Sizing AC Generator CTs for 1875 kVA, 380 VAC Systems

The number that matters is the current in each line conductor. For a balanced 1875 kVA, 380 VAC three-phase generator, that current is approximately 2848.5 A per phase. The three currents are equal in magnitude under balanced loading; they are not added into a separate “total current.”

Current and thermal limit

Generator kVA is the total three-phase apparent-power rating. With 380 V interpreted as line-to-line voltage, calculate the balanced line current from:

Iline = S3phase / (sqrt(3) × VLL)
Iline = 1,875,000 VA / (sqrt(3) × 380 V)
Iline = 2,848.5 A

Therefore, at rated balanced apparent power:

Ia = Ib = Ic ≈ 2,848.5 A

Each phase CT must measure approximately 2.85 kA at full rated load. This is just under 3000 A, but that observation alone does not finalize the CT ratio. The CT must also accommodate the generator nameplate current, permitted operating range, meter input, burden, accuracy requirement, conductor size, and expected transient current.

This is heat, not logic. Line current drives conductor, winding, switchgear, and CT thermal loading, while real power also depends on power factor. A generator can approach its current or kVA limit without reaching its real-power limit.

Quantity	Value or formula	Where to confirm it
Total apparent power	1875 kVA	Generator nameplate
Line-to-line voltage	380 VAC	Nameplate and phase-to-phase measurement
Balanced line current	2848.5 A	Calculation and generator full-load-current marking
Approximate rule	1.5 A/kVA at 380 V	Use only for an initial estimate
Calculated factor	1.519 A/kVA at 380 V	1/(sqrt(3) × 0.38)

Phase quantities in a four-wire system

The 2848.5 A result is the magnitude of each line current when the load is balanced. It is not one-third of a separate current rating. Dividing the total apparent power by three gives the apparent power associated with each balanced phase:

Sphase = 1875 kVA / 3 = 625 kVA

If the generator is a 380/220 V wye system, its calculated line-to-neutral voltage is:

VLN = 380 V / sqrt(3) = 219.4 V

That result explains the nominal 220 V line-to-neutral loads. Each balanced phase then carries 625 kVA / 219.4 V ≈ 2848.5 A. Confirm the winding arrangement and terminals on the generator documentation before applying the wye relationship.

Under ideal balance, the three current phasors sum to zero and neutral current is zero. The arithmetic sum of the three magnitudes—approximately 8545.5 A—has no useful meaning for selecting a phase CT. Unequal quantities or distributions of 220 V loads make Ia, Ib, and Ic different and create neutral current. A load-shedding system must therefore evaluate all three phase currents rather than multiply one measured current by three.

Line-to-neutral loads use the neutral conductor as their normal return path. Protective ground is not a substitute for the neutral. If measured phase-to-ground voltage is being used as shorthand for load voltage, verify the actual load connection before commissioning the measurement system.

Measurement approaches

Approach	Measured information	Advantages	Limitations
Three CTs with compatible current inputs or transducers	Individual values of Ia, Ib, and Ic	Direct input for simple overcurrent or phase-loading decisions	Current alone does not report kVA, kW, power factor, or voltage loss
Three CTs and a three-phase power monitor	Phase currents plus voltage-derived loading quantities supported by the monitor	Better fit for generator loading, imbalance, and load-shedding logic	Requires correct voltage connections, CT polarity, scaling, and communications mapping

Use direct current measurement when the control decision is strictly based on the highest phase current. Use a three-phase power monitor when the program must distinguish current limit, apparent power, real power, phase imbalance, or abnormal voltage. Variable 220 V line-to-neutral loads favor the power-monitor approach because a balanced estimate derived from one CT can hide an overloaded phase.

The recommended architecture is one CT on each phase conductor feeding a compatible three-phase monitor, with the monitor supplying phase currents and loading quantities to the shedding program. Retain the individual phase currents in the control interface even when the program also uses total kVA or kW.

CT primary-rating decision

A primary rating near 3000 A appears numerically close to the calculated 2848.5 A full-load current. It provides only about 5.3% numerical margin, so select it only after checking the actual generator nameplate current and the CT manufacturer’s continuous-current and accuracy data.

Selection item	Decision rule	Source of the required value
Primary current	Cover generator nameplate full-load current and the approved operating range without losing required resolution	Generator nameplate and operating specification
Secondary interface	Match the monitor or transducer input exactly	Monitor input label and manual
Accuracy	Meet the accuracy required at the shedding threshold, not merely at full scale	CT accuracy table and control tolerance
Burden	Include the monitor input and the complete secondary wiring circuit	CT and monitor datasheets plus cable calculation
Saturation behavior	Keep the expected measurement range within the CT’s specified performance	CT excitation or performance data
Mechanical fit	Fit the phase conductor or bus while preserving insulation clearances	Switchgear layout and CT dimensional drawing
Polarity	Orient all three CTs consistently from source toward load	CT markings and monitor wiring diagram

The shortcut 1.5 A/kVA gives 2812.5 A for 1875 kVA. It is useful for a quick plausibility check but is about 36 A below the formula result. Base the final ratio and control scaling on the nameplate and exact three-phase calculation.

Load-shedding decision criteria

Choose the controlled quantity from the limit being protected. Use the maximum of Ia, Ib, and Ic to protect against phase thermal overload. Use total kVA when generator apparent-power capacity is the governing limit, and use kW when prime-mover real-power capacity is the governing limit. A practical system can supervise all three and act on whichever reaches its configured threshold first.

Set thresholds from the generator and switchgear documentation rather than from the CT full-scale value. CT range is an instrument property; it is not the allowable generator load. Apply pickup and dropout separation, and use an approved delay where needed to prevent switching chatter during normal load steps. Read the permitted thresholds and delays from the generator operating requirements and the process load-shedding study.

For unbalanced 220 V loads, prioritize shedding by the affected phase when the distribution design permits it. A total kVA value can remain below rating while one phase is heavily loaded. Record all three currents before and after each shedding stage so the program can verify that the commanded action relieved the limiting quantity.

Installation and configuration procedure

1. Read the generator nameplate full-load current and confirm that 380 VAC is the line-to-line rating used for the calculation.
2. Identify the generator winding and verify the phase-to-neutral voltage if 220 V loads are present. Confirm that those loads return through neutral rather than protective ground.
3. List the maximum normal operating current, required measurement accuracy at the shedding threshold, conductor dimensions, secondary cable route, and monitor input specification.
4. Select three matching CTs whose primary range covers the approved operating current and whose secondary interface, burden, accuracy, and mechanical dimensions match the installation.
5. Install one CT around each phase conductor only. Keep polarity direction identical on phases A, B, and C.
6. Wire each secondary through the approved termination and shorting arrangement to the monitor. Never open-circuit a CT secondary while primary current is flowing; hazardous voltage and CT damage can result.
7. Connect the monitor’s voltage inputs according to the documented four-wire configuration. Use the monitor’s stated phase order and neutral terminal assignments.
8. Enter the installed CT ratio and voltage configuration. Map Ia, Ib, Ic, and the required power quantities into the control program.
9. Configure shedding pickup, dropout, delay, and stage priority from the approved operating limits. Keep the current threshold separate from the CT full-scale setting.

Commissioning and verification

Begin at the lowest practical load. Compare the monitor’s phase-to-phase and phase-to-neutral readings with an independent suitable instrument and the generator indication. A voltage mismatch points to configuration or wiring; a current mismatch points to CT ratio, phase assignment, polarity, burden, or secondary wiring.

Apply a known balanced load increment when the installation permits it. The measured phase currents should rise together, and the calculated relationship S = sqrt(3) × VLL × Iline should agree with the monitor within the specified instrument tolerances. With unequal line-to-neutral loads, compare each phase separately and verify that the most heavily loaded phase is the one reported by the control system.

Test each shedding stage below the generator’s limiting condition by simulation or an approved controlled-load test. Confirm the correct output operates, the intended load disconnects, the limiting current or power falls, and the stage resets only at its configured dropout condition. Also verify loss-of-signal handling so an invalid current or communications value cannot be interpreted as a lightly loaded generator.

Frequently asked questions