TryBuildCalcGenerator Wire Size Calculator (Generator-to-Panel Feeder Sizing)
Size generator feeder wire.
🕒 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: 8 AWG (Copper)
Sized by ampacity, not voltage drop — recommended breaker/OCPD: 40A
Ampacity Check
Design current: 40.0 A
Minimum size by ampacity: 8 AWG
Derating factor applied: 1.00×
Final wire's derated ampacity: 50.0 A
Voltage Drop Check
Actual load current: 40.0 A
Minimum size by voltage drop: 8 AWG
Allowed drop: 3.0%
Final wire's actual drop: 1.95% (4.69 V)
Circuit Summary
| System | 240 V, single-phase |
| One-way run length | 75.0 ft (22.9 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.
Generator feeder wire sizing
A standby or large portable generator is often installed some distance from the house's transfer switch or panel — this page is pre-set to a 75ft run at a common 40A generator output.
Edit the load (matching your specific generator's rated output), voltage, and length above to size the actual feeder for your installation.
- Use the generator's actual rated output current or wattage, not an assumed figure.
- Outdoor generator-to-panel runs are a common case where voltage drop governs over ampacity.
- Confirm local code requirements for generator interconnection — this typically needs a permitted transfer switch, not just a feeder wire.
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 | 40.0 A, 240 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), 75.0 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 | 40.0 A |
| Minimum size by ampacity | 8 AWG |
Step 2 — Voltage Drop Check
| Calculation | Result |
|---|---|
| Minimum size by voltage drop | 8 AWG |
| Final recommended size | 8 AWG (governed by ampacity) |
Therefore, for a 40.0A load over 75.0 ft at 240V, you need approximately 8 AWG copper wire on a 40A 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.