TryBuildCalcMax Wire Length Calculator (Maximum Distance by Voltage Drop)
Find the maximum wire run length.
🕒 Last updated: August 27, 2026
Inputs
ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).
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.
ℹ️The target — this calculator solves for the length that hits exactly this percentage.
Cost
Maximum Run Length: 45.6 ft (13.9 m)
Within your 3.0% allowed limit
Circuit Details
| Wire size | 12 AWG (Copper) |
| Load current | 20.0 A |
| System | 120 V, single-phase |
| Length at max drop | 45.6 ft (13.9 m) |
| Receiving-end voltage | 116.4 V |
| Ampacity sanity check (75°C, no derating) | Looks adequate (25 A base) |
Compare Across Wire Sizes
| AWG | Max Length | Result |
|---|---|---|
| 14 AWG | 28.7 ft | Pass |
| 12 AWG (selected) | 45.6 ft | Pass |
| 10 AWG | 72.4 ft | Pass |
| 8 AWG | 115.2 ft | Pass |
| 6 AWG | 183.1 ft | Pass |
| 4 AWG | 291.2 ft | Pass |
| 3 AWG | 367.1 ft | Pass |
| 2 AWG | 463.0 ft | Pass |
| 1 AWG | 583.9 ft | Pass |
| 1/0 AWG | 736.7 ft | Pass |
| 2/0 AWG | 928.6 ft | Pass |
| 3/0 AWG | 1,170.7 ft | Pass |
| 4/0 AWG | 1,476.3 ft | 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 →
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Voltage Drop Calculator.
Maximum wire run length by voltage drop
This page is pre-set to solve for maximum length — the direct answer to "how far can this wire go?" given a load current, voltage, and your allowed drop percentage.
Edit the wire size, load, or maximum drop above and the maximum length recalculates immediately.
- Solves the voltage-drop formula for distance instead of voltage.
- Comparison table shows the maximum length for every standard wire size at the same load.
- Switch Solve For back to "Voltage Drop" to check a specific length instead.
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 | Maximum length, 3.0% max | Sets which formula direction runs and the pass/fail threshold |
Step 1 — Maximum Length
| Calculation | Result |
|---|---|
| Maximum length at 3.0% drop | 45.6 ft (13.9 m) |
| Receiving-end voltage | 116.4 V (passes the 3.0% limit) |
Therefore, 12 AWG copper carrying 20.0A at 120V can run up to 45.6 ft before exceeding 3.0% drop.
Related Calculators
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.