TryBuildCalcGenerator Voltage Drop Calculator (Generator-to-Panel Feeder Drop Check)
Check generator feeder voltage drop.
🕒 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.
ℹ️Distance from the panel/breaker to the load, one way — not round trip.
ℹ️The threshold your result is checked against.
Cost
Voltage Drop: 1.95% (4.69 V)
Within your 3.0% allowed limit
Circuit Details
| Wire size | 8 AWG (Copper) |
| Load current | 40.0 A |
| System | 240 V, single-phase |
| One-way run length | 75.0 ft (22.9 m) |
| Receiving-end voltage | 235.3 V |
| Ampacity sanity check (75°C, no derating) | Looks adequate (50 A base) |
Compare Across Wire Sizes
| AWG | Drop % | Result |
|---|---|---|
| 14 AWG | 7.85% | Fail |
| 12 AWG | 4.94% | Fail |
| 10 AWG | 3.11% | Fail |
| 8 AWG (selected) | 1.95% | Pass |
| 6 AWG | 1.23% | Pass |
| 4 AWG | 0.77% | Pass |
| 3 AWG | 0.61% | Pass |
| 2 AWG | 0.49% | Pass |
| 1 AWG | 0.39% | Pass |
| 1/0 AWG | 0.31% | Pass |
| 2/0 AWG | 0.24% | Pass |
| 3/0 AWG | 0.19% | Pass |
| 4/0 AWG | 0.15% | 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.
Generator feeder voltage drop check
A standby or large portable generator is often installed some distance from the transfer switch or panel — this page is pre-set to a 75ft run at a common 40A generator output on 8 AWG copper.
Edit the load (matching your specific generator's rated output), wire size, and length above to check your actual installation.
- Use the generator's actual rated output current or wattage, not the connected appliance loads.
- Outdoor generator-to-panel runs are a common case where voltage drop, not ampacity, ends up governing.
- Compare wire sizes in the table below before running conduit — upsizing later is expensive.
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 | 8 AWG, copper | Sets circular-mil area and resistivity constant K |
| Load current / voltage | 40.0 A, 240 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 75.0 ft | 4.69 V (1.95%) |
| Receiving-end voltage | 235.3 V (passes the 3.0% limit) |
Therefore, 8 AWG copper carrying 40.0A at 240V over 75.0 ft has a 1.95% drop, which passes your 3.0% limit.
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.