In short. Every DG set needs four things matched to its full-load current: a generator breaker, a four-pole changeover or AMF panel, power cables from the set to the panel, and an earthing system. This chart gives all of them for every common rating from 7.5 kVA to 3,000 kVA at 415 V, 50 Hz, worked out by one consistent rule: the breaker is at or above the full-load current, the changeover is at least the breaker rating, and the cable carries at least the breaker rating after derating. It then shows how far each cable can run before voltage drop reaches 3%, how to correct the cable for a hot DG room or a crowded tray, and when busduct takes over from cables. Use it for budgeting, checking a quotation or a first layout; the final selection should always be confirmed against the actual site, route and fault level.
Quick answers at 415 V
- 62.5 kVA DG: 87 A full load, 100 A MCCB, 100 A four-pole changeover, 1 × 35 sq mm aluminium XLPE armoured cable
- 125 kVA DG: 174 A full load, 200 A MCCB, 200 A four-pole changeover, 1 × 95 sq mm aluminium XLPE armoured cable
- 250 kVA DG: 348 A full load, 400 A MCCB, 400 A four-pole changeover, 1 × 300 sq mm aluminium XLPE armoured cable
- 320 kVA DG: 445 A full load, 500 A MCCB, 630 A four-pole changeover, 2 × 150 sq mm aluminium XLPE armoured cable
- 500 kVA DG: 696 A full load, 800 A ACB, 800 A four-pole changeover, 2 × 300 sq mm aluminium XLPE armoured cable
- 750 kVA DG: 1,043 A full load, 1,250 A ACB, 1,250 A four-pole changeover, 4 × 240 sq mm aluminium XLPE armoured cable
- 1,010 kVA DG: 1,405 A full load, 1,600 A ACB, 1,600 A four-pole changeover, 4 × 300 sq mm aluminium XLPE armoured cable
- 1,500 kVA DG: 2,087 A full load, 2,500 A ACB, 2,500 A four-pole changeover, 7 × 300 sq mm aluminium XLPE armoured cable (busduct is usually the better choice)
DG breaker, changeover and cable size chart (415 V, 50 Hz)
All figures are for a three-phase, four-wire DG set at 415 V. Cable sizes are 1.1 kV aluminium-conductor, XLPE-insulated, armoured cables (IS 7098 Part 1), laid in air on a perforated or ladder tray at 40 °C ambient. A single run is not derated; two or more parallel runs are spaced one cable diameter apart and given a 0.9 allowance. The last column is the longest one-way route that keeps voltage drop within 3% at full load. Tap or click any rating to get a link straight to that row.
| Rating (kVA) | kW at 0.8 PF | Full load (A) | Generator breaker (4-pole) | Changeover (4-pole) | Al XLPE armoured cable (runs × sq mm) | Max route for 3% drop |
|---|---|---|---|---|---|---|
| 7.5 | 6 | 10 | 16 A MCCB | 40 A | 1 × 16 | 281 m |
| 10 | 8 | 14 | 16 A MCCB | 40 A | 1 × 16 | 211 m |
| 15 | 12 | 21 | 25 A MCCB | 40 A | 1 × 16 | 141 m |
| 20 | 16 | 28 | 32 A MCCB | 40 A | 1 × 16 | 106 m |
| 25 | 20 | 35 | 40 A MCCB | 40 A | 1 × 16 | 84 m |
| 30 | 24 | 42 | 50 A MCCB | 63 A | 1 × 16 | 70 m |
| 40 | 32 | 56 | 63 A MCCB | 63 A | 1 × 16 | 53 m |
| 45 | 36 | 63 | 63 A MCCB | 63 A | 1 × 16 | 47 m |
| 50 | 40 | 70 | 80 A MCCB | 100 A | 1 × 25 | 67 m |
| 62.5 | 50 | 87 | 100 A MCCB | 100 A | 1 × 35 | 74 m |
| 75 | 60 | 104 | 125 A MCCB | 125 A | 1 × 50 | 83 m |
| 82.5 | 66 | 115 | 125 A MCCB | 125 A | 1 × 50 | 76 m |
| 100 | 80 | 139 | 160 A MCCB | 160 A | 1 × 70 | 90 m |
| 125 | 100 | 174 | 200 A MCCB | 200 A | 1 × 95 | 99 m |
| 140 | 112 | 195 | 200 A MCCB | 200 A | 1 × 95 | 89 m |
| 160 | 128 | 223 | 250 A MCCB | 250 A | 1 × 150 | 117 m |
| 180 | 144 | 250 | 315 A MCCB | 315 A | 1 × 185 | 127 m |
| 200 | 160 | 278 | 315 A MCCB | 315 A | 1 × 185 | 115 m |
| 250 | 200 | 348 | 400 A MCCB | 400 A | 1 × 300 | 138 m |
| 275 | 220 | 383 | 400 A MCCB | 400 A | 1 × 300 | 125 m |
| 320 | 256 | 445 | 500 A MCCB | 630 A | 2 × 150 | 117 m |
| 380 | 304 | 529 | 630 A MCCB | 630 A | 2 × 240 | 152 m |
| 400 | 320 | 556 | 630 A MCCB | 630 A | 2 × 240 | 144 m |
| 450 | 360 | 626 | 630 A MCCB | 630 A | 2 × 240 | 128 m |
| 500 | 400 | 696 | 800 A ACB | 800 A | 2 × 300 | 138 m |
| 600 | 480 | 835 | 1,000 A ACB | 1,000 A | 3 × 240 | 144 m |
| 625 | 500 | 870 | 1,000 A ACB | 1,000 A | 3 × 240 | 139 m |
| 650 | 520 | 904 | 1,000 A ACB | 1,000 A | 3 × 240 | 133 m |
| 750 | 600 | 1,043 | 1,250 A ACB | 1,250 A | 4 × 240 | 154 m |
| 1,000 | 800 | 1,391 | 1,600 A ACB | 1,600 A | 4 × 300 | 138 m |
| 1,010 | 808 | 1,405 | 1,600 A ACB | 1,600 A | 4 × 300 | 136 m |
| 1,250 | 1,000 | 1,739 | 2,000 A ACB | 2,000 A | 5 × 300 | 138 m |
| 1,500 | 1,200 | 2,087 | 2,500 A ACB | 2,500 A | 7 × 300 or busduct | 161 m |
| 1,750 | 1,400 | 2,435 | 2,500 A ACB | 2,500 A | 7 × 300 or busduct | 138 m |
| 2,000 | 1,600 | 2,782 | 3,200 A ACB | 3,200 A | 8 × 300 or busduct | 138 m |
| 2,250 | 1,800 | 3,130 | 3,200 A ACB | 3,200 A | 8 × 300 or busduct | 122 m |
| 2,500 | 2,000 | 3,478 | 4,000 A ACB | 4,000 A | 10 × 300 or busduct | 138 m |
| 3,000 | 2,400 | 4,174 | 5,000 A ACB | 5,000 A | 13 × 300 or busduct | 149 m |
Full-load current = kVA × 1000 ÷ (√3 × 415). Breaker and changeover ratings are the next standard frame at or above the current; MCCBs are shown up to 630 A and ACBs from 800 A, although MCCBs are made up to 1,600 A. Below 25 sq mm four-core cable is used, as 3.5-core cable starts at 25 sq mm; 16 sq mm is shown as the smallest practical armoured aluminium size, and small sets are often wired in copper of a smaller size. For the full-load current alone at 415 V, 11 kV and 230 V, see the generator full-load current chart.
How the chart is worked out
The chart follows the coordination rule used in IEC 60364-4-43 and the Indian wiring code: IB ≤ In ≤ IZ. The design current IB is the generator full-load current, In is the breaker rating, and IZ is what the cable can carry continuously after derating. Each column is one step of that rule.
- Full-load current. IB = kVA × 1000 ÷ (√3 × V). At 415 V that is 1.391 A per kVA. A 750 kVA set: 750,000 ÷ (1.732 × 415) = 1,043 A.
- Generator breaker. The next standard rating at or above IB: 1,043 A becomes a 1,250 A ACB. The overload release is then set close to the full-load current, so the frame size gives headroom while the setting protects the alternator.
- Changeover. At least the breaker rating, four-pole, and at least the mains incomer rating if that is larger: 1,250 A for the 750 kVA set.
- Cable. Enough parallel runs of one size so that runs × tabulated rating × derating ≥ breaker rating. For 1,250 A: four runs of 240 sq mm aluminium give 4 × 392 A × 0.9 = 1,411 A. The cable is sized to the breaker rating, not just the full-load current, so the breaker protects it whatever its setting.
- Voltage drop. Drop (V) = current per run × mV/A/m × length ÷ 1000. The chart gives the length at which this reaches 3% of 415 V (12.45 V) at full load. Beyond it, add a run or go up a size.
Correct the cable for your site
The chart assumes 40 °C air and cables that do not heat each other. A DG room in a Telangana summer is often 45 °C or more, and cables are frequently bunched on one tray. Multiply the cable rating by every factor that applies, then check it still carries the breaker rating.
Air temperature (XLPE, 90 °C conductor)
| Ambient air | Factor |
|---|---|
| 30 °C | 1.10 |
| 35 °C | 1.05 |
| 40 °C | 1.00 |
| 45 °C | 0.95 |
| 50 °C | 0.89 |
| 55 °C | 0.84 |
Factor = √((90 − T) ÷ 50), relative to the 40 °C table.
Grouping on a tray, cables touching
| Multicore cables in one layer | Factor |
|---|---|
| 1 | 1.00 |
| 2 | 0.88 |
| 3 | 0.82 |
| 4 | 0.77 |
| 6 | 0.73 |
Single layer on a perforated tray (IEC 60364-5-52, Table B.52.20). Spacing cables one diameter apart on a ladder tray recovers most of this loss.
Parallel runs of the same circuit heat each other too, so they count as grouped cables. Cables laid directly in the ground carry less than in air for the larger sizes (300 sq mm aluminium: 369 A in ground against 452 A in air), so a buried route usually needs one more run.
Aluminium cable ratings used in this chart
| Size (sq mm) | In ground, 30 °C (A) | In air, 40 °C (A) | Voltage drop (mV/A/m) |
|---|---|---|---|
| 16 | 74 | 69 | 4.24 |
| 25 | 95 | 93 | 2.67 |
| 35 | 114 | 114 | 1.94 |
| 50 | 134 | 138 | 1.43 |
| 70 | 164 | 175 | 0.99 |
| 95 | 197 | 216 | 0.72 |
| 120 | 223 | 249 | 0.58 |
| 150 | 249 | 284 | 0.48 |
| 185 | 282 | 329 | 0.39 |
| 240 | 327 | 392 | 0.31 |
| 300 | 369 | 452 | 0.26 |
1.1 kV XLPE-insulated, aluminium-conductor, armoured multicore cables to IS 7098 (Part 1); conductor 90 °C, ground at 30 °C with soil thermal resistivity 1.5 K·m/W and 750 mm laying depth, air at 40 °C. Figures are typical of published Indian manufacturers’ tables and vary slightly between makes; use your chosen manufacturer’s table for the final design.
Choosing the generator breaker
- Four-pole. The generator breaker switches the neutral as well as the phases where the set has its own earthed neutral.
- MCCB or ACB. MCCBs are compact and economical up to 630 A and are made up to 1,600 A. ACBs are normal from 800 A upwards: they are withdrawable for maintenance, accept electrical operation for AMF panels, and offer adjustable long-time, short-time, instantaneous and earth-fault settings.
- Release setting. Use an adjustable (thermal-magnetic or microprocessor) release and set Ir close to the full-load current. A fixed release equal to the frame rating does not protect the alternator.
- Breaking capacity. The Icu/Ics rating must exceed the prospective fault current at the panel. That is usually set by the mains transformer (often 25 to 50 kA), because a generator’s own fault current is only about 6 to 10 times its full-load current and decays within cycles.
- Short-time setting. Because the generator’s sustained fault current is low, the short-time pickup must sit below it or a fault on the DG supply may not trip the breaker fast enough. This is the most common protection error on DG panels.
Choosing the changeover: manual, ATS or AMF
Indian electrical safety regulations require a standby generator to be interlocked so that it cannot run in parallel with, or feed back into, the utility supply unless it is designed and approved to synchronise. The changeover is what guarantees this.
| Arrangement | How it works | Where it fits |
|---|---|---|
| Manual changeover switch | Hand-operated, three positions (mains, off, DG), mechanically prevents both sources closing together. | Small and medium sets at homes, shops and sites with staff on hand to switch over. |
| Automatic transfer switch (ATS) | Motorised four-pole switch that transfers to DG when mains fails and back when it returns, to IS/IEC 60947-6-1. | Offices, hospitals, apartments and plants where the set is started automatically; 40 A to 3,200 A. |
| AMF panel | Auto-mains-failure controller that senses mains failure, starts the engine, closes the DG breaker and returns on mains restoration; switching by two interlocked contactors, MCCBs or ACBs. | Unattended standby on any size; ACB-based AMF panels are standard from about 800 A. |
| Synchronising panel | Parallels two or more sets, or sets and mains, with load sharing. | Where one set is not enough, or where redundancy (N+1) is required. |
- Rating. At least the larger of the mains incomer current and the generator breaker rating.
- Four poles. Keeps the generator and transformer neutrals separate (see the FAQ below).
- Utilisation category. AC-33A or AC-33B where the load includes motors (IS/IEC 60947-6-1); AC-31 is only for non-inductive or slightly inductive load.
- Interlocking. Mechanical and electrical interlock on ACB-based panels, so that both breakers can never be closed together even if the control system fails.
- Transition time. An open transition with a short delay lets motor back-EMF decay before reconnection; UPS-fed loads ride through the break.
Cable practice that the chart assumes
- Aluminium or copper. Aluminium is the normal choice for DG power cables from 25 sq mm upwards on cost and weight. Copper is used for small sets, tight panel entries and vibrating or flexible connections; it is usually one size smaller for the same current.
- 3.5-core or four-core. 3.5-core cable has a reduced neutral and suits balanced linear loads. For UPS, drives, LED lighting and IT loads use four-core cable with a full neutral, because triplen harmonics add in the neutral.
- Parallel runs. All runs must be the same size, type and length and follow the same route, or they will not share current. Terminate each run on its own lug of the correct size and torque.
- Armour earthing. Earth the armour at both ends through the cable gland; it is part of the earth-fault return path, not a substitute for a protective earth conductor on long or large runs.
- Flexible connection at the set. The set sits on anti-vibration mounts, so the final connection to the alternator terminal box needs a flexible loop or a short flexible copper cable, not a rigid armoured cable pulled tight.
- Rental sets. Temporary connections use flexible rubber or PCP-sheathed copper cables with plug connectors, sized on the same full-load current.
Above 2,000 A: busduct
From about 1,500 kVA at 415 V, the breaker is 2,500 A or more and the chart needs seven to thirteen parallel runs of 300 sq mm cable. That is heavy, slow to terminate and difficult to keep balanced. A sandwich-type busduct of the same rating is the normal choice between the set and the main panel; it shares current evenly by construction, takes less space and is easier to inspect with a thermal camera. Larger installations may also generate at 11 kV, where the current falls to about one twenty-sixth (a 2,000 kVA set gives 105 A at 11 kV).
Earthing the DG set
Indian electrical safety regulations require the generator neutral to be earthed by two separate and distinct connections, and every metal body to be earthed. The usual arrangement, following IS 3043, is:
- Two dedicated earth electrodes for the generator neutral, connected by separate conductors.
- Two earth electrodes for the set body, alternator frame, control panel and changeover enclosure, interconnected.
- Neutral earth conductor sized for the earth-fault current; body earth conductors sized to IS 3043 for the fault level and disconnection time.
- Earth resistance measured at commissioning and recorded, then rechecked every year before the monsoon.
Worked examples
125 kVA set, panel 60 m away. Full-load current 174 A; breaker 200 A MCCB; changeover 200 A four-pole. One run of 95 sq mm aluminium carries 216 A in air, more than the 200 A breaker. Voltage drop: 174 A × 0.72 mV/A/m × 60 m = 7.5 V, which is 1.8% of 415 V. One run of 95 sq mm is right.
750 kVA set, 120 m route. Full-load current 1,043 A; breaker 1,250 A ACB; changeover 1,250 A. Four runs of 240 sq mm aluminium carry 4 × 392 × 0.9 = 1,411 A. Voltage drop: 1,043 ÷ 4 = 261 A per run; 261 × 0.31 × 120 ÷ 1000 = 9.7 V, which is 2.3%. Within limits.
320 kVA set in a 45 °C DG room, two runs touching on one tray. Breaker 500 A. The chart’s two runs of 150 sq mm give 2 × 284 × 0.88 (grouping) × 0.95 (air temperature) = 475 A, less than 500 A, so they are not enough here. Two runs of 185 sq mm give 2 × 329 × 0.88 × 0.95 = 550 A. Use 2 × 185 sq mm, or keep 2 × 150 sq mm and space the runs on a ladder tray.
1,010 kVA set, 200 m route. Breaker 1,600 A ACB. Four runs of 300 sq mm pass on heating (1,627 A), but voltage drop decides: 1,405 ÷ 4 = 351 A per run; 351 × 0.26 × 200 ÷ 1000 = 18.3 V, which is 4.4%. Six runs bring it down to 2.9%. On long routes, check voltage drop before ordering cable.
Common mistakes
- Sizing the cable to the full-load current but fitting a bigger breaker. The cable must carry the breaker rating, or the breaker setting must be locked at or below the cable’s rating.
- Three-pole changeover on a set with an earthed neutral. The neutrals of the two sources are then tied together, earth-fault protection misreads, and neutral current takes unintended paths.
- Ignoring the DG room temperature. A room at 50 °C cuts the cable rating by about 11%.
- Unequal parallel runs. A shorter run takes more than its share of current and overheats first.
- Breaker breaking capacity chosen for the generator alone. The panel also sees the mains fault level when the mains is connected.
- Short-time pickup above the generator’s sustained fault current. The breaker then fails to clear a fault on DG supply.
- Reduced neutral on a UPS or drive load. Harmonic neutral current overheats the 3.5-core neutral.
What to send us
For a breaker, changeover and cable recommendation for your site, send the generator nameplate kVA and voltage, the mains incomer rating, the route length and how the cable is laid (tray, trench or buried), the DG room temperature in summer, the type of load (motors, UPS, drives, lighting), and the fault level of the main panel if known. Use the application engineering enquiry, call 098852 01608 or WhatsApp us on the same number.
Questions engineers ask
What size breaker does a 250 kVA DG set need?
A 250 kVA set delivers about 348 A per phase at 415 V, so the generator breaker is a 400 A four-pole MCCB with its overload release set close to 348 A. The breaking capacity (Icu) must suit the fault level of the panel it sits in, which is usually decided by the mains transformer, not by the generator.
What cable size is used for a 125 kVA DG set?
One run of 95 sq mm aluminium XLPE armoured cable (3.5-core) laid in air at 40 °C carries the 200 A breaker rating of a 125 kVA set, and keeps voltage drop within 3% up to about 99 m at full load. A hotter room, cables bunched on a tray or a longer route each push the size up; the derating tables on this page show by how much.
What changeover switch is needed for a 500 kVA generator?
A 500 kVA set delivers about 696 A at 415 V and is protected by an 800 A ACB, so the changeover is rated 800 A, four-pole. That can be an 800 A on-load changeover switch or ATS, or an AMF panel built with two mechanically and electrically interlocked 800 A ACBs. If the mains incomer is larger than the generator, the changeover is sized for the mains current instead.
Why should a DG changeover switch be four-pole?
In most Indian installations the generator neutral is earthed at the set and the transformer neutral is earthed at the substation. Switching the neutral with the phases keeps the two earthed neutrals apart, so neutral current returns to the source that is supplying it, earth fault and RCD protection see the true fault current, and the generator cannot back-feed the utility neutral.
Can 3.5-core cable be used on a generator?
Yes, where the load is mostly balanced three-phase motors and heaters, because the neutral then carries little current. Where the generator supplies UPS systems, variable-speed drives, LED lighting or IT loads, third-harmonic currents add up in the neutral and can exceed the phase current, so use four-core cable with a full-size neutral.
When should busduct be used instead of cables?
From about 2,000 to 2,500 A, which is roughly 1,500 kVA and above at 415 V. At that current a cable connection needs seven or more parallel runs of 300 sq mm, each needing its own termination and current-sharing check, whereas a single busduct run is compact, shares current by design and is quicker to install and inspect.
Is copper cable one size smaller than aluminium?
Usually, yes. A copper conductor carries roughly a quarter more current than an aluminium conductor of the same size, so the copper size is often one step smaller. Always confirm with the cable manufacturer's own copper table for the core count and laying method, and recheck voltage drop, because the smaller copper cable has a different drop per metre.
Does the chart change for a 400 V generator?
Current is about 3.75% higher at 400 V than at 415 V for the same kVA. Most rows keep the same breaker, changeover and cable because the standard sizes leave a margin, but four rows move up a size because their full-load current is within 4% of the breaker rating: 140 kVA (202 A at 400 V against a 200 A breaker), 450 kVA (650 A against 630 A), 1,750 kVA (2,526 A against 2,500 A) and 2,250 kVA (3,248 A against 3,200 A). For those, take the next breaker, changeover and cable size up.
Related guides and tools
- Generator full-load current chart, 7.5 to 3,000 kVA
- Cable sizing and voltage drop calculator
- Generator cable sizing guide
- ACB, MCCB and MCB selection
- ATS and changeover switchgear engineering
- AMF panels
- Generator fault current
- Earthing and neutral grounding
Standards referred to
IS 7098 (Part 1), XLPE-insulated cables up to 1.1 kV. IS/IEC 60947-2, circuit-breakers. IS/IEC 60947-3, switches and switch-disconnectors. IS/IEC 60947-6-1, transfer switching equipment. IEC 60364-4-43 and IEC 60364-5-52, overcurrent protection, current-carrying capacity and grouping. IS 732, code of practice for electrical wiring installations. IS 3043, code of practice for earthing. ISO 8528 and IS/IEC 60034-1, generating set and machine ratings. Central Electricity Authority (Measures relating to Safety and Electric Supply) Regulations, 2023.
Reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators, in generator service since 2001. Last reviewed 1 November 2026.