How to measure a run of wire
Measure the route the cable will really take, from the panel to the load, one way. A run to a shed is rarely straight: it goes down the wall, along the trench and back up. Add those together, and be generous. Do not double it for the return, because the formula does that for you. That is the most common error with these sums.
Then find the load in amps, on the nameplate of whatever you are powering. For a shed, add up what might run at once. Use the circuit's real voltage: 120 for a regular branch and 240 for larger loads. Most homes are single phase, and three phase is mainly for commercial buildings.
Keep in mind what this does not do. Voltage drop is about how much power reaches the far end. It says nothing about whether a wire can safely carry the current, which comes under ampacity and overcurrent rules. A wire can pass this sum and still be too small for its breaker.
A worked example
Mike Holt's EC&M column prints these two. These lines come from the same formula the calculator above runs.
Holt's two examples
- Single phase: 2 × 12.9 × 44 × 160 ÷ 26240 = 6.92 V
- That is 2.9% of 240 V, so 233.1 V at the load
- Three phase: 1.732 × 21.2 × 100 × 80 ÷ 83690 = 3.51 V
- That is 1.7% of 208 V, so 204.5 V at the load
Both match Holt's 2.9 and 1.7 percent, and they fall within the 3 percent the NEC's notes recommend for a branch circuit. If you are planning backup power for that shed, the generator size calculator uses the same load list, and so does the electricity cost calculator.
Smallest copper wire for 3 percent
On a 120 volt single phase circuit, by voltage drop alone. Ampacity and breaker rules may ask for larger wire than any cell here, particularly at the short distances, so this is not a chart to wire by.
| Load | 50 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|
| 10 amps | 14 | 10 | 8 | 8 |
| 15 amps | 12 | 8 | 8 | 6 |
| 20 amps | 10 | 8 | 6 | 4 |
| 30 amps | 8 | 6 | 4 | 3 |
| 40 amps | 8 | 4 | 3 | 2 |
Mistakes that lead to the wrong wire
- Doubling the distance. The 2 in the formula covers the return. Enter it one way.
- Trusting drop alone. Passing this sum says nothing about whether the wire can carry the current without overheating.
- Treating 3 percent as law. EC&M says the notes are not requirements. But your equipment will not care, and a long, low-voltage run can leave motors struggling.
- Percent and volts mixed up. 3 percent is 3.6 volts at 120, but 7.2 volts at 240. The same run of wire looks much better at the higher voltage.
- Looking up one note number. Sources number the NEC notes differently by edition, so we do not print one. Ask your electrician about the code your area adopted.
Questions
What size wire for 20 amps at 100 feet?
8 AWG copper at 120 volts stays within 3 percent for voltage drop, at about 2.6 percent. 10 AWG is about 4.1 percent, and at 240 volts 10 AWG is about 2.1. Have an electrician confirm ampacity and breaker size, since those are separate checks.
What size wire for 50 amps at 100 feet?
6 AWG copper at 240 volts gives about 2.05 percent drop, which is within 3 percent. But that is voltage drop only. Whether 6 AWG can carry 50 amps depends on its insulation and how it runs, and that check is separate.
How much voltage drop is acceptable?
The NEC's informational notes recommend no more than 3 percent on a branch circuit, and 5 percent for feeder and branch together, as EC&M reports. Those are not requirements. On a 120 volt circuit, 3 percent is 3.6 volts.
Do I measure the wire there and back?
No, enter the distance one way. The 2 in the formula already counts the return. This is the most likely input error, and doubling it again would make your drop look twice as bad.
Why use aluminum at all?
It is cheaper and lighter for long, large runs, but it has more resistance: K is 21.2 against 12.9 for copper, so it needs a larger size for the same drop. It also calls for rated terminations, so ask an electrician.
When do I need a professional?
You need a licensed electrician for any new circuit or subpanel, and most places require a permit. Wire size must meet ampacity and overcurrent rules that this calculator does not check. Getting them wrong can start a fire, even when the voltage drop looks fine.
This voltage drop figure is for planning, and it says nothing about whether a wire can safely carry the current, which is a separate ampacity check. An undersized conductor can overheat and start a fire. Sizing is governed by the National Electrical Code as your area adopts it, so a licensed electrician should confirm the wire, breaker and conduit before any of it goes in.
Sources and review
- Code calculations, EC&M, 2000.
- Watch out for that voltage drop, EC&M, 2015.
- Voltage drop calculations and conductor properties, Open Exam Prep, 2026.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Single phase
Volts lost = 2 × K × Amps × Feet ÷ Circular mils
- K
- 12.9 for copper, 21.2 for aluminum
- Feet
- Distance one way
Worked through: 2 × 12.9 × 44 × 160 ÷ 26240 = 6.92 V
Three phase
Volts lost = 1.732 × K × Amps × Feet ÷ Circular mils
- 1.732
- The square root of 3, in place of the 2
Worked through: 1.732 × 21.2 × 100 × 80 ÷ 83690 = 3.51 V
The constants, and where they come from
K_BY_METAL
Published figureResistance constant by metal, in ohm-circular-mils per foot
- copper
- 12.9
- aluminum
- 21.2
Applies to: Common wire at normal temperatures. Long AC runs with large wire differ
Source: Code calculations, EC&M, published 2000-02, read 2026-11. This source is about 26 years old, and it is the one the figure rests on.
12.9 for copper and 21.2 for aluminum, from Mike Holt's EC&M column. K is an approximation, and exact values vary with stranding.
CIRCULAR_MILS
Published figureArea of each wire size, in circular mils
- 0
- 14, 4110
- 1
- 12, 6530
- 2
- 10, 10380
- 3
- 8, 16510
- 4
- 6, 26240
- 5
- 4, 41740
- 6
- 3, 52620
- 7
- 2, 66360
- 8
- 1, 83690
- 9
- 1/0, 105600
- 10
- 2/0, 133100
- 11
- 3/0, 167800
- 12
- 4/0, 211600
- 13
- 250, 250000
Applies to: NEC Chapter 9 Table 8, as reproduced in a study guide
Source: Voltage drop calculations and conductor properties, Open Exam Prep, published 2026, read 2026-11.
Also from: Code calculations, EC&M, published 2000-02, read 2026-11.
We could not read NFPA 70 itself. Two rows also appear in Holt's column, and they match.
SINGLE_PHASE_FACTOR
DefinitionalFactor for a single phase circuit
Applies to: Single phase circuits. It already counts the return
Source: Code calculations, EC&M, published 2000-02, read 2026-11. This source is about 26 years old, and it is the one the figure rests on.
Current goes out and back, so the one-way length is doubled, which is why you enter the distance one way only.
THREE_PHASE_FACTOR
DefinitionalFactor for a three phase circuit
Applies to: Three phase circuits
Source: Code calculations, EC&M, published 2000-02, read 2026-11. This source is about 26 years old, and it is the one the figure rests on.
The square root of 3, rounded as Holt's column prints it. It replaces the 2, and is not an extra multiplier.
RECOMMENDED_BRANCH_PERCENT
Published range, narrowedVoltage drop recommended for a branch circuit, in percent
Applies to: A recommendation and not a requirement. Feeder and branch together are 5 percent
Source: Watch out for that voltage drop, EC&M, published 2015, read 2026-11. This source is about 11 years old, and it is the one the figure rests on.
EC&M reports these from informational notes in the NEC, and says they are not requirements. Sources number the notes differently across editions, so we do not print a note number.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
- Published range, narrowed:
- a source publishes a range and this calculator uses a representative value from it.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the voltage drop calculator
- 1
Choose the metal and the wire size
- 2
Measure the distance from the panel to the load, one way
- 3
Enter the load in amps and the circuit voltage
- 4
Read the drop, and the smallest wire that meets your target
- 5
Have an electrician check the ampacity and the breaker
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- The distance is one way. The formula already allows for the return
- K is an approximation for common wire at normal temperatures. Long AC runs in very large wire differ
- 3 percent for a branch circuit and 5 percent in all are recommendations in the NEC's notes, not requirements
- It does not check ampacity or the breaker, so a wire that passes here may still be too small
Pro tips
- Go one size up on a long run if you may add load later
- Measure along the real route, as conduit rarely runs straight
- Size for the running current, and ask about motor starts separately