Declared, Undeclared, Inferred: Reading a Cummins QSK vs KOHLER-SDMO D830 Quote for an N+1 Data Hall
A colocation operator building an N+1 standby plant for a single data hall asked for two like-for-like quotes: a Cummins QSK set in its lower band and a KOHLER-SDMO generator D830 (about 750 kW prime / 825 kVA standby), the top of the D-series. Both documents are full of numbers. The decision does not turn on whose numbers are bigger — it turns on whose numbers are knowable. This teardown sorts every claim into three buckets and shows how the provenance of a figure, not its value, is what should move the buying decision.
Dimension 1 — Transient performance class
Mechanism — why the provenance matters here
A data hall's UPS rectifiers and cooling plant impose step loads on the generator the instant it picks up the building. Whether the set holds frequency and voltage through that step is governed by ISO 8528-5 performance class, alternator transient reactance, excitation headroom and governor response. For a colocation SLA you cannot design the protection scheme around a number you cannot see.
Cummins generator: PowerCommand 3.3 with isochronous load-sharing and AmpSentry is a declared control platform, and Cummins publishes standby/prime ratings and transient behaviour against the standard.
SDMO D830: the APM403 control and base-engine governor are documented, but a specific ISO 8528-5 G-class for the packaged set is typically not published in the standard data — it must be requested.
Worked consequence
The data-hall designer must commit a transient envelope to the client SLA before ordering. With a declared class, the envelope is contractible: the protection and the UPS ride-through are designed to a number the manufacturer stands behind. With an undeclared class, the designer is choosing between two bad options — design to an inferred figure (illustratively a deeper, slower dip than a G2-class set) and carry that risk, or hold the order while a written G-class is extracted from the vendor, slipping the schedule.
Decision force: the declared number is worth a measurable schedule and risk premium. For an SLA-backed hall, prefer the set whose transient class is on paper; do not let an inferred number anchor a contractual envelope.
When this reverses: for a non-SLA internal lab bus with generous UPS ride-through and no step-load near the set's limit, the transient class barely binds; an undeclared figure costs nothing because nothing downstream depends on it. Provenance only carries weight where a contract leans on the number.
Dimension 2 — Heat rejection and hot-aisle derate
Mechanism — why the provenance matters here
Usable output at a hot generator yard is nameplate minus derate minus parasitics. Derate is driven by the three heat-rejection streams — jacket water, charge-air after-cooler, radiator airflow — against intake temperature and altitude, plus the cooling-fan power drawn off the shaft. A data hall's standby set often sits in a walled enclosure where intake air is warmer than ambient.
Both: rated kW at reference conditions is declared by each maker.
Heat-rejection rates and fan parasitic draw at the specific enclosure restriction are frequently undeclared at quote stage and must be pulled from the technical data pack.
The actual hot-day usable kW is then inferred from standard derate behaviour.
Worked consequence
Size the hall's essential load at 700 kW. Both sets are 825 kVA standby on paper. Apply an illustrative hot-enclosure derate of, say, 6–9% plus fan parasitics, and usable output lands somewhere in the 700–760 kW window — but where in that window depends on the undeclared heat-rejection margin. A large, slow-turning displacement with thermal headroom (the QSK case) tends to land at the top of the window; a smaller engine nearer its thermal ceiling tends to land at the bottom, possibly under the 700 kW the SLA promises.
| Figure | Provenance | What you can rely on |
|---|---|---|
| 825 kVA standby rating | Declared | Reference-condition only |
| Heat-rejection / fan draw | Undeclared | Must request before order |
| Hot-day usable kW | Inferred | Carry margin; verify |
Decision force: never size an N+1 hall on a declared reference rating alone; demand the undeclared heat-rejection data and size on the inferred hot-day kW with margin to the SLA load.
When this reverses: in a temperature-controlled generator room held near reference conditions with unrestricted airflow, derate shrinks toward zero and the declared rating is close to the usable rating — the undeclared heat-rejection figures stop being load-bearing on the decision.
Dimension 3 — Protection coordination on the N+1 bus
Mechanism — why the provenance matters here
N+1 means two or more sets share a paralleling bus so any one can be serviced live. Coordination requires the source to hold fault current along a known curve while downstream breakers clear selectively, and the sets to load-share without hunting. The relevant facts are the relaying philosophy and the load-sharing method.
Cummins: isochronous load-sharing and AmpSentry protective relaying, with paralleling arrays from 2 MW to 20+ MW in N+1 or 2N and black-start, are declared platform capabilities.
SDMO D830: the standard APM403 panel provides control, metering and breaker management; integrated multi-set isochronous paralleling and a source-matched relay are not the published standard-package behaviour and would need to be specified and confirmed.
Worked consequence
On a declared isochronous-paralleling platform, the N+1 bus is a catalogue capability: two sets share load evenly, either can drop out for service, and the relay holds fault current to the alternator's damage curve so a branch fault clears without collapsing the bus. Where multi-set paralleling and source-matched relaying are undeclared in the standard package, the operator is buying integration risk — the synchronising and load-share scheme becomes a bespoke engineering exercise whose behaviour is inferred until commissioned and tested.
Decision force: for an N+1 SLA bus, a declared, catalogued paralleling-and-relay capability is worth a premium over an inferred one; the cost of a paralleling scheme that misbehaves on a fault is the whole hall, not one branch.
When this reverses: if the redundancy is achieved another way — a single set plus a separate UPS-backed ride-through, with no generator-to-generator paralleling — then multi-set isochronous capability is not on the critical path and its declared/undeclared status stops driving the choice.
How to read the two quotes
Line the documents up and the asymmetry is the story. The Cummins quote tends to put the load-bearing numbers — control platform, paralleling scale, relaying, ratings against the standard — in the declared column. The D830 is a genuinely capable 825 kVA machine, but several figures a data-hall SLA leans on — transient class, package heat rejection at the real enclosure, standard-package multi-set paralleling behaviour — sit in the undeclared column until you extract them.
That is not a verdict that one engine is better; it is a procurement rule. Pay for declared numbers where a contract depends on them; treat undeclared numbers as risk to be priced or extracted in writing before order; never let an inferred figure quietly become the basis of an SLA. For a single non-contractual bus, the cheaper undeclared route is fine. For an N+1 hall with uptime penalties, the provenance of the number is the specification.
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.
