Generator Wire Size Calculator
// SCREEN FEEDER CONDUCTOR SIZE FOR GENERATOR INLETS, TRANSFER SWITCHES, AND STANDBY DISTRIBUTION RUNS FROM KW, VOLTAGE, DISTANCE, AND DROP TARGET //
Enter generator nameplate real-power output in kilowatts for a first-pass conductor screen.
Pick the actual generator output or feeder voltage so current and drop are checked on the right basis.
Use 1.00 for resistive or nameplate kW planning, or enter a lower PF when generator loading is motor-heavy.
Enter one-way conductor length in feet between the generator source and the connected equipment.
Enable this when you want a 125% planning screen for sustained generator loading instead of a straight nameplate check.
12 kW at 240 V, single-phase, PF 1.00
100% design screen shown for intermittent or nameplate-based planning.
8.4 mm² equivalent, 50.0 A ampacity screen, 5.73 V estimated drop (2.39%).
Each row checks the selected conductor material and terminal rating against both required ampacity and estimated voltage drop for the entered run length.
| Size | Ampacity | Resistance | Voltage Drop | Status |
|---|---|---|---|---|
14 AWG 2.1 mm² | 20.0 A | 3.0700 ohm/kft | 23.0 V / 9.59% | Upsize |
12 AWG 3.3 mm² | 25.0 A | 1.9300 ohm/kft | 14.5 V / 6.03% | Upsize |
10 AWG 5.3 mm² | 35.0 A | 1.2100 ohm/kft | 9.07 V / 3.78% | Upsize |
8 AWG 8.4 mm² | 50.0 A | 0.7640 ohm/kft | 5.73 V / 2.39% | Pass |
6 AWG 13.3 mm² | 65.0 A | 0.4910 ohm/kft | 3.68 V / 1.53% | Pass |
4 AWG 21.1 mm² | 85.0 A | 0.3080 ohm/kft | 2.31 V / 0.96% | Pass |
3 AWG 26.7 mm² | 100 A | 0.2450 ohm/kft | 1.84 V / 0.77% | Pass |
2 AWG 33.6 mm² | 115 A | 0.1940 ohm/kft | 1.46 V / 0.61% | Pass |
1 AWG 42.4 mm² | 130 A | 0.1540 ohm/kft | 1.16 V / 0.48% | Pass |
1/0 AWG 53.5 mm² | 150 A | 0.1220 ohm/kft | 0.91 V / 0.38% | Pass |
2/0 AWG 67.4 mm² | 175 A | 0.0967 ohm/kft | 0.73 V / 0.30% | Pass |
3/0 AWG 85.0 mm² | 200 A | 0.0766 ohm/kft | 0.57 V / 0.24% | Pass |
4/0 AWG 107.2 mm² | 230 A | 0.0608 ohm/kft | 0.46 V / 0.19% | Pass |
250 kcmil 126.7 mm² | 255 A | 0.0515 ohm/kft | 0.39 V / 0.16% | Pass |
300 kcmil 152.0 mm² | 285 A | 0.0429 ohm/kft | 0.32 V / 0.13% | Pass |
350 kcmil 177.3 mm² | 310 A | 0.0367 ohm/kft | 0.28 V / 0.11% | Pass |
400 kcmil 202.7 mm² | 335 A | 0.0321 ohm/kft | 0.24 V / 0.10% | Pass |
500 kcmil 253.3 mm² | 380 A | 0.0258 ohm/kft | 0.19 V / 0.08% | Pass |
Use this calculator for fixed wiring between a generator, inlet, transfer switch, disconnect, or downstream distribution equipment. If you are sizing a portable cord set instead, use the extension cord tool and verify the cord listing.
- Generator feeder conductors still need final review for overcurrent protection, transfer equipment listings, and any neutral or grounding rules that apply to the installation.
- Voltage-drop limits here are planning targets, not direct NEC mandates. Sensitive equipment and motor starting often justify a tighter drop limit.
- This tool is for fixed wiring between a generator and downstream equipment, not for flexible portable cord sets or temporary wiring approvals.
7.5 kW portable generator to 30A inlet
At 240 V single-phase and 60 ft one way, the calculated current is about 31.3 A. 10 AWG copper has enough ampacity but usually pushes above a tight 2% drop limit, so 8 AWG becomes the cleaner fixed-wiring choice.
20 kW standby generator to transfer switch
A 20 kW, 120/240 V single-phase unit delivers about 83.3 A at PF 1.00. With a 100 ft run and 3% target, 3 AWG copper can be a practical planning result before final OCPD, neutral sizing, and grounding checks.
60 kW three-phase generator feeder
On 480 V three-phase at PF 0.9, load current is roughly 80 A. Voltage drop is modest at this voltage, so the selection is often driven by ampacity, terminals, and installation conditions rather than distance alone.
Voltage Drop Calculator
Run a broader AC voltage-drop check after the generator circuit layout is known.
Ampacity Calculator
Review terminal temperature, bundling, and derating before final conductor release.
Conduit Fill Calculator
Check raceway occupancy once phase, neutral, and grounding conductors are selected.
Cable Size Calculator
Compare the generator feeder result with a broader residential or commercial sizing workflow.
Wire Resistance Calculator
Audit raw conductor resistance if you want to inspect the voltage-drop math directly.
Wire Weight Calculator
Estimate pull weight and reel handling for larger standby generator feeders.