In short. A generator feeder cable passes three checks, not one. It must carry the generator’s full-load current after derating for ambient temperature, grouping and installation method; it must keep voltage drop within the allocation for the circuit, including during motor starting; and it must withstand the fault current until protection clears. The neutral, earth conductor and terminations are sized with it. Current alone, read from a kVA figure, is a starting point and never a specification.
Start from the design current
I = S × 1000 ÷ (√3 × VLL)
I is the full-load line current in amperes, S is the generator rating in kVA, and VLL is the line-to-line voltage in volts, 415 V for Indian low-voltage sets. For single-phase sets the divisor is simply the line-to-neutral voltage of 230 V. Worked example (illustrative): a 500 kVA set gives 500,000 ÷ (1.732 × 415), or about 696 A. The feeder is sized for the generator’s rated current rather than the connected load, because the set may one day carry its full rating, and many designers add a margin so that the cable is not the limiting element during the 10 percent overload that standby ratings permit for short periods. The generator current guide tabulates the arithmetic for common ratings.
| Generator rating | Full-load current at 415 V, three phase | Typical arrangement (illustrative, copper XLPE) |
|---|---|---|
| 125 kVA | 174 A | One 3.5-core 95 or 120 mm² |
| 250 kVA | 348 A | One 3.5-core 240 mm² or two 3.5-core 120 mm² |
| 500 kVA | 696 A | Two or three 3.5-core 300 mm², or single cores in trefoil |
| 1,000 kVA | 1,391 A | Four to five parallel runs of single-core 400 mm², or busduct |
| 1,500 kVA | 2,087 A | Busduct is usually the better answer |
The right-hand column shows the scale only; the actual size depends on the derating factors and the route, and a cable schedule is produced from the manufacturer’s current ratings for the installation method in question.
Derating for the real installation
Published current ratings under IS 7098 for XLPE and IS 1554 for PVC assume a reference ambient, typically 40 °C in air and 30 °C in ground, a single circuit, and a stated installation method. Each departure applies a factor. A generator room at 50 °C reduces an XLPE rating by roughly 10 percent; six circuits touching on a tray reduce it by 25 to 30 percent; cables buried in hot, dry soil with a thermal resistivity above the assumed 1.5 K·m/W derate further. The factors multiply, and it is not unusual for a cable to deliver only 60 percent of its tabulated rating on a crowded tray through a hot room. Aluminium conductors, common for large feeders on cost grounds, need about 1.6 times the copper cross-section for the same current and require bimetallic lugs at copper terminals. Parallel runs must be identical in length, size and routing so that they share current equally, and single-core cables in parallel are laid in trefoil groups with each group containing all three phases to limit reactance and armour heating.
Voltage drop and motor starting
A cable that carries the current comfortably can still deliver poor voltage at the far end. Common Indian practice limits total drop from source to the final load to about 3 to 5 percent, and the generator feeder is usually allocated 1 to 2 percent of that so that the distribution beyond it has room. The check that is most often missed is motor starting: a motor drawing six times its full-load current during direct-on-line starting produces six times the steady-state drop in the feeder, which adds to the generator’s own transient dip. A 2 percent feeder becomes a 12 percent feeder for a few seconds, the motor sees perhaps 75 percent voltage, and its starting torque, which falls with the square of voltage, is barely half of nominal. The method and an example are in voltage drop, and the generator side of the same problem is in motor starting.
Fault withstand and protection
S = If × √t ÷ k
S is the minimum conductor cross-section in square millimetres, If is the fault current in amperes, t is the clearing time of the protection in seconds, and k is a constant for the conductor and insulation, about 143 for copper XLPE, 94 for aluminium XLPE and 115 for copper PVC. Worked example (illustrative): if the generator feeder can also be energised from the utility side with a prospective fault current of 25 kA cleared in 0.2 seconds, the minimum copper XLPE section is 25,000 × 0.447 ÷ 143, or about 78 mm², which the feeder easily exceeds but a small control or earth conductor on the same bus might not. On the generator side the fault current is different in character: the subtransient contribution is typically 6 to 10 times rated current for the first cycles, decaying to a sustained value of about 2.5 to 3 times rated with PMG excitation and far less without it. Instantaneous elements set for utility fault levels may never see a generator fault, and the cable must then survive the longer clearing time of a thermal or overcurrent element. The study method is in fault current and protection coordination.
Neutral, earth and terminations
The neutral is sized full size, not reduced, wherever the load contains single-phase electronic equipment, because third-harmonic currents add in the neutral and can exceed the phase current. The earth conductor between the generator frame, the neutral earthing point and the switchboard is sized under IS 3043 to the fault current and clearing time, and the neutral-earth link location is decided once, in accordance with the switching arrangement, as explained in earthing and neutral grounding. At the alternator terminal box, check that the number and size of lugs fit the palms, that single-core cables enter through a non-magnetic gland plate, and that there is space to form the bends without stressing the terminals. Rental and temporary feeders use flexible cables with mechanical protection, ramps at crossings and connectors rated for the current, as described under temporary power cables.
Related photographs and videos
Common mistakes
- Selecting the cable from a current table without derating; on a hot, crowded tray it runs above its insulation temperature and ages in a few years.
- Checking voltage drop at running load only; the first large motor start on generator stalls or trips.
- Reducing the neutral on a feeder to a data centre or office; the neutral overheats on third-harmonic current.
- Running parallel single-core cables with all three phases in separate groups; the unequal reactance unbalances the sharing and heats the armour.
- Ignoring the utility-side fault level on a cable that the ATS can connect to either source; the cable is sized for the softer generator fault and fails on the harder one.
What to send us
Send the generator rating and voltage, the route length and how the cable will be installed, the ambient temperature along the route, how many other circuits share the tray or trench, the switchboard’s fault level and protection settings, the largest motor started direct-on-line, and the load mix. A sketch of the route with the terminal box and switchboard positions helps. Send your application details and we will prepare the cable schedule with the derating, voltage-drop and fault checks shown.
Questions engineers ask
What cable size is needed for a 250 kVA generator?
The full-load current is about 348 A at 415 V. Depending on route, ambient and grouping that is typically a single 3.5-core 240 mm² copper XLPE cable or two 120 mm² cables in parallel, before the voltage-drop and fault checks. The schedule should be produced from the manufacturer’s ratings for the actual installation.
Can aluminium cable be used for a generator feeder?
Yes, and it is common above about 95 mm². It needs roughly 1.6 times the copper cross-section, bimetallic lugs at copper terminals, and care with termination torque and creep. Alternator terminal boxes sometimes lack space for the larger lugs, so check before ordering.
Should the generator feeder be rated for the generator or the load?
For the generator’s full rated current, because the set can be loaded to its rating and the cable should not be the element that limits or overheats first. The downstream distribution is then sized for its own loads.
Why is a half-size neutral not acceptable on generator feeders?
Because modern loads are rich in single-phase electronic equipment whose third-harmonic currents add arithmetically in the neutral. A reduced neutral can carry more than the phase conductors and overheat. Full-size neutrals are the norm, and oversized neutrals are specified where the harmonic content is severe.
Looking for a specific rating? See the generator full-load current chart for every common size from 7.5 to 3,000 kVA at 415 V and 11 kV.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.