Voltage Drop Calculator
Calculate the voltage drop in volts and percent for a given wire size, load, run length and voltage, plus the longest run that stays inside the NEC 3% recommendation.
By CalcBuilt Editorial TeamUpdated September 10, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Voltage drop
- 5.93 V
- 4.94% of 120 V, over 3% but within the 5% feeder-plus-branch total
- Percent drop
- 4.94%
- 12 AWG copper carrying 15 A over 100 ft one way
- Voltage at the load
- 114.07 V
- Starting from 120 V at the panel
- Longest one-way run for 3% drop
- 61 ft
- At 15 A on 12 AWG copper; 101 ft for a 5% drop
- NEC recommendation
- 3% branch, 5% total
- Informational notes to 210.19 and 215.2. Planning estimate only: a licensed electrician and local code govern, and ampacity must be checked separately.
Voltage Drop formula
Voltage drop (V) = 2 x K x Amps x One-way distance (ft) / Circular mils (K = 12.9 copper, 21.2 aluminum; use 1.732 instead of 2 for three-phase)
Percent drop = Voltage drop / System voltage x 100
Voltage at the load = System voltage - Voltage drop
Longest run for 3% = 0.03 x System voltage x Circular mils / (2 x K x Amps)Worked example
A 30 A, 240 V single-phase load 150 ft from the panel on 10 AWG copper (10,380 circular mils): 2 x 12.9 x 30 x 150 = 116,100. Divide by 10,380 to get 11.18 V, which is 11.18 / 240 = 4.66%, leaving 228.8 V at the load. The longest run that holds 3% at 30 A is 0.03 x 240 x 10,380 / (2 x 12.9 x 30) = 74,736 / 774 = 96.6 ft, so this run needs 8 AWG.
A 100 A three-phase feeder at 480 V, 200 ft long in 2 AWG aluminum (66,360 circular mils): 1.732 x 21.2 x 100 x 200 = 734,368. Divide by 66,360 for 11.07 V, or 2.31% of 480 V. The same wire could run 0.03 x 480 x 66,360 / (1.732 x 21.2 x 100) = 955,584 / 3,671.84 = 260 ft before reaching 3%.
Quick reference
| One-way distance | Drop at 15 A | Percent at 15 A | Drop at 20 A | Percent at 20 A |
|---|---|---|---|---|
| 50 ft | 2.96 V | 2.47% | 3.95 V | 3.29% |
| 100 ft | 5.93 V | 4.94% | 7.90 V | 6.58% |
| 150 ft | 8.89 V | 7.41% | 11.85 V | 9.88% |
| 200 ft | 11.85 V | 9.88% | 15.80 V | 13.17% |
How voltage drop is calculated
Every conductor has resistance, and current through resistance loses voltage. The standard field formula multiplies the K constant (ohms per circular-mil foot, 12.9 for copper and 21.2 for aluminum at 75°C) by the current and the round-trip length, then divides by the wire's cross section in circular mils. The factor of 2 covers the outgoing and return conductors on a single-phase circuit; a balanced three-phase circuit uses 1.732 instead.
The result is volts lost between the panel and the load. Divide by the system voltage to get the percentage, which is what the NEC informational notes reference: 3% on a branch circuit and 5% for feeder and branch combined. This calculator answers the question in reverse from a wire size calculator: you already know the wire and want to see how much it drops.
What to do about a high drop
If the drop is over 3%, you have four levers: shorten the run, lower the current, raise the voltage or increase the wire size. Going from 12 AWG to 10 AWG cuts the drop by 37%, and to 8 AWG by 60%. For a detached garage or shop, run a 240 V feeder to a subpanel instead of a long 120 V circuit. A 60 A feeder in 6 AWG copper holds 3% to about 120 ft at full load; past that, 4 AWG copper or 2 AWG aluminum reaches about 190 ft. Underground that is THWN-2 in PVC at 18 in or URD direct-buried at 24 in (Table 300.5), 4-wire with a separate ground. Figure the drop on the calculated load, not the breaker.
The formula uses DC resistance at 75°C and ignores conductor reactance, power factor and temperature, which is accurate to within a few percent for residential wire sizes. The drop calculated here says nothing about whether the wire can carry the current; ampacity is a separate check, and the table row flags a load above the 75°C rating. Upsizing for drop never changes the breaker: a 20 A circuit pulled in 10 AWG stays on a 20 A breaker and gets a 10 AWG ground (250.122(B)). The remote building needs its own disconnect (225.31) and ground rods (250.32), with neutral and ground kept apart in its panel.
What this calculator leaves out
The result is a drop on one conductor size. Not included: ampacity and derating, the ground size, conduit fill, the trench (typically $5 to $15 per ft by machine) and the 811 locate before digging. Also GFCI on every outbuilding receptacle (210.8(A)), the subpanel, and the permit with a trench inspection before backfill.
Code limits that apply
- NEC 210.19(A) IN 4checked
Branch circuit voltage drop held to 3%, and 5% for feeder plus branch circuit combined
If you miss it: 12 AWG at 20 A run 100 ft at 120 V drops 6.6%; lights dim and the garage fridge compressor stalls, so the run is repulled in 8 AWG
- NEC Table 310.16checked
75°C copper 8 AWG 50 A, 6 AWG 65 A, 4 AWG 85 A; aluminum 8 AWG 40 A, 6 AWG 50 A, 4 AWG 65 A
If you miss it: A wire that holds 3% drop can still be under the load ampacity; 8 AWG aluminum on a 50 A feeder is refused and repulled in 6 AWG
- NEC 240.4(D)checked
14 AWG copper capped at 15 A, 12 AWG at 20 A, 10 AWG at 30 A whatever the table ampacity
If you miss it: 25 A on 12 AWG shows a drop figure but is not a legal circuit; the breaker is held to 20 A or the run repulled in 10 AWG
- NEC 250.122(B)
Equipment ground upsized in proportion when the hots are upsized for voltage drop
If you miss it: A 20 A circuit pulled in 10 AWG hots with a 12 AWG ground is written up at rough-in; the ground is repulled to 10 AWG
- NEC Table 300.5
Direct-buried UF 24 in deep, PVC conduit 18 in, 12 in only for a GFCI 20 A circuit
If you miss it: A feeder trench dug 12 in for URD fails the trench inspection before backfill; it is dug to 24 in or the cable moved into PVC at 18 in
- NEC 250.32
Detached building on a 4-wire feeder gets ground rods, with neutral and ground kept apart
If you miss it: A 3-wire feeder with the shed panel neutral bonded puts return current on the ground path; the feeder is repulled 4-wire before final
- NEC 225.31
A detached building fed from the house has a disconnect at the building
If you miss it: A shed subpanel with no main breaker is refused at final; a main breaker kit or a separate disconnect is added at the building
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Upsize the hots for drop and the ground goes up in proportion (NEC 250.122(B)): 20 A on 10 AWG needs a 10 AWG ground.
- A detached garage feeder is 4-wire with neutral and ground separated, plus ground rods and a disconnect at the building (NEC 250.32, 225.31).
- Direct-buried UF goes 24 in deep, PVC conduit 18 in (NEC Table 300.5); 12 in is allowed only for a GFCI-protected 20 A circuit.
- A 100 A feeder 150 ft to a shop needs 1/0 aluminum to hold 3 percent drop, one size above what ampacity alone requires.
- The 5 percent is a total: a subpanel feeder that already drops 3 percent leaves only 2 percent for every branch circuit off it.
- Measure the cable path, not the site plan: 100 ft across the yard is 130 ft with the trench drops and wall runs added.
Key facts
- 12 AWG copper has a resistance of about 1.93 ohms per 1,000 ft at 75°C (NEC Chapter 9 Table 8), so a 100 ft circuit, which has 200 ft of conductor, adds about 0.39 ohm to the loop.
- Voltage drop is proportional to both current and length: doubling the load or doubling the run doubles the drop, while moving up three wire sizes roughly halves it.
- Incandescent light output falls about 3.5% for every 1% of voltage drop, so a 5% drop cuts a lamp's light output by roughly 17%.
- NEMA MG-1 rates motors for plus or minus 10% of nameplate voltage; below that they draw more current, run hotter and lose torque, which is why long motor circuits are sized for drop rather than ampacity.
- At 240 V a circuit drops the same number of volts as at 120 V with the same wire, amps and length, but half the percentage, which is why long runs to outbuildings favor 240 V.
Frequently asked questions
How far can I run 12 gauge wire on a 20 amp circuit?
At 120 V and a full 20 A load, about 46 ft one way before the drop passes 3%; at 15 A the limit is 61 ft. At 240 V those distances double.
Is a 5% voltage drop acceptable?
The NEC recommends no more than 3% on a branch circuit and 5% for the feeder and branch circuit together, but those are informational notes, not enforceable rules. Inspectors rarely fail a residential job on drop alone. What fails is the equipment: motors and heat pumps are rated for plus or minus 10% of nameplate, so a 5% feeder drop plus a 5% branch drop under starting load is where compressors stall and trip.
Why does 240 V have less voltage drop than 120 V?
It does not drop fewer volts; the same wire, amps and length lose the same voltage. But the loss is a smaller share of 240 V, so the percent drop is half, and a 240 V load also draws half the amps for the same power, which halves the volts lost as well.
What size wire do I need to run power to a shed 150 ft away?
A 20 A, 120 V circuit at full load needs 8 AWG to stay near 4% at 150 ft and 6 AWG for 3%, which is a lot of copper for a few lights. The better job is a 30 A, 240 V feeder in 8 AWG copper (2.9%) or 4 AWG aluminum (1.9%) to a small subpanel in the shed: 4-wire, a disconnect and ground rods at the shed, GFCI on the receptacles, and a permit.
Sources and references
- NFPA 70, National Electrical Code (NFPA)
- Voltage Drop Calculations, IAEI Magazine (IAEI)
- Re3 Voltage Drop Calculator (Southwire)
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