Get in touch with us
Leave a message
Generator Cable Sizing
What Size Wire for a Generator?
Wire size depends on three things: generator amps, cable distance, and system voltage. Higher amps and longer runs need thicker wire; higher voltage lets you use thinner wire for the same power. A 40 kW 120/240 V set draws 175 A — 3/0 AWG copper. A 100 kVA 400 V three-phase set draws 144 A — 35 mm². The full chart below covers both AWG and mm².
Amps
Generator Output
The generator's full-load current at your voltage. This is the primary number that determines your minimum conductor size.
Distance
Cable Run Length
Voltage drop grows with distance. A wire that works at 30 m may be undersized at 60 m. Always check the drop for your run.
Voltage
System Voltage
Higher voltage means lower current for the same power. A 480 V feeder can use smaller wire than a 240 V feeder for the same load.
Planning Values — Then Verify
A 40 kW single-phase generator at 120/240 V draws about 175 amps and needs 3/0 AWG copper. A 100 kVA three-phase set at 400 V draws about 144 amps and needs 35 mm². These are starting points — distance, temperature, and local code can push the size up. Always confirm the final conductor with a qualified electrician.
What You Need
Three Numbers Decide Your Wire Size
01
Generator Amps
The full-load output current of your genset. This is the starting point for every wire size calculation.
02
Cable Distance
The one-way run from generator to transfer switch or panel. Distance drives voltage drop and often forces a size up.
03
System Voltage
120 V, 240 V, 400 V, or 480 V. Higher voltage means lower current and smaller cable for the same power.
04
Distance Changes Everything
A conductor sized correctly for ampacity can still be undersized for voltage drop on a long run. Always check the distance.
NEC 310.16
Generator Wire Size Chart
This chart is the quick reference for sizing the conductor from a generator to a transfer switch or distribution board, based on the NEC 310.16 ampacity table at 75 °C with not more than three current-carrying conductors.
| Wire Size (AWG/kcmil) | Copper 75 °C | Aluminum 75 °C | Typical Breaker |
|---|---|---|---|
| 14 AWG | 20 A | – | 15 A |
| 12 AWG | 25 A | 20 A | 20 A |
| 10 AWG | 35 A | 30 A | 30 A |
| 8 AWG | 50 A | 40 A | 40–50 A |
| 6 AWG | 65 A | 50 A | 60 A |
| 4 AWG | 85 A | 65 A | 70–80 A |
| 3 AWG | 100 A | 75 A | 100 A |
| 2 AWG | 115 A | 90 A | 100–110 A |
| 1 AWG | 130 A | 100 A | 110–125 A |
| 1/0 AWG | 150 A | 120 A | 125–150 A |
| 2/0 AWG | 175 A | 135 A | 150–175 A |
| 3/0 AWG | 200 A | 155 A | 175–200 A |
| 4/0 AWG | 230 A | 180 A | 200–225 A |
| 250 kcmil | 255 A | 205 A | 225–250 A |
| 350 kcmil | 310 A | 250 A | 300 A |
| 500 kcmil | 380 A | 310 A | 350–400 A |
Values per NEC 310.16, 75 °C column, three conductors or fewer, 30 °C ambient. Smaller conductors, higher ambient temperatures, and more conductors in one raceway require derating. Read the chart across: a 100 amp generator feeder needs 3 AWG copper or 1 AWG aluminum at the panel. A 175 amp feeder needs 3/0 AWG copper or 250 kcmil aluminum. Distance can push you down a row.
Metric Three-Phase
Generator kVA to Cable Size Chart
For metric markets and three-phase diesel sets, the chart below gives the copper cable size in mm² for standard 415 V, 50 Hz generators at a 0.8 power factor. These are the values used across the Middle East, Africa, South Asia, and most of Asia for generator-to-switchboard feeders.
| Generator kVA | Full-Load Amps | Copper Cable (mm²) |
|---|---|---|
| 15 kVA | 20 A | 4 |
| 25 kVA | 35 A | 6 |
| 40 kVA | 55 A | 10 |
| 62.5 kVA | 87 A | 16 |
| 82.5 kVA | 115 A | 25 |
| 100 kVA | 140 A | 35 |
| 125 kVA | 175 A | 50 |
| 160 kVA | 225 A | 70 |
| 200 kVA | 280 A | 95 |
| 250 kVA | 350 A | 120 |
| 320 kVA | 445 A | 150 |
Approximate planning values for XLPE-insulated armored copper cable at 415 V, 3-phase, 0.8 PF. Short runs only; distance increases the size. The full-load current in the middle column comes from the standard three-phase formula: amps = kVA × 1000 ÷ (1.732 × voltage). A 100 kVA set at 400 V draws about 144 amps, and a 200 kVA set about 289 amps.
Distance Planning
Generator Cable Size by Distance
Ampacity picks a starting wire size. Distance decides whether you move up, and in practice the generator cable size on your drawing often ends up one row heavier than the ampacity table alone suggests. Voltage drop grows with cable length, and the NEC and most codes hold feeders to 3% maximum drop, with 5% as the total through the system. Cords and generator-to-panel runs are often held to 2%.
The practical effect is a distance rule you can use on site: for a 120 V circuit, plan to go up one conductor size for roughly every 100 feet beyond a short run. The penalty halves at 240 V and drops to about a third at 480 V, because higher voltage carries more power for the same current. Three-phase feeds drop less than single-phase at the same distance and current.
| 3-Phase 50 A Load | Distance | Copper Size |
|---|---|---|
| 400 V | up to 100 ft (30 m) | 6 AWG |
| 400 V | 100–200 ft (30–60 m) | 2 AWG |
| 400 V | 200–400 ft (60–120 m) | 1/0 AWG |
| 400 V | 400–600 ft (120–180 m) | 3/0 AWG |
Planning values for a 50 A three-phase feeder at 400 V. Your exact size depends on code, conductor type, and ambient temperature. Every generator installer has a story about a feeder that measured fine on paper and sagged badly on site because nobody checked the drop.
Transfer Switch Wiring
Generator to Transfer Switch Wiring
The most common permanent generator wiring job, and the heart of most generator transfer switch wiring diagrams, is the run from the generator to the automatic transfer switch, and then to the distribution board. The conductor between the genset and the ATS is sized to the generator's output amps, in both the phases and the neutral, and the ground conductor follows the code for your system.
| Generator | Voltage / Phase | Amps | Copper | Aluminum |
|---|---|---|---|---|
| 30 kW | 120/240 V 1-ph | 125 A | 1/0 AWG | 3/0 AWG |
| 40 kW | 120/240 V 1-ph | 175 A | 3/0 AWG | 250 MCM |
| 40 kW | 277/480 V 3-ph | 60 A | 4 AWG | 3 AWG |
| 100 kW | 277/480 V 3-ph | 150 A | 2/0 AWG | 3/0 AWG |
Generator-to-transfer-switch conductor sizing per the NEC 310.16 75 °C column, 30 °C ambient. Confirm with your ATS documentation and a qualified installer. When you buy a genset from us, we can supply the matching transfer switch and the cable-connection package, so the wire chart and the equipment arrive together.
Download the Generator Wire Size Chart
The full generator wire size chart, in both AWG and mm², is available as a free download in three formats: Excel (.xlsx) for editing, CSV for import into your own tools, and PDF for printing and keeping in the site folder.
The spreadsheet version carries the complete NEC 310.16 ampacity table with the 75 °C copper and aluminum columns, the kVA-to-mm² chart for metric three-phase sets, and the distance/voltage-drop notes. You can filter by your generator's amps, compare copper against aluminum, and print a site-specific copy for the electrician.
To download: enter your details below, and we'll send the current chart to your email or WhatsApp. No spam, and our engineers are available if you want a second opinion on your specific install.
Let's talk
Get a Genset + Cable Quote
Our engineers will review your generator size, run length, and voltage drop, then deliver a factory-direct price — no obligation, no hidden fees. Every quote includes the generator, matching transfer switch, and cable-connection package sized to your exact install.
Frequently Ask Questions
Q: What size wire do I need for my generator?
A: Match the wire to the generator’s output amps: a 40 kW 120/240 V single-phase set draws about 175 A and needs 3/0 AWG copper; a 100 kVA 400 V three-phase set draws about 144 A and needs 35 mm². Distance can push the size up.
Q: What generator wire gauge for a 100 amp generator?
A: A 100 amp generator feeder needs 3 AWG copper or 1 AWG aluminum at the panel per the NEC 310.16 75 °C table, and 2 AWG copper if the run is long enough for voltage drop to matter. Confirm with a qualified electrician.
Q: What size cable for a 50 kW generator?
A: A 50 kW three-phase generator at 400 V, 0.8 power factor draws about 90 amps, which calls for 25 mm² copper cable on short runs. A 50 kW single-phase set at 120/240 V draws about 208 amps and needs 4/0 AWG copper. Distance increases both.
Q: How far can you run generator wire before voltage drop?
A: Feeders should hold voltage drop to 3%, and generator-to-panel runs are often held to 2%. For 120 V circuits, expect to go up one conductor size every 100 feet; the penalty is about half at 240 V and a third at 480 V.
Q: What size wire from generator to transfer switch?
A: Size the conductor to the generator’s output amps, in both phases and neutral. A 30 kW 120/240 V set needs 1/0 AWG copper, a 40 kW set needs 3/0 AWG, and the terminal lug range on the switch must accept the size you choose.
Q: Is a 10 gauge extension cord enough for a generator?
A: A 10 AWG cord carries up to 30 A at the breaker and is fine for a 30 A generator outlet on short runs. Above that, or for long runs, step up to 8 AWG or heavier, and always use a cord rated for outdoor use.
Q: Do generators use bigger wire than mains?
A: Often yes, for two reasons: a generator feeder is sized to full-load amps with headroom for motor-start inrush, and the run to a standby or remote genset is frequently long enough that voltage drop forces a larger conductor than a same-ampacity mains circuit.
Q: Can I use aluminum wire for my generator?
A: Yes, aluminum is common on long feeders and costs less, but it must be sized up per the ampacity chart and terminated with anti-oxidant compound. Copper is preferred for short runs and inside panels.