.46%.
>
> On aluminum, 8 AWG would drop 15.41 V (6.42%); you'd need 4 AWG to stay under 3%.

## What does the NEC say about voltage drop?

For most circuits, the NEC recommends rather than requires a limit. Informational Notes to 210.19(A) (branch circuits) and 215.2(A) (feeders) say that conductors sized to keep voltage drop at the farthest outlet to 3%, with a total of 5% for feeders and branch circuits combined, provide reasonable efficiency of operation. Under NEC 90.5(C), Informational Notes are explanatory material and are not enforceable. Two places do set enforceable limits: 647.4(D) for sensitive electronic equipment (1.5% branch, 2.5% combined) and 695.7 for fire pump motor circuits. Energy codes and local amendments may impose their own limits, so check with the authority having jurisdiction.

## Voltage drop by wire size

**Drop for the example feed (40 A, 240 V single phase, 150 ft one way) by conductor**

| Size | Circular mils | Copper drop | Aluminum drop |
| --- | --- | --- | --- |
| 10 AWG | 10,380 | 14.9 V (6.2%) | 24.5 V (10.2%) |
| 8 AWG | 16,510 | 9.4 V (3.9%) | 15.4 V (6.4%) |
| 6 AWG | 26,240 | 5.9 V (2.5%) | 9.7 V (4%) |
| 4 AWG | 41,740 | 3.7 V (1.5%) | 6.1 V (2.5%) |
| 3 AWG | 52,620 | 2.9 V (1.2%) | 4.8 V (2%) |
| 2 AWG | 66,360 | 2.3 V (1%) | 3.8 V (1.6%) |
| 1 AWG | 83,690 | 1.8 V (0.8%) | 3 V (1.3%) |

_Computed with the calculator above. Circular mil areas are NEC Chapter 9, Table 8 values. Ampacity is not considered; 10 AWG is shown only to illustrate the drop._

## How far can you run wire before voltage drop is a problem?

**Maximum one-way run for 3% drop on copper, single phase, at full breaker load**

| Copper size | 120 V, 15 A | 120 V, 20 A | 240 V, 30 A | 240 V, 50 A |
| --- | --- | --- | --- | --- |
| 14 AWG | 38 | 28 | 38 | 22 |
| 12 AWG | 60 | 45 | 60 | 36 |
| 10 AWG | 96 | 72 | 96 | 57 |
| 8 AWG | 153 | 115 | 153 | 92 |
| 6 AWG | 244 | 183 | 244 | 146 |
| 4 AWG | 388 | 291 | 388 | 232 |

_Feet, rounded down, computed from the formula with K = 12.9. Loads rarely draw the full breaker rating; use the actual load current for a real circuit. Some size and amperage combinations exceed the conductor's ampacity and are shown for the math only._

## Copper vs. aluminum

Aluminum's K of 21.2 is about 64% higher than copper's 12.9, so at the same size it drops about 64% more voltage. Aluminum feeders are usually one or two sizes larger than the copper equivalent for ampacity anyway, which closes most of the gap. Use the calculator to compare the actual sizes you'd install, with listed terminations rated for the metal you choose.

## Common voltage drop mistakes

1. **Doubling the length yourself.** The 2 in the formula already accounts for the return path. Enter the one-way distance.
2. **Using breaker size instead of load.** Drop depends on the current that actually flows. A 20 A circuit feeding a 6 A load drops a third as much.
3. **Treating 3% as code.** It's an Informational Note, not a requirement, except where 647.4(D), 695.7 or a local amendment applies. Many designers still hold to it because motors, LEDs and electronics behave better.
4. **Sizing for drop before ampacity.** Ampacity, terminal temperature ratings and overcurrent protection set the minimum. Drop only ever increases it.
5. **Ignoring reactance on big wire.** For 2/0 and larger on AC, the Table 9 impedance method is more accurate than the K approximation.
6. **Forgetting the feeder.** A 2% drop on the feeder plus 3% on the branch is 5% at the receptacle. Check the whole path.

When the run calls for bigger wire than the customer expected, BRIX lets you revise the quote on site with the new conductor, conduit and labor, and follows up until it's approved.

- [Quotes and estimates](https://www.brix.so/features/quotes-estimates): Builds accurate quotes on site and follows up on every open estimate.
- [Sales automation](https://www.brix.so/features/sales-automation): Follows up every lead and open estimate automatically by text and email, and asks every customer for a review after the job.

## Frequently asked questions

### What is the formula for voltage drop?

For single-phase or DC circuits, VD = 2 × K × I × L ÷ CM; for three-phase, VD = 1.732 × K × I × L ÷ CM. K is 12.9 for copper and 21.2 for aluminum, I is amps, L is one-way length in feet and CM is circular mils from NEC Chapter 9, Table 8.

### What is the NEC's voltage drop requirement?

For general branch circuits and feeders the NEC has recommendations, not requirements: Informational Notes to 210.19(A) and 215.2(A) suggest 3% on a branch circuit and 5% total. Informational Notes are not enforceable under 90.5(C). Enforceable limits exist for sensitive electronic equipment (647.4(D)) and fire pumps (695.7), and local codes may add their own.

### What size wire do I need for a 100 ft run?

It depends on the load and voltage. A 20 A load at 120 V, 100 ft away, drops 6.6% on 12 AWG copper; 8 AWG keeps it under 3%. Enter your own amps and voltage in the calculator, and always size for ampacity first.

### Does voltage drop matter on a 240 V circuit?

Less than on 120 V for the same current and distance, because the same volts lost are a smaller percentage of the supply. That's one reason long runs to outbuildings, pumps and EV chargers are often fed at 240 V.

### Can I use aluminum wire to reduce cost on a long run?

Yes, if the terminations are listed for aluminum and it's installed per the NEC. Aluminum has about 64% more resistance per circular mil (K = 21.2 vs. 12.9), so you'll typically go up one or two sizes. Compare the installed sizes in the calculator.

### Is this calculator accurate for motors and three-phase feeders?

It gives a close estimate for most runs with conductors 1/0 and smaller. For large conductors, long runs and loads with low power factor, inductive reactance adds to the drop; use NEC Chapter 9, Table 9 impedance values or an engineer's calculation for those.

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## Sources

- [IAEI Magazine — Voltage Drop Formulas (K values, single- and three-phase formulas)](https://iaeimagazine.org/electrical-fundamentals/voltage-drop-formulas/) (checked 2026-10-12)
- [Mike Holt — Voltage Drop Calculations (formulas, K, Informational Notes not enforceable)](https://www.mikeholt.com/technnical-voltage-drop-calculations-part-one.php) (checked 2026-10-12)
- [EC&M — The Basics of Calculating Voltage Drop](https://www.ecmweb.com/content/article/20889505/the-basics-of-calculating-voltage-drop) (checked 2026-10-12)
- [EC&M — Calculating Voltage Drop in Power Distribution Systems (NEC 647.4(D) limits)](https://www.ecmweb.com/content/article/20890701/calculating-voltage-drop-in-power-distribution-systems) (checked 2026-10-12)
- [EC&M — Article 695 and Fire Pumps (NEC 695.7 voltage drop limits)](https://www.ecmweb.com/code-basics/article-695-and-fire-pumps) (checked 2026-10-12)
- [Mike Holt — Summary of the 2023 Changes to the NEC (210.19 reorganized, 90.5(C))](https://www.mikeholt.com/files/PDF/23CC_Summaries.pdf) (checked 2026-10-12)

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Written by BRIX Editorial Team. Last updated 2026-10-12.
Canonical: https://www.brix.so/tools/voltage-drop-calculator