In short. Power leaves the alternator through cables, busbars, joints, breakers and panels before it reaches a motor or a server, and every element adds impedance and a thermal limit. A correctly sized generator can still fail the load because the feeder drops too much voltage during a motor start, a joint runs hot, or a rental cable is left coiled on its drum. Cable and busbar design is part of the generator system, not an afterthought.
Start with the current, then correct it
The full-load current of a three-phase generator is I = S / (√3 × V), so a 500 kVA set at 415 V delivers about 696 A and a 1,000 kVA set about 1,391 A. That number is the beginning of cable selection, not the end. Tabulated cable ratings assume a reference ambient, a reference installation method and a single circuit. Real installations in Telangana differ on every count: air temperatures of 45 °C in summer, cables bunched on a tray, runs buried in dry soil with poor thermal resistivity, and harmonic-rich loads that heat the neutral. Each condition applies a derating factor, and the product of those factors against the tabulated rating gives the current the cable can actually carry. A grouping factor of 0.8 and an ambient factor of 0.87 together leave about 70 percent of the catalogue rating, which is why a feeder that looks generous on paper runs warm in May. The method is set out in cable sizing for generators.
Voltage drop decides whether the load works
A healthy generator can show 415 V at its terminals while a pump 150 metres away sees 385 V, because current through the cable impedance costs voltage. For a balanced three-phase circuit the drop is calculated as follows.
ΔV = √3 × I × L × (R cos φ + X sin φ)
ΔV is the line-to-line voltage drop in volts, I the current in amperes, L the one-way route length in kilometres, R and X the cable resistance and reactance in ohms per kilometre at operating temperature, and cos φ the load power factor. Worked example (illustrative): a 500 kVA set feeds a panel 80 metres away at 696 A and 0.8 power factor through two parallel aluminium cables with a combined R of 0.10 Ω/km and X of 0.08 Ω/km. The drop is 1.732 × 696 × 0.08 × (0.10 × 0.8 + 0.08 × 0.6), about 12.3 V, or 3 percent of 415 V. That is acceptable for a main feeder. Double the distance and the drop reaches 6 percent before any downstream circuit has been added, and during a direct-on-line motor start the current, and therefore the drop, is six times larger for a second or two. Design limits of around 3 percent on main feeders and 5 percent total to the load are common practice, with motor-starting dips checked separately; the voltage drop guide and the screening calculator work through both.
The factors a cable schedule must record
| Factor | What to record | Effect on selection |
|---|---|---|
| Conductor and insulation | Copper or aluminium, PVC or XLPE | Aluminium needs a larger section and bimetallic lugs; XLPE allows 90 °C conductors against 70 °C for PVC |
| Installation method | Tray, trench, duct, buried, clipped | Changes the base rating and the grouping factor |
| Ambient and soil | Design air temperature, soil thermal resistivity | Derating of 10 to 20 percent is normal in Indian summers |
| Route length | Measured, including rises and bends | Drives voltage drop and the minimum earth-fault current at the far end |
| Fault withstand | Prospective fault current and clearing time | Section must satisfy I²t for the protection in place |
| Neutral and earth | Harmonic content, earthing arrangement | Full-size or oversized neutral for drive and UPS loads |
Busbars and parallel cables for high current
Above about 1,600 A, which is a single 1,250 kVA set or two paralleled 630 kVA sets, cables become awkward: four or five single cores per phase, large gland plates, and current sharing between parallel cables that depends on identical length and symmetrical arrangement. Busbar trunking between the generator room and the switchboard solves the sharing problem, reduces the footprint and simplifies terminations, at a higher material cost and with its own requirements for joint torque, expansion and ingress protection. Inside the switchboard, busbar section is governed by temperature rise under the assembly standard, treated under switchboard thermal and busbar engineering. Where parallel cables are used, they should be the same type, size and length, laid in trefoil groups each containing one cable of each phase, so that the reactance seen by each cable is equal and the current divides evenly.
Joints and terminations fail before conductors do
Most cable failures we have investigated on generator installations began at a termination: a lug crimped with the wrong die, aluminium terminated on copper without a bimetallic lug, a bolt below torque, or a gland that let monsoon moisture in. At 1,000 A a joint resistance of a tenth of a milliohm dissipates 100 W into a confined space, and the resistance climbs as the joint oxidises. The defence is procedural: specified lugs and dies, torque values recorded at installation, and a thermographic survey of every termination under load in the first months and then annually.
Temporary and rental distribution
Rental power is where distribution discipline is most often lost. Long flexible runs to a site office or a pump mean voltage drop nobody calculated, cables left partly coiled on drums overheat because the inner turns cannot shed heat, distribution boards sit on wet ground and earthing is improvised. A temporary system deserves the same current, drop and fault checks as a permanent one, the drum must be fully paid out, and the earth path back to the generator neutral must be continuous. Our rental cables and distribution page describes what is supplied with a rental set and how it should be laid.
Distribution shapes protection too
A long feeder does not only drop voltage under load; it limits the fault current that reaches the far end. The earth-fault loop impedance may be high enough that the breaker protecting the feeder never sees its instantaneous pickup, and clears, if at all, on its slow thermal curve. This is why cable selection and protection coordination are done together, and why a feeder from a generator, whose fault contribution decays within cycles, is checked for minimum fault current and not only maximum.
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Common mistakes
- Selecting cables from the catalogue rating without derating for ambient and grouping. The feeder runs above its insulation temperature every summer afternoon and the sheath hardens and cracks.
- Checking voltage drop at running current only. The motor start pulls the far-end voltage below contactor hold-in and the drive resets.
- Terminating aluminium cable directly onto copper busbar. The joint corrodes, heats and eventually arcs.
- Leaving rental cable on the drum while on load. The inner turns overheat and the insulation fails before the hire ends.
What to send us
Send the route length from generator to switchboard, the generator rating, the installation method and any existing cable schedule, the largest motor on the feeder with its starting method, and photographs of the cable route, the termination box at the generator and the switchboard incomer. For rental, tell us the distances to each point of use. Use the application engineering enquiry page.
Questions engineers ask
What voltage drop is acceptable on a generator feeder?
Common design practice holds the main feeder to about 3 percent and the total from generator to the furthest load to about 5 percent at full load. Motor-starting dips are judged separately against what the connected equipment tolerates, usually 10 to 15 percent at the motor terminals. The governing design standard for the project sets the formal limit.
Is aluminium cable acceptable for generator feeders?
Yes, and it is widely used in India for cost reasons. It needs roughly one size larger than copper for the same current, bimetallic lugs at every copper interface, and more careful crimping and torque control. For short, high-current runs inside a generator room copper is often still preferred because of the space it saves.
When should I use busbar trunking instead of cables?
Generally above 1,600 A, where cables would need four or more cores per phase, where the route is straight and short, or where the generator room sits directly beside the switchboard. Below that, cables are cheaper and easier to modify. Trunking also helps where future generators will be added, because tap-off points can be built in.
Why does the neutral need to be full size on generator circuits?
Because the loads most often placed on generator supply, UPS systems, drives, LED lighting and IT equipment, draw third-harmonic currents that add in the neutral rather than cancel. The neutral current can approach or exceed the phase current. A full-size neutral is the minimum; where such loads dominate, a neutral of larger section than the phases is specified.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.