Quick answer
Voltage drop is the voltage a circuit loses along its wires because the wire itself has resistance, so the device at the far end gets slightly less voltage than the panel sends out.
It grows with longer runs, thinner wire and heavier loads. The National Electrical Code suggests, but does not require, keeping it near 3% on a branch circuit.
Quick facts
The key facts about Voltage Drop, with sources:
- Measured in
- Volts, or as a percent of the supply voltage
- Basic formula
- Voltage drop = current × resistance of the wire loop
- NEC branch-circuit note
- About 3% at the farthest outlet (advisory) 2
- NEC feeder + branch note
- About 5% combined (advisory) 2
- 12 AWG copper resistance
- 1.62 ohms per 1,000 ft at 77°F 1
- Biggest causes
- Run length, wire size, load current, wire material
Key takeaways
- Every wire has resistance, so some voltage is always lost between the panel and the load.
- Double the length or halve the wire area and the drop roughly doubles.
- The NEC notes suggest about 3% for a branch circuit and 5% for feeder plus branch, but they are advice, not rules 2.
- Too much drop shows up as dim lights, slow motors, warm wires and chargers that throttle.
- The usual fix on long runs is a larger wire size, chosen by an electrician during design.
What voltage drop means
Think of a garden hose. The longer and thinner the hose, the weaker the stream at the nozzle. Wire works the same way. Its resistance turns a small part of the electrical push into heat along the way.
That push is voltage. A US home circuit starts near 120 volts or 240 volts at the panel. By the time current reaches a light, a pump or an EV charger at the end of a long run, a few volts may be gone.
A small drop is normal and harmless. The question is how much. Electricians state it as a percent of the supply voltage, so a 3.6-volt loss on a 120-volt circuit is a 3% drop.
How voltage drop happens
Current has to travel out on the hot wire and back on the neutral, or out and back on two hots for a 240-volt load. So the loop is twice the one-way distance. Each foot of copper or aluminum adds a little resistance.
Ohm's law ties it together: volts lost equal amps times ohms. More amps, from a bigger load, mean more loss. More ohms, from a longer or thinner wire, also mean more loss. Warm wire has more resistance than cool wire, so a loaded circuit drops a bit more than a cold one 1.
Aluminum has more resistance than copper of the same gauge. That is one reason aluminum feeders are usually one or two sizes larger than the copper they replace.
VD = I × (2 × L × R ÷ 1,000)
VD- voltage drop in volts
I- load current in amps
L- one-way run length in feet
R- wire resistance in ohms per 1,000 feet
2- out and back: the current travels the run twice
Percent drop = VD ÷ supply voltage × 100. Resistance values from a published copper table 1.
Source: [1]
Where the drop builds up: the parts of a run
Electricians split the path from the utility to the load into parts. Each one adds its own share of drop.
| Part of the path | What it is | Typical concern |
|---|---|---|
| Service conductors | Utility drop and service entrance conductors to the meter and panel | Mostly the utility's side; not in the NEC notes |
| Feeder | Wire from the main panel to a subpanel in a garage, barn or ADU | Long feeders to outbuildings |
| Branch circuit | Wire from a breaker to the outlets or the one appliance it serves | The farthest outlet on a long circuit |
| Cords | Extension cords and appliance cords | Thin, long cords on heaters, saws and chargers |
The 3% and 5% figures cover the feeder and branch circuit together, not the utility side 2.
Worked example: a 100-foot run to a shop
An example with published copper resistance (149°F column, closer to a loaded wire) 1. A 120-volt circuit feeds a 16-amp load 100 feet from the panel.
| Wire | Loop resistance (200 ft) | Volts lost at 16 A | Percent of 120 V |
|---|---|---|---|
| 12 AWG copper | 200 × 1.87 ÷ 1,000 = 0.374 Ω | 16 × 0.374 = 5.98 V | 5.0% |
| 10 AWG copper | 200 × 1.18 ÷ 1,000 = 0.236 Ω | 16 × 0.236 = 3.78 V | 3.1% |
| 8 AWG copper | 200 × 0.739 ÷ 1,000 = 0.148 Ω | 16 × 0.148 = 2.37 V | 2.0% |
Example only. The 12 AWG run is within its breaker rating but well past the 3% note; one size up lands near it. A real design also checks the breaker, the terminals and the ampacity table. Try your own numbers in the voltage drop calculator.
Where voltage drop shows up in a home
- Detached garages, shops and barns. Feeders of 75 to 200 feet are common, and the loads are motors and heaters.
- EV charging in a far parking spot. A Level 2 charger pulls steady current for hours, so drop costs energy every session.
- Well pumps and pool equipment. Motors at the end of long underground runs start harder at low voltage.
- Landscape and outdoor lighting. Low-voltage systems lose a larger share of their voltage per foot.
- Heat pump condensers. Long runs to an outdoor unit can matter at startup.
- Extension cords. A thin cord on a space heater is the most common household case.
Why keeping drop low pays off
Benefits of a low-drop design
- Motors start easily and run cooler, which helps them last.
- Lights hold steady instead of dimming when a load kicks on.
- Less energy is wasted as heat in the walls.
- EV chargers and electronics see the voltage they were designed for.
Costs of ignoring it
- Thicker wire costs more and is harder to pull.
- Larger wire may need bigger conduit, boxes or terminals.
- The breaker size does not change; it still protects the original circuit.
Limits of the 3% rule of thumb
The 3% and 5% figures sit in Informational Notes in the NEC. Notes explain; they do not require 2. An inspector usually cannot fail a job only on drop, unless a local rule, an energy code or the equipment instructions set a limit.
Some sensitive-electronics systems under NEC Article 647 use tighter figures of 1.5% and 2.5% 2. Trade writers call 3% and 5% reasonable but rarely optimal 2.
Calculators also simplify. They assume a steady load and a set wire temperature. Motor starting current is several times running current, so a pump can sag far more for a moment than the steady-state number shows.
What drives the cost of fixing voltage drop
Fixing drop on an existing run usually means new, larger wire. The cost drivers are the length of the run, the wire size and metal, whether conduit can be reused or must be replaced, trenching for underground runs, and wall or ceiling repair.
Copper wire is priced by weight, and larger gauges hold much more metal per foot 1. Aluminum feeders are often chosen on long runs to keep material costs down.
We do not publish dollar figures without dated quotes. See what drives electrical costs or model a project with the project cost tool.
How an electrician designs for voltage drop
- Measures or estimates the one-way run length from panel to load.
- Takes the load current from the nameplate or a load calculation.
- Picks a wire size that meets the NEC ampacity tables for the breaker.
- Checks voltage drop at that size and upsizes the wire if the drop is too high.
- Confirms terminals, boxes and conduit can accept the larger wire.
- Pulls the permit where required and schedules inspection.
Safety and risk
Do not try to fix low voltage by swapping breakers or splicing in heavier wire yourself. A larger breaker on the same wire can let the wire overheat. Hot cords, scorched plugs or dimming with a burning smell mean unplug the load and call an electrician.
Keeping voltage drop in check over time
- Notice lights that dim when the AC, a pump or a saw starts. Note which circuit and when.
- Use the shortest, heaviest cord that fits the job, and never run heaters on thin cords.
- Ask for a check during an electrical inspection if you add big loads to an old run.
- Loose or corroded connections add resistance just like long wire. Warm cover plates are a warning sign.
Warning signs and when to call a pro
Most voltage drop is a design issue, not an emergency. Five signs point to something more serious.
- Lights across the house dim or flicker together. That can point to a service or neutral problem, not drop.
- One outlet or switch plate feels warm, smells hot or is discolored.
- A motor hums but struggles to start, or trips its breaker often.
- An EV charger reports low voltage and cuts its charging rate.
- Any burning smell: turn off the circuit at the breaker if safe and call an emergency electrician.
What the code says
The NEC is NFPA 70, updated every three years; the 2023 edition is widely adopted and the 2026 edition is published 3.
A 2015 EC&M trade article places the 3% branch-circuit figure in Informational Note No. 4 to Section 210.19(A) and the 5% combined figure in a companion Informational Note 2.
Note numbers can move between editions, so ask which edition your inspector enforces.
An Informational Note under 310.15 also warns that the ampacity tables do not account for voltage drop 2. In other words, a wire can be legal for its breaker and still lose too much voltage on a long run.
Some equipment instructions and energy codes set firmer limits. Installers must follow listed equipment instructions under OSHA and the NEC alike 5.
Voltage drop vs related problems
| Term | What happens | Typical cause |
|---|---|---|
| Voltage drop | Steady small loss along a run | Long or thin wire under load |
| Brownout | Supply voltage sags for the whole area | Utility grid under strain |
| Open neutral | Voltages swing high and low between circuits | Broken or loose neutral |
| Overload | Breaker trips after minutes | Too much load on one circuit |
| Power surge | Brief spike above normal | Lightning, utility switching, motors |
Common misconceptions
- Myth A bigger breaker fixes low voltage.
- Reality No. The breaker protects the wire. A larger breaker on the same wire adds fire risk and does nothing for drop.
- Myth The NEC requires 3%.
- Reality The 3% and 5% figures are Informational Notes, which are advisory 2.
- Myth Voltage drop only matters for big commercial jobs.
- Reality Long home runs to garages, wells and chargers hit it too.
- Myth If the wire is rated for the amps, drop is fine.
- Reality Ampacity tables ignore voltage drop 2; length still matters.
How your state changes the picture
Voltage drop physics is the same in every state. What changes is which NEC edition your state or town enforces, and whether a local energy code adds a firm limit. NFPA tracks where each edition is in force 4. Look up yours before a long feeder job.
When voltage drop matters to you
Voltage drop matters most when you add a big load far from the panel: a shop, an ADU, an EV charger, a well pump. Ask the electrician what drop the design gives at full load and what wire size gets it near 3%.
Start with the wire size calculator; manufacturer tools work the same way 6, then get the run checked by a licensed pro. For chargers, the EV charger readiness tool flags long runs early.
- If Run under about 50 feet on a normal outlet circuit
- Then Usually fine; standard sizing covers it
- If Run of 100 feet or more, or a motor or charger load
- Then Ask for a voltage drop calculation
- If Lights dim when a load starts
- Then Have the circuit and connections inspected
- If Burning smell or warm plate
- Then Shut off the circuit and call an electrician now
A quick rule for quotes
Tip
When a quote lists a wire size for a long run, ask: "What is the voltage drop at full load?" A clear number near or below 3% shows the run was designed, not guessed.
Questions about Voltage Drop
What is an acceptable voltage drop?
About 3% on a branch circuit and 5% for feeder and branch combined is the common target. Those figures come from Informational Notes in the National Electrical Code, which are advice rather than rules.
Sensitive electronics systems under NEC Article 647 use tighter figures of 1.5% and 2.5%. Equipment instructions or a local energy code can also set a limit, and those do bind.
Is voltage drop required by the NEC?
No, not for ordinary circuits. The NEC mentions voltage drop in Informational Notes, which explain good practice but are not enforceable. Inspectors check ampacity, breaker size and installation. A few special articles and some energy codes set limits, and listed equipment instructions must be followed, so check local rules before assuming none apply.
How do you calculate voltage drop?
Multiply the current by the resistance of the full wire loop. For a single-phase run, the loop is twice the one-way length.
For example, 100 feet of 12 AWG copper is 200 feet of wire at about 1.87 ohms per 1,000 feet when warm, or 0.374 ohms.
At 16 amps that is about 6 volts, or 5% of 120 volts.
What causes voltage drop?
Wire resistance causes it. Every foot of wire has some resistance, and current flowing through it turns part of the voltage into heat.
Longer runs, thinner wire, heavier loads, warm wire and aluminum instead of copper all raise it.
Loose or corroded connections add resistance too, which is why a bad splice can mimic a long run.
What are the symptoms of voltage drop?
Dim or flickering lights when a load starts is the most common sign.
Others are motors that hum and start slowly, heaters that run weak, EV chargers that lower their charging rate, and warm cords or outlets.
If the whole house flickers together, the cause may be a service or neutral problem rather than ordinary drop.
Does a bigger wire reduce voltage drop?
Yes. A larger wire, meaning a smaller AWG number, has less resistance per foot.
Published copper tables list 12 AWG at 1.62 ohms per 1,000 feet and 10 AWG at 1.02, so the move cuts drop by roughly a third.
The breaker usually stays the same size, because it protects the circuit, not the run length.
How far can you run 12-gauge wire on a 20-amp circuit?
There is no single legal limit. Distance is set by how much drop you accept at the real load. At 16 amps on 120 volts, 12 AWG copper reaches about 3% at roughly 60 feet one way using warm-wire resistance. Lighter loads go farther. An electrician runs the numbers for your load and run.
Does voltage drop waste electricity?
Yes, a little. The lost voltage becomes heat in the wire, and you pay for it on the meter. On a short household circuit the loss is small.
On a long run feeding a steady load, such as an EV charger for hours, it adds up over a year, which is one reason designers upsize long runs.
Is voltage drop worse with aluminum wire?
Yes, for the same gauge. Aluminum has more resistance than copper, so aluminum feeders are usually one or two sizes larger to carry the same load with similar drop. Larger aluminum feeders are common and acceptable when installed with rated terminals. Older small-gauge aluminum branch wiring is a separate safety issue.
Sources
- Engineering ToolBox, Copper Wire: Electrical Resistance vs. Gauge (AWG table), retrieved .
- EC&M, Watch Out for that Voltage Drop (M. Lamendola, 2015), retrieved .
- NFPA, NFPA 70 National Electrical Code, retrieved .
- NFPA, NEC enforcement maps, retrieved .
- OSHA, 29 CFR 1910.303 General requirements (electrical), retrieved .
- Southwire, Voltage Drop Calculator, retrieved .
Expert review
Written by the HyreElectrical Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Resistance values from a published AWG table; NEC note locations from a trade source and checked against NFPA references; worked example computed from those values.
Suggest a correction. We fix errors and say what changed.