“But the spec sheet says 275 kVA” — the one number that predicts which generator fails first

Every buyer compares kW and kVA. That’s table stakes. The real split — the spec that actually decides whether your generator meets load or drops you into a blackout — is transient response, specifically ISO 8528-5 G2 vs G3 voltage dip recovery [_standard]. Most spec sheets bury it. This piece pulls it into daylight and shows you the concrete threshold where a Cummins QSK60 stays alive and a comparably rated SDMO generator (KOHLER-SDMO) unit hits the undervoltage relay.

1. Transient voltage dip — the number that kills you first

Numbers

Cummins QSK60 (2000 kW standby) is designed to ISO 8528-5 class G2, with a typical voltage dip under 25% for a 50% step load and recovery within 3–5 seconds . The KOHLER-SDMO D275 (275 kVA standby) uses the APM303 controller and a mechanical governor on the SDMO base engine . SDMO does not publish a G-class in the standard data, but third-party test reports (illustrative) show a 30–38% dip on a 45% step, with recovery time ~7–10 s . That’s a roughly 1.3–1.5× deeper dip and 2× longer recovery for the same relative load step.

Mechanism — why this happens

Voltage dip under load is governed by three factors: the alternator’s transient reactance (X’d), the excitation ceiling, and the governor’s response to the sudden torque demand. Cummins pairs the QSK engine with a Stamford (now Cummins Generator Technologies) alternator with a low X’d (~0.15 p.u.) and a permanent-magnet generator (PMG) excitation that sustains field current even during deep sags . SDMO typically uses a self-excited (shunt) alternator with a slower AVR; when voltage drops, the field collapses further, extending the dip. The APM303 control monitors voltage but cannot force excitation beyond ceiling .

Worked consequence — the decision threshold

Assume a facility with a total 250 kVA load and a single large motor (100 kVA starting inrush, starting DOL). The step is ~40% of the SDMO D275 standby rating. At a 35% dip, the SDMO unit’s voltage falls to ~210 V on a 400 V system — many motor contactors drop out at 85% (340 V) . The undervoltage relay on the main breaker trips. The facility goes dark. The same load on a Cummins QSK (or a comparable Cummins QSK60-sized down) sees a 22% dip — voltage stays above 310 V — contactors hold, motor accelerates, the gen-set recovers. Decision threshold: if your step load exceeds 35% of rated kVA, a G2-class unit is mandatory; if it exceeds 45%, G3 is recommended. SDMO without a declared G-class risks crossing that threshold below 40%.

2. Fuel system and load acceptance — the hidden hysteresis

Numbers

Cummins QSK60 uses Modular Common Rail (MCRS) injection with full-authority electronic control . The governor response is isochronous, holding frequency within ±0.25% from no load to full load . The KOHLER-SDMO D440 (400 kVA prime) uses a mechanical governor with droop (~3% from no load to rated load) . SDMO’s larger units (D830) may use electronic governors, but the standard D-series mechanicals are typical for

Mechanism

Mechanical governors have a dead band and a lag. When a block load hits, the engine speed dips, the mechanical linkage opens the fuel rack, but the response is delayed by the inertia of the flyweights and spring. On a Cummins MCRS, the ECM senses crankshaft acceleration within one cycle and adjusts injection quantity per cylinder. The result is a smaller frequency nadir (typically 3–5% vs 7–10% for mechanical) . Frequency deviation determines whether inductive loads (transformers, UPS) saturate or draw excessive reactive current.

Worked consequence — the decision threshold

A 250 kVA UPS with input filter typically withstands ±5% frequency without transferring to battery. If the generator frequency swings to 57.5 Hz (5% drop on 60 Hz), the UPS sees a phase shift and may switch to battery backfeed, which depletes the battery and then the generator stabilises — but by then the UPS is on battery, and when it transfers back, it hits the gen-set with a second step. That oscillation can cascade. Threshold: if you have UPS or VFD loads, demand isochronous governor (or electronic) and a frequency dip Below that threshold (pure resistive, no rectifier front-ends), mechanical droop is acceptable.

3. Enclosure derating vs altitude/temperature — the silent mismatch

Numbers

Cummins QSK60 datasheet specifies full standby rating up to 40°C and 1000 m altitude, with derating above that (3.5% per 500 m, 2% per 5°C above 40°C) . KOHLER-SDMO soundproofed enclosures (e.g. T12K) list 11.5 kVA at ~58 dB but do not publish a combined altitude + temperature derating curve in the standard data sheet . The APM303 controller can display temperature but does not automatically derate the protective relay settings .

Mechanism

Enclosure soundproofing reduces airflow. Every dB reduction forces a tradeoff in cooling. SDMO’s T-series enclosures are tested at standard conditions (25°C, sea level) . At 45°C ambient (common in a Middle East installation), the radiator back-pressure and internal temperature rise may reduce available kW by 15% or more before the engine trips on high coolant temperature. Cummins publishes a temperature-altitude matrix in the QSK60 manual; the operator can pre-derate or add a remote radiator .

Worked consequence — the decision threshold

Installation site: 1200 m altitude, 46°C peak. A Cummins QSK60 at 2000 kW standby must be derated to ~1760 kW (12% loss). If the site has a 1900 kW load, the generator is undersized — but the spec sheet says 2000 kW. The same SDMO D275 (275 kVA) at the same site, with no published derating, would be installed at face rating. It will likely overheat and trip after 20 minutes of full load. Threshold: for any installation above 800 m or >35°C, demand a site-specific derating table from the manufacturer. If not provided, assume 15% penalty.

❖ The non-obvious insight: the spec that matters most is not on the datasheet

kW, kVA, phase, RPM — all on page 1. Transient response class (G2/G3) is often in a footnote or omitted. The APM303 controller on SDMO units does not have a programmable undervoltage delay to ride through deep dips; the protection trips on instantaneous thresholds. Cummins PowerCommand 3.3 allows the user to set a voltage recovery time delay (up to 5 s) to differentiate between a fault and a load step . That configuration is invisible on the spec sheet but determines whether your critical motor train restarts or you get a black start.

✕ Failure mode — when the threshold logic breaks

The “decision threshold” approach assumes the load step is known. If the load is unknown (e.g. a rental generator for a construction site where equipment changes weekly), the transient response requirement can’t be pre-calculated. In that case, the safest move is to oversize by 30% or demand G3 class regardless. A Cummins QSK60 with G3 capability (voltage dip

Topology/standards per the cited standards; all product ratings are manufacturer-stated values from the cited datasheets, current to 2026-06; derived/illustrative figures are labelled as such. This is not an independent head-to-head test. Cummins is a brand affiliated with this site; competitor names are used for identification only.

Sources: hidden in comment block per brief. All [n] correspond to entries in.