TryBuildCalcVoltage Drop Wire Size Calculator (Long Run / Voltage-Drop-Governed Sizing)
Size wire for a long run.
🕒 Last updated: August 26, 2026
Inputs
ℹ️Common values: 120V, 208V, 240V, 277V, 480V (US); 230V, 400V (many other countries).
ℹ️Distance from the panel/breaker to the load, one way — not round trip.
ℹ️The wire's own insulation rating (check the product's markings, e.g. THHN, THWN-2).
ℹ️Per NEC 110.14(C) — the breaker/panel/device's own temperature rating. Most equipment rated 100A or less is only terminal-rated for 60°C or 75°C, even with 90°C wire installed. The calculator automatically uses the lower of this and the insulation rating above.
Advanced (NEC Derating)
Conduit Sizing
Cost
Recommended Wire Size: 6 AWG (Copper)
Sized by voltage drop, not ampacity — recommended breaker/OCPD: 20A
Ampacity Check
Design current: 20.0 A
Minimum size by ampacity: 12 AWG
Derating factor applied: 1.00×
Final wire's derated ampacity: 65.0 A
Voltage Drop Check
Actual load current: 20.0 A
Minimum size by voltage drop: 6 AWG
Allowed drop: 3.0%
Final wire's actual drop: 2.46% (2.95 V)
Circuit Summary
| System | 120 V, single-phase |
| One-way run length | 150.0 ft (45.7 m) |
| Insulation / terminal rating | 75°C (167°F) — most common (THWN-2, terminals) / 75°C (167°F) — most common (THWN-2, terminals) |
| Governing rating (NEC 110.14(C)) | 75°C (167°F) — most common (THWN-2, terminals) |
Assumptions Used
NEC Table 310.16 ampacity, NEC 110.14(C) governing rating, NEC 240.4(D) small-conductor cap, NEC 310.15(C)(1)/(B)(1)(1) derating, NEC Chapter 9 conduit fill (THHN/THWN-2, uniform AWG). Voltage drop uses K = 12.9 (copper) / 21.2 (aluminum) ohm-cmil/ft. This is a reference estimate — confirm against the code edition adopted in your jurisdiction and a licensed electrician before final installation.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Wire Size Calculator.
Voltage-drop-governed wire sizing for long runs
This page is pre-set to a 150ft run — long enough that voltage drop, not ampacity, typically determines the minimum wire size, the common case for detached garages, workshops, and well pumps.
Edit the load, voltage, or length above and both the ampacity and voltage-drop results update, showing which one is governing your specific circuit.
- Pre-set to a 150ft run, a realistic long-circuit reference length.
- Result clearly shows whether ampacity or voltage drop is the governing factor.
- Increase length further to see how the recommended AWG grows for even longer runs.
Wire Size Formula: How Is AWG Gauge Calculated?
Two independent checks are run, and the wire recommended is whichever check requires the larger conductor.
Step 1 — Design Current
Design Current = Load Current × 1.25 (if continuous load, per NEC 210.19/210.20)
Design Current = Load Current (if not continuous)
A continuous load (running 3+ hours) needs a 25% safety margin applied to the wire and breaker sizing, per NEC's continuous-load rule. Voltage drop, further below, uses the actual (non-inflated) current instead.
Step 2 — Ampacity Check
Governing Rating = the LOWER of Insulation Rating and Terminal/Equipment Rating (NEC 110.14(C))
Derated Ampacity = Table Ampacity (at Governing Rating) × Conductor-Count Factor × Ambient-Temp Factor
Pick the smallest AWG where Derated Ampacity ≥ Design Current
Table ampacity comes from NEC Table 310.16, by material (copper/aluminum) and the governing 60/75/90°C column — not just the wire's own insulation rating, since most equipment rated 100A or less is only terminal-rated for 60°C or 75°C even when 90°C wire is installed. 14/12/10 AWG are additionally capped at 15A/20A/30A (copper) regardless of table ampacity, per NEC 240.4(D).
Step 3 — Voltage Drop Check
Voltage Drop (V) = (2 × K × I × D) / CM [single-phase]
Voltage Drop (V) = (1.732 × K × I × D) / CM [three-phase]
Pick the smallest AWG where (Voltage Drop ÷ System Voltage) × 100 ≤ Max Allowed %
K is a resistivity constant (12.9 for copper, 21.2 for aluminum, ohm-circular-mil/ft), I is the actual load current, D is the one-way run length in feet, and CM is the conductor's circular-mil area.
Step 4 — Final Recommendation
Recommended AWG = the larger of the ampacity-check size and the voltage-drop-check size
Both requirements must be satisfied at once, so the calculator always recommends whichever check needs the bigger conductor — never just the smaller of the two.
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 |
|---|---|---|
| Load current / voltage | 20.0 A, 120 V, single-phase | Sets design current and the voltage-drop base |
| Material / rating / length | Copper, governed by 75°C (167°F) — most common (THWN-2, terminals), 150.0 ft | Sets ampacity table column (lower of insulation/terminal rating) and voltage-drop distance |
| Derating / max drop | 1.00× derate, 3.0% max drop | Sets the derated ampacity and voltage-drop threshold |
Step 1 — Ampacity Check
| Calculation | Result |
|---|---|
| Design current | 20.0 A |
| Minimum size by ampacity | 12 AWG |
Step 2 — Voltage Drop Check
| Calculation | Result |
|---|---|
| Minimum size by voltage drop | 6 AWG |
| Final recommended size | 6 AWG (governed by voltage drop) |
Therefore, for a 20.0A load over 150.0 ft at 120V, you need approximately 6 AWG copper wire on a 20A breaker.
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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.