A generator sizing calculator converts your electrical loads into a practical generator capacity recommendation using three connected values: kW, kVA and amps.
The real electrical power your connected equipment consumes while operating.
The total apparent power the generator must provide after power factor is considered.
The current at your selected voltage, useful for cable, breaker and distribution planning.
The calculator combines your running loads, the starting surge of the largest motor, a sizing margin and relevant site conditions such as altitude and temperature. The result is intended to give you a usable generator recommendation rather than a simple total-load estimate.
Supports single-phase and three-phase systems at both 50 Hz and 60 Hz.
Includes power factor, motor starting method, altitude derating and future expansion.
Recommendations can be matched against ShanHua generator sets from 8 kVA to 4,000 kVA.
Use the calculated requirement as the basis for a factory-direct quotation.
Enter your current, load, or generator size. The calculator returns the correct AWG and mm² conductor, checks voltage drop over your run length, and flags when you need to upsize.
Enter the circuit current in amps. The calculator returns the smallest AWG and mm² conductor with sufficient ampacity, plus a voltage-drop check.
Enter your load in kW or kVA. The calculator converts to current, then sizes the conductor and checks voltage drop.
Select a generator size. The calculator returns the full-load output current and the recommended output cable for your run length.
This chart shows the allowable ampacity of copper conductors from NEC Table 310.16 at the three insulation temperature ratings, assuming up to three current-carrying conductors at 30 °C ambient. Use it to sanity-check a calculation; the wire size calculator above applies the same values with derating and voltage-drop checks.
| AWG | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 14 | 20 A | 25 A | 30 A |
| 12 | 25 A | 30 A | 35 A |
| 10 | 35 A | 40 A | 45 A |
| 8 | 50 A | 55 A | 65 A |
| 6 | 55 A | 65 A | 75 A |
| 4 | 70 A | 85 A | 95 A |
| 3 | 85 A | 100 A | 110 A |
| 2 | 95 A | 115 A | 130 A |
| 1 | 110 A | 130 A | 145 A |
| 1/0 | 125 A | 150 A | 170 A |
| 2/0 | 145 A | 175 A | 195 A |
| 3/0 | 165 A | 200 A | 225 A |
| 4/0 | 195 A | 230 A | 260 A |
Wire sizing is a two-step process. First size for ampacity: the conductor must carry the circuit current without overheating. Then check voltage drop: the conductor must keep the voltage loss within limits over the actual run length. The final size is the larger of the two, and on runs longer than about 30–50 metres, voltage drop is usually the size that wins.
Select the conductor from the ampacity table whose rating equals or exceeds the circuit current after any derating for ambient temperature and conductor count. For a continuous load (3+ hours), size for 125% of the full-load current.
Voltage drop (V) = (mV/A/m × current × length) ÷ 1,000
Recommended limits (NEC, BS 7671): 3% on a branch circuit, 5% total from source to load. If the drop exceeds the limit, move up one conductor size and re-check.
The final conductor size is the larger of the ampacity size and the voltage-drop size. On runs longer than about 30–50 metres, voltage drop is usually the deciding factor and forces a size up.
The National Electrical Code (NFPA 70) sets out the ampacity tables and derating rules. BS 7671 (IET) sets out the equivalent for metric cable. Both agree: ampacity first, voltage drop second, and the bigger conductor wins.
A conductor protecting a load that runs for three hours or more must be sized for 125% of the full-load current. A 90 A continuous load needs a conductor rated at least 112.5 A.
Ampacity sets the floor — the conductor must carry the current without overheating. Voltage drop sets the ceiling — the same conductor must deliver the load at the far end of the run. Miss either one and you get a cable that trips, a conductor that overheats, or equipment that never runs at full power.
This is the mistake we see most often in the field, and it is the one the simple charts always miss. A conductor that is correctly sized for ampacity can still be undersized for voltage drop on a long run. The result is a circuit that works on paper but delivers too little voltage at the load — motors run hot, starters struggle, and lights dim.
For any run longer than about 30–50 metres, check voltage drop before you order. The voltage lost along the cable is proportional to both current and length — doubling the distance doubles the drop.
Limits: no more than 3% voltage drop on a branch circuit and 5% total from source to load — the same limits in NEC and BS 7671.
Move up one conductor size and re-check. Each AWG size up cuts the resistance per metre by about a fifth, and roughly three sizes halve it. The calculator applies the 3%/5% limits automatically for your exact run length.
The cable from a genset to the building is often the longest single run on site. A 100 kW generator at 400 V pushes about 180 A; over a 100-metre run you'll almost always need a bigger cable than the ampacity table alone suggests.
If your run is long, size the wire for distance, not just current. Voltage drop grows with every metre, and on long feeds it is routine for the voltage-drop size to be two or three sizes larger than the ampacity size. Always include the distance in your calculation.
Copper and aluminum both carry current; they differ in price, weight, and the size needed for the same job. The choice matters most on long generator feeders and large services, where the material decides the cost of the run.
Carries more current per cross-section, is more ductile, and is the default for circuits under 100 A and everything indoors. It costs more, but it is the standard for branch circuits and generator control wiring.
Needs roughly two AWG sizes larger (or about 1.6× the mm²) for the same ampacity. It is lighter and significantly cheaper per amp-metre. Common on large services and long feeder runs where the savings are real.
Aluminum requires rated lugs and anti-oxidant compound at terminations. Copper-to-aluminum connections must use proper bi-metallic fittings or corrosion forms at the joint. Never skip the termination hardware.
Use copper for branch circuits and generator control wiring. Consider aluminum for long feeder runs above 100 A where the price difference is meaningful. Let an engineer confirm the lugs and termination method.
The wire size calculator above has a copper/aluminum toggle so you can compare the two sizes for your exact run before you decide. See the price difference and the size difference side by side.
Not sure which material to choose? Our engineers will review your run length, current, and site conditions and recommend the best material and size for your installation. Talk to our engineers.
Our engineers will review your wire sizing, confirm the voltage drop, and deliver a factory-direct price — no obligation, no hidden fees. Every quote includes the generator, output cable, and automatic transfer switch for a complete power solution.
A: It depends on three things: the current, the voltage, and the distance. For a 20 A circuit use 12 AWG (3.31 mm²) copper; for 50 A use 6 AWG (13.3 mm²); for 100 A use 2 AWG (33.6 mm²). Then check voltage drop for the run length, because long runs need larger wire.
A: Size for ampacity first, the conductor must carry the current without overheating, applying the 125% factor for continuous loads. Then check voltage drop over the run length and confirm it stays under 3% for a branch circuit and 5% total. The larger of the two sizes wins.
A: 10 AWG is approximately 5.26 mm². The full conversion: 6 AWG ≈ 13.3 mm², 4 AWG ≈ 21.2 mm², 2 AWG ≈ 33.6 mm², 1/0 ≈ 53.5 mm², 2/0 ≈ 67.4 mm².
A: For a 100 A service, 2 AWG copper (33.6 mm²) at 75 °C is the standard choice for a normal run. Over about 50 metres, upsize to 1/0 (53.5 mm²) to keep voltage drop within limits. Aluminum requires 1/0 or larger.
A: A 100 kW generator at 400 V three-phase produces about 180 A. A normal run uses 70 mm² copper (≈ 2/0 AWG). A long run, or a generator placed far from the building, needs 95 mm² or larger, run it through the calculator with your exact distance.
A: Yes, and it is often the deciding factor. Generator feeders are frequently the longest runs on site, and the drop grows with every metre. On runs over 50 metres the voltage-drop size is usually larger than the ampacity size, so always include the distance in the calculation.
A: For runs under 100 A, use copper. For long, large feeders above 100 A, aluminum is a legitimate cost-saving choice at roughly two AWG sizes larger, provided the terminations use rated lugs and anti-oxidant compound. Our engineers confirm the material and terminations with your run before you order.
A: Start from the current: 30 kW at 400 V three-phase at 0.8 PF draws about 54 A, which is 16 mm² copper for a short run. A 100 kW supply draws about 180 A, needing 70 mm². Verify with the calculator for your exact distance.
