Voltage Drop Calculator (Drop %, Volts & Max Length by Wire Size)

Calculate voltage drop instantly.

Use this voltage drop calculator to check a specific wire size's voltage drop, or solve for the maximum run length that stays within your allowed drop percentage — with a side-by-side comparison across every standard AWG size.

🕒 Last updated: August 27, 2026

Inputs

Solve For

Load Entered As

Load Current

System Voltage

ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).

Phase

▾

Conductor Material

▾

Wire Size (AWG)

▾

ℹ️Not sure what size you have (or need)? Use the Wire Size Calculator instead — this tool checks a specific size you already have in mind.

One-Way Run Length

ℹ️Distance from the panel/breaker to the load, one way — not round trip.

Max Voltage Drop Allowed

▾

ℹ️The threshold your result is checked against.

Cost

Enable Cost Estimation?

Voltage Drop: 3.29% (3.95 V)

Exceeds your 3.0% allowed limit

Circuit Details

Wire size	12 AWG (Copper)
Load current	20.0 A
System	120 V, single-phase
One-way run length	50.0 ft (15.2 m)
Receiving-end voltage	116.1 V
Ampacity sanity check (75°C, no derating)	Looks adequate (25 A base)

Compare Across Wire Sizes

AWG	Drop %	Result
14 AWG	5.23%	Fail
12 AWG (selected)	3.29%	Fail
10 AWG	2.07%	Pass
8 AWG	1.30%	Pass
6 AWG	0.82%	Pass
4 AWG	0.52%	Pass
3 AWG	0.41%	Pass
2 AWG	0.32%	Pass
1 AWG	0.26%	Pass
1/0 AWG	0.20%	Pass
2/0 AWG	0.16%	Pass
3/0 AWG	0.13%	Pass
4/0 AWG	0.10%	Pass

Row highlighted in blue is your selected wire size. "Pass"/"Fail" is against your 3.0% allowed drop only — always confirm ampacity separately with the Wire Size Calculator.

Assumptions Used

Voltage drop uses K = 12.9 (copper) / 21.2 (aluminum) ohm-cmil/ft — the same formula and constants as the Wire Size Calculator, so results stay consistent between the two tools. The ampacity sanity check is informational only (base 75°C table, no derating) — use the Wire Size Calculator for a full ampacity and breaker sizing check.

Need to find the right wire size from scratch instead of checking one? Wire Size Calculator →

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What Is a Voltage Drop Calculator?

A voltage drop calculator checks how much voltage is lost between a source (panel or breaker) and a load over a specific wire size and run length — telling you the drop in volts and percent, the actual voltage the load receives, and whether that stays within an acceptable limit (commonly 3% for a branch circuit).

Unlike a wire-sizing tool that recommends a gauge from your load, this calculator starts from a wire size you already have in mind (existing wiring, a specific product, a size you're considering) and tells you whether it actually works for your run — or, in reverse, how far that wire can run before it doesn't.

Why checking voltage drop matters:

Excessive drop causes dim lighting, motor overheating, and malfunctioning voltage-sensitive electronics — even on a wire that's perfectly fine on ampacity
Long runs to detached garages, workshops, and well pumps are the most common case where drop, not ampacity, is the limiting factor
Knowing the maximum length for a given wire size upfront avoids an expensive mid-project surprise once conduit and walls are already closed up
Comparing drop across several wire sizes side by side makes the cost-vs-performance tradeoff of upsizing a wire concrete, not guesswork

Voltage Drop Formula: How Is It Calculated?

The same underlying resistance-based formula runs in both directions — solving for drop given a length, or solving for the maximum length given a target drop percentage.

Step 1 — Voltage Drop (given length)

Voltage Drop (V) = (2 × K × I × D) / CM [single-phase]

Voltage Drop (V) = (1.732 × K × I × D) / CM [three-phase]

Drop % = (Voltage Drop ÷ System Voltage) × 100

K is a resistivity constant (12.9 for copper, 21.2 for aluminum, ohm-circular-mil/ft), I is the load current, D is the one-way run length in feet, and CM is the selected wire size's circular-mil area. This is the same formula and constants used by the Wire Size Calculator, so both tools always agree on the same inputs.

Step 2 — Maximum Length (given target drop %)

Allowed Voltage Drop (V) = System Voltage × Max Drop % ÷ 100

Maximum Length (ft) = (Allowed Voltage Drop × CM) ÷ (2 × K × I) [single-phase, 1.732 for three-phase]

This is the same formula solved for distance instead of voltage — useful when the question is "how far can this wire go?" rather than "what's the drop at this length?"

Step 3 — Receiving-End Voltage & Pass/Fail

Receiving Voltage = System Voltage − Voltage Drop

Passes = Drop % ≤ Max Allowed %

The result is checked against your selected maximum (commonly 3% for a single branch circuit or feeder, per NEC's recommendation) and flagged as passing or failing that threshold.

Worked Example

This example walks through your current inputs above, using the same steps as the Formula section.

Input Values Used

Input	Value	Why it is used
Wire size / material	12 AWG, copper	Sets circular-mil area and resistivity constant K
Load current / voltage	20.0 A, 120 V, single-phase	Sets current and the drop-percent base
Solve for / max drop	Voltage drop, 3.0% max	Sets which formula direction runs and the pass/fail threshold

Step 1 — Voltage Drop

Calculation	Result
Voltage drop at 50.0 ft	3.95 V (3.29%)
Receiving-end voltage	116.1 V (fails the 3.0% limit)

Therefore, 12 AWG copper carrying 20.0A at 120V over 50.0 ft has a 3.29% drop, which fails your 3.0% limit.

Copper Voltage Drop Reference (Single-Phase, 120V)

Approximate voltage drop percent at 120V single-phase for a 20A load, by wire size and run length — this calculator's own output, for a quick sanity check before a detailed calculation with your actual current and voltage.

AWG	50 ft	100 ft	150 ft
12 AWG	3.3%	6.6%	9.9%
10 AWG	2.1%	4.1%	6.2%
8 AWG	1.3%	2.6%	3.9%
6 AWG	0.8%	1.6%	2.5%

Figures assume 20A copper, single-phase, 120V — your actual current, voltage, and phase change these numbers directly. Use the calculator above for your exact circuit.

When should you use this voltage drop calculator?

Checking whether a specific, already-chosen wire size will have acceptable voltage drop for your run.
Finding the maximum run length a given wire size supports before exceeding your allowed drop.
Comparing voltage drop across several wire sizes to decide whether upsizing is worth it.
Diagnosing dim lighting, motor overheating, or flaky electronics on an existing long circuit run.
Sizing a generator, well pump, or detached-structure feeder where distance is the dominant constraint.

Quick Voltage Drop Tips

Use Solve For "Max Length" when planning a new long run — it tells you the ceiling before you even pick a length to test.
Check the comparison table before committing to a wire size — the next size up often costs less than expected for a meaningfully lower drop.
3% is the NEC-recommended maximum for a single branch circuit; use 5% only when it's the combined feeder-plus-branch total, not one segment alone.
This tool's ampacity note is a sanity check only — always confirm ampacity, derating, and breaker size with the Wire Size Calculator before finalizing.
Measure run length as the actual routed distance, not the straight-line distance between source and load.

Common Mistakes

Assuming a wire that passes ampacity automatically passes voltage drop — they're two independent checks, and long runs commonly fail drop while passing ampacity easily.
Using round-trip distance instead of one-way distance — this formula's constant already accounts for the return conductor, so doubling the length again overstates the drop.
Applying the single-phase 2x multiplier to a three-phase circuit (or vice versa) — the two use genuinely different multipliers (2 vs 1.732), not interchangeable defaults.
Treating this calculator's informational ampacity note as a full ampacity check — it skips derating, continuous-load factors, and breaker sizing entirely.
Forgetting that aluminum needs a meaningfully larger gauge than copper for the same drop — its resistivity constant is over 60% higher.

Limitations

Voltage drop uses a simplified resistance-only formula (no reactance) — standard practice for most branch/feeder sizing, but not exact for very long, high-current three-phase runs.
The ampacity note is informational only (base 75°C table, no derating, no continuous-load factor, no breaker sizing) — use the Wire Size Calculator for a full ampacity check.
Covers standard AWG conductors up to 4/0 (~260A) — larger service/feeder conductors sized in kcmil are outside this calculator's scope.
NEC-based only (US) — other regions use different tables and conventions (IEC/BS 7671 and similar) and this calculator does not model them.

Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.

FAQ

This calculator starts from a wire size you already have in mind (existing wiring, a specific product, a size you're considering) and tells you the voltage drop for a given length, or the maximum length before drop becomes excessive. The Wire Size Calculator works the other direction — starting from your load, it searches every standard AWG size and recommends the smallest one that satisfies both ampacity and voltage drop together, plus derating, continuous-load factors, and breaker/conduit sizing. Use this tool to check or compare specific sizes; use the Wire Size Calculator to find the right size from scratch.

Set "Solve For" to "Max Length", enter your wire size, load current, voltage, and phase, and choose your maximum allowed drop percentage — the calculator solves the same formula for distance instead of voltage, giving you the length at which the drop hits exactly that percentage. This is the direct answer to "how far can this wire go?" without having to guess a length and check it repeatedly.

They're two completely independent checks. Ampacity is about heat — how much current a conductor can carry without overheating — and doesn't care how long the run is. Voltage drop is about resistance accumulating over distance — the longer the run, the more voltage is lost, regardless of whether the wire is running hot. A wire can be comfortably within its ampacity rating and still lose too much voltage over a long run, or (less commonly) be marginal on ampacity while easily passing drop on a short run. Both checks need to pass independently — this calculator focuses on drop, with only an informational ampacity sanity note; use the Wire Size Calculator for the full ampacity check.

3% is the NEC-recommended maximum for a single branch circuit or feeder considered on its own. 5% is the recommended combined maximum when a feeder and its branch circuits are considered together as one path from the service to the load — don't apply 5% to a single segment in isolation, since that leaves no margin for the rest of the path. For voltage-sensitive equipment (some electronics, certain motor loads), check the manufacturer's own tolerance, which may call for a tighter 2% target.

Aluminum has meaningfully higher electrical resistivity than copper — reflected in this calculator's K constant (21.2 for aluminum vs. 12.9 for copper, ohm-circular-mil/ft), a difference of over 60%. For the same current and length, an aluminum conductor of the same AWG size will show a noticeably higher voltage drop than copper — which is why aluminum feeders are typically specified one or more sizes larger than the copper equivalent for the same job.

Yes, significantly, for the same power delivered. Voltage drop in volts stays similar for a given current and length, but drop *percent* is that voltage divided by the system voltage — so the same current at 240V shows half the drop percentage it would at 120V, and delivering the same power (watts) at a higher voltage also means a proportionally lower current in the first place, compounding the improvement. This is a core reason higher-voltage circuits are often specified for long runs.

It can inform that decision by showing the drop or max length for each individual wire size in the comparison table, but it doesn't model splitting a single load across two circuits directly — that's a load-balancing and code-compliance decision (with its own panel capacity and circuit-count considerations) that goes beyond a single-circuit voltage-drop check, and is worth discussing with a licensed electrician for anything beyond a simple single run.

Yes — turn on "Enable Cost Estimation", choose a currency, and enter a rate per foot of wire. The estimate multiplies your rate by the one-way run length (or the calculated maximum length, in Max Length mode). This is a rough material-cost estimate only — it doesn't include conduit, boxes, labor, or any other part of the installation.

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