In short. Generator current follows from apparent power and voltage. For a balanced three-phase set, I = (kVA x 1000) / (1.732 x V), so a 500 kVA set at 415 V delivers about 696 A per phase at full load. For single-phase, I = (kVA x 1000) / V. The result is a steady-state figure for sizing cables, breakers and transfer switches as a starting point; it says nothing about starting current, fault current or unbalance.
Why current matters more than kVA on site
Generators are sold in kVA, but everything downstream of the alternator terminals is chosen in amperes. The outgoing breaker, the cable to the changeover panel, the automatic transfer switch, the busbars and the earthing conductor are all rated by current. Getting the current right, and understanding what the figure does and does not represent, is the link between the generator and the installation that the CEA regulations and IS 732 expect to be designed as a whole.
Current also drives losses and heat. Cable losses rise with the square of current, so an undersized cable from a generator in a yard to a panel 80 metres away wastes fuel and delivers low voltage at the load. The voltage drop guide deals with that consequence.
The current formulas
Three-phase: I = (kVA x 1000) / (1.732 x V). Single-phase: I = (kVA x 1000) / V
Here I is line current in amperes, kVA is apparent power and V is voltage in volts, line-to-line for three-phase and line-to-neutral for single-phase. The factor 1.732 is the square root of three. If only kW is known, convert first using kVA = kW / PF as explained in the kW to kVA guide, because current depends on apparent power, not real power.
For a motor, where the nameplate gives mechanical kW, the input kVA is kW / (efficiency x PF). A 55 kW motor at 0.93 efficiency and 0.86 PF draws about 69 kVA, which at 415 V is roughly 96 A at full load (illustrative). Motor data sheets usually state full-load current directly, and that figure should be preferred.
Reference table at 415 V, three-phase
| Generator rating | Full-load current at 415 V | Typical outgoing breaker frame |
|---|---|---|
| 25 kVA | 35 A | 63 A MCCB |
| 62.5 kVA | 87 A | 100 or 125 A MCCB |
| 125 kVA | 174 A | 200 or 250 A MCCB |
| 250 kVA | 348 A | 400 A MCCB |
| 500 kVA | 696 A | 800 A MCCB or ACB |
| 750 kVA | 1043 A | 1250 A ACB |
| 1000 kVA | 1391 A | 1600 A ACB |
| 1500 kVA | 2087 A | 2500 A ACB |
The currents are arithmetic from the formula and apply at exactly 415 V. The breaker frames are typical choices for the generator outgoing feeder, and the actual rating and trip settings must be set by the protection design, not read from a table. Generator breakers are commonly set at or slightly above the alternator’s rated current with a long-time delay that matches the alternator’s short-time overload capability.
What the calculated current does not tell you
Starting and inrush current
A direct-on-line motor draws five to eight times its full-load current for the first second or two of acceleration. A transformer energised from the generator can draw an inrush of several times its rated current. These short events do not change the continuous cable rating but they do affect voltage drop, breaker instantaneous settings and the alternator’s transient response. The motor starting guide covers the mechanics.
Fault current
A diesel alternator is a weak source compared with the DISCOM transformer. Its sustained short-circuit current is typically limited to about three times rated current by the excitation system, after an initial sub-transient peak that may reach eight to twelve times rated for a few cycles. This matters because downstream breakers and fuses that trip quickly on grid supply may trip slowly, or not at all, on generator. Protection coordination therefore has to be checked for both sources.
Unbalance and neutral current
The formula assumes the three phases carry equal current. Single-phase loads distributed unevenly produce different currents in each phase, and the alternator must be judged on the most heavily loaded phase, not on the average. Manufacturers typically allow around 10 percent unbalance before derating. Unbalance and triplen harmonics from single-phase electronic loads also put current in the neutral, which on some installations needs a full-sized or oversized neutral conductor rather than the reduced section once customary.
From current to cable, breaker and transfer switch
The current figure is the entry point to the distribution design, not the exit. Cable size is chosen for current-carrying capacity under the actual installation method and ambient, derated for grouping and for Hyderabad soil and air temperatures, and then checked for voltage drop over the route and for the thermal withstand of the fault current until the protection clears. The cable sizing guide describes the sequence. The transfer switch must be rated for the larger of the generator and utility currents and for the fault level of the stronger source, which is nearly always the grid. The earthing conductor and neutral arrangement follow from the earthing design rather than from the phase current alone.
A worked example (illustrative)
A 320 kVA canopy set is to feed a panel 60 metres away through buried aluminium cable. Full-load current is 320 x 1000 / (1.732 x 415) = 445 A. On installation conditions typical of Telangana, with soil at 35 degrees Celsius and two circuits in the same trench, a cable with a tabulated rating comfortably above 445 A after derating is needed. Checking voltage drop over 60 metres at 445 A shows whether a single run is adequate or two parallel runs per phase are preferable. The outgoing breaker is a 630 A frame set near 450 A with a long-time delay that respects the alternator’s overload curve, and its instantaneous setting is checked against the alternator’s sub-transient fault contribution so that it trips on a genuine short circuit but rides through the compressor start.
Common mistakes
- Calculating current from kW instead of kVA, which understates the figure by 20 percent at 0.8 power factor and leaves the cable undersized.
- Choosing a cable because its catalogue rating exceeds the calculated current, without derating for buried, grouped or hot conditions, so it runs hot every summer.
- Setting the generator breaker from the current table rather than the alternator’s overload and short-circuit curves, which produces either nuisance trips on motor starts or no protection on a fault.
- Assuming balanced phases, so one phase runs at 115 percent of rating while the panel meter shows an acceptable average.
What to send us
Send the generator rating or a nameplate photograph, the supply voltage and whether the load is three-phase or single-phase, the cable route length and how it will be installed, and a list of large single-phase loads if any. If you are reviewing an existing installation, add a photograph of the outgoing breaker label and the cable marking. Send your application engineering enquiry and we will return the current, the suggested breaker frame and the points that need a cable and protection check.
Questions engineers ask
How many amps does a 125 kVA generator produce?
At 415 V three-phase, about 174 A per phase at full load. At 400 V the figure is 180 A, and at 440 V it is 164 A, so always use the actual rated voltage of the set.
Why is the current so high on a single-phase generator?
All the power flows through one phase and the neutral at 230 V, so for the same kVA the current is about three times the per-phase current of a three-phase set at 415 V. That is why single-phase sets are generally limited to small ratings.
Can I size the cable directly from the calculated current?
Only as a first pass. The cable must be derated for installation method, ambient and grouping, then checked for voltage drop along the route and for fault withstand. In practice the voltage drop check often governs for runs beyond 50 to 100 metres.
Does the current change when the load is less than full?
Yes, current is proportional to the actual kVA being drawn. A 100 kVA set carrying 60 kVA draws about 83 A at 415 V. The cable and breaker, however, are sized for the full rating because the load may grow to it.
What happens to the current during a short circuit on generator?
It rises briefly to perhaps eight to twelve times rated current and then falls to roughly three times rated as the excitation system limits it. Breakers must be chosen and set to clear within that window, which is a protection coordination task rather than a current calculation.
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.