Imported production lines: turn motor ratings into an auditable factory P/Q load schedule

An imported production line often arrives with an equipment list that looks complete: 75 kW motor, 45 kW mixer, 120 kW oven, 55 kW compressor. It is not yet an electrical demand schedule. The ratings may refer to motor shaft output, installed heating elements, electrical input at the nameplate, or the maximum setting of a variable-speed drive. Adding those numbers gives a convenient total but not a defensible basis for transformer capacity, switchboard loading, cable design, or an energization request.

This engineering guide provides an auditable method for an FDI factory’s owner, machine supplier, and Vietnamese electrical EPC contractor to turn incomplete machine information into a P/Q schedule. It includes a worked hypothetical process line, uncertainty classes, an independent mathematical check, and an RFQ data register. It is not an approved design for any specific factory.

Prepared for QuangAnhcons · Editorial focus: factory MV/LV power system inputs · Sources checked: 10 October 2026. All machine capacities, loading assumptions and duty cycles in the worked example are hypothetical.

The first design decision: what does each nameplate kW mean?

A motor rated at 75 kW normally denotes mechanical output at the shaft under specified rating conditions. The electrical power it absorbs is greater because the motor has losses. For an idealized steady-state motor operating at the specified output, input active power equals shaft power divided by efficiency. Its apparent electrical demand additionally depends on power factor. However, an oven marked 120 kW may denote heating-element electrical rating, while a complete imported machine marked 120 kVA may already be specifying apparent input. A VFD panel’s nominal kW can be a drive frame rating, not the process power observed during normal production. Classify every incoming value before calculating anything.

A machinery schedule must preserve the source units and the document revision. For instance, label a 75 kW rated motor shaft_kW rather than transforming it silently into input_kW. Record when the efficiency or power factor is measured versus assumed; do not inherit typical motor efficiency data across different motor loads and speeds. Harmonic-producing loads require special attention because the total power factor measured at the supply can differ from the fundamental displacement power factor often shown for a motor. A VFD’s upstream current, harmonic distortion, front-end technology and filtering must come from the correct model documentation.

Incoming document	Essential fields to obtain	Why they affect the decision
OEM process machine schedule	Machine ID, quantity, mechanical/electrical rating type, line voltage, phases, frequency, duty	Prevents mixing shaft and electrical ratings or 400 V and other supply levels
Motor / drive documentation	Input current, efficiency and PF at operating point, inrush or ramp profile, permitted starts per hour	Separates thermal demand from supply disturbance
Operating sequence	Which motors and ovens can run together, maximum product throughput, cleaning and restart mode	Establishes real coincidence rather than an invented diversity factor
Energy and controls architecture	UPS, controls, emergency stop mode, ride-through duration, power-quality sensitivity	Distinguishes continuous loads from essential or transient loads
Interface and delivery register	Party providing data, source revision, evidence, due date and open assumptions	Makes unresolved issues visible to the factory owner and EPC bidder

The principal output should be a version-controlled register in which each calculation cell traces to a drawing or OEM schedule. If the machine supplier declines to disclose a start-current value, identify the uncertainty explicitly rather than inserting a guessed factor and calling the design complete.

Convert shaft power into active, reactive and apparent power

For a three-phase induction motor in a simplified steady-state model, use P_in = P_shaft × load_fraction / η, where η is operating efficiency at that load. For a declared displacement power factor cosφ, the fundamental reactive component is Q = P_in × tan(arccos(cosφ)), and S = sqrt(P_in² + Q²). If the supply is balanced and sinusoidal, the corresponding RMS line current is I = S × 1000 / (sqrt(3) × U_LL), with S in kVA and U_LL in volts. Keep this assumption explicit; converter harmonic current may make these relations inadequate for actual total RMS current or cable heating.

Never sum the apparent kVA of heterogeneous circuits and then pretend that the result is the exact total. When fundamental PF differs across simultaneous loads, aggregate active P and signed reactive Q separately, then calculate total S. This avoids a recurring specification mistake: averaging equipment power factors, multiplying by total kW and treating the result as an engineering study. If a load has power-factor correction or leading reactive power, its Q sign may be negative. If nonlinear current distorts the waveform, add an explicit distortion assessment; the simple P/Q vector model is a first-pass balance only.

Worked example: six load groups on a hypothetical 400 V factory bus

Assume a packaging factory is considering a new imported production line. The line contains two 75 kW motors, three 45 kW motors, an electrical oven, a compressor, a chilled-water auxiliary and lighting/controls. On the worst normal-production case under review, both larger motors run at 85% shaft output, two smaller motors at 90%, the oven at its listed electrical rating, and auxiliary loads according to the stated scenario. All values below are illustrative and must be replaced by OEM or measured data.

Load group	Simultaneous units	Operating basis	Input P (kW)	Fundamental PF	Q (kVAr)
Motor A, 75 kW shaft each	2	0.85 load, η=0.94	135.64	0.86	80.48
Motor B, 45 kW shaft each	2 of 3	0.90 load, η=0.92	88.04	0.88	47.52
Resistive oven	1	120 kW electrical input	120.00	0.98 assumed	24.37
Compressor, 55 kW shaft	1	0.75 load, η=0.93	44.35	0.85	27.49
Chilled-water package	1	95 kW electrical × 0.80	76.00	0.92	32.37
Lighting and controls	1 group	35 kW electrical input	35.00	0.95	11.50

For this illustrative scenario, P_total ≈ 499.03 kW, Q_total ≈ 223.74 kVAr, S_total ≈ 546.9 kVA, and at 400 V three-phase I ≈ 789 A. These quantities describe simultaneous steady-state fundamental power demand, not a transformer or ACB selection, short-circuit result, motor starting acceptance, or busbar temperature verification. A 630 kVA transformer might seem close by nameplate alone, but its actual suitability cannot be decided without existing loads, ambient and cooling conditions, harmonics, losses, voltage variation, future expansion, and the approved distribution arrangement.

Independent checks are important. The two 75 kW motors must contribute 2 × 75 × 0.85 / 0.94 ≈ 135.64 kW rather than 150 kW or 2 × 75/0.85. The three 45 kW motor nameplates are installed capacity, but only two were assumed to be running in the peak scenario. Adding the third changes the result; do not hide that fact inside a generic 0.8 diversity allowance. Similarly, every auxiliary power factor is a project-specific input and should be revisited if a supplier provides a different operating point.

Separate duty cases instead of applying one diversity multiplier

The factory’s operational peak may be an entirely different instant from the maximum demand of a single machine. A credible study needs an operating case register, including normal peak throughput, reduced production, overnight housekeeping, start-up after a grid interruption, maintenance, and emergency load shedding. The line may stop its motors when the oven heats up, or all drives may accelerate simultaneously after a controls reset. Neither behaviour should be inferred from the BOM. Ask the production planner which events can coincide and the automation vendor which sequence is programmed.

Case	Expected main consumers	Special examination	Design consequence
Full production	Two main drives, selected mixers, oven and utilities	Coincidence and sustained thermal P/Q	Initial transformer, MSB and feeder loading envelope
Shift start	Compressor, pumps and multiple motor accelerations	Voltage dip, frequency transient, VFD current limitation	Starting sequence, generator and utility interface
Washdown / maintenance	Few motors but heaters and cleaning pumps possible	Unusual simultaneity, equipment isolation	Smaller feeder and protection settings may govern
Grid restoration	Auto-restart groups, contactors, UPS recharge	Staggered restoration and upstream voltage	Interlocks and sequencing requirements
Critical operation	Controls, safe-stop loads, cooling as needed	Ride-through, essential supply duration	Emergency board and generator/UPS scope

An owner’s demand estimate is not made safer by simply increasing every group by a large undefined percentage. A visible load margin can be useful, but it should sit in a separate column with a named reason and approval owner. Such reasons may include a production line that has not yet been specified, a contractual reserve, seasonal cooling demand, or a verified future machine addition. Avoid classifying unapproved growth projections as simultaneously connected at commissioning.

Reconcile the LV board, transformer and utility boundary

Place the load schedule on an up-to-date single-line diagram: actual electricity provider or industrial-park distributor → metering/connection point → MV switchgear → transformer(s) → MSB → MDB → process feeder → machine terminals. A machine supplier may supply a local MCC with an isolator while the electrical EPC scope ends at its incoming lugs, or the EPC may be expected to terminate each motor. Those are different contracts. For imported production lines the voltage and earthing requirements must also be checked against the actual Vietnamese supply and the transformer vector group; do not assume every machine has a 400 V line input.

The total load at the PCC may include older factory operations, pumps, compressors, HVAC, distribution losses, and auxiliary supplies not present in the imported-line spreadsheet. A transformer may supply more than one bus section. If the equipment schedule totals approximately 547 kVA for the new line only, the building’s total demand still requires a separate calculation. Sizing feeder conductors also needs installation method, grouping, ambient, fault clearing time, voltage-drop, neutral harmonics and cable-manufacturer data. The illustrative line current is a screening number, not a verified cable ampacity.

Short-circuit levels require maximum and minimum supply cases, network impedance and upstream fault data; one transformer percentage impedance does not constitute an IEC 60909-compliant fault study. A short-circuit study may need contributions from motors, inverter-based sources and approved switching arrangements. The appropriate edition of IEC 60909-0 and the model assumptions must be established before issuing protective-device duty ratings. A vendor brochure showing a circuit breaker’s high breaking capacity is not proof that selectivity and short-circuit protection of the entire installation are adequate.

Procurement deliverables that expose missing information

Before an RFQ becomes a fixed-price electrical package, ask each supplier to label the maturity of its data. A simple four-state status works: Confirmed by signed OEM document, Provided but revision pending, Engineering assumption for comparison only, or Missing—RFI open. Keep the supplier evidence link or file name in the same row as the rating, rather than a detached collection of PDFs. For ambiguous or conflicting values, include both the older value and the new proposed value with their revision identifiers; do not silently overwrite the provenance.

The machinery RFQ should request at minimum: an electrical input schedule for every machine and local subassembly; exact line voltage and supply arrangement; steady input kW/kVA and total PF or true current as applicable; start-up sequence and allowable voltage disturbance; harmonic/emission information for drives; emergency stop / restart behaviour; protective-device coordination data; control and communication drawings; earthing and EMC requirements; incoming terminal details; witnessed test records and the factory-acceptance scope. Owners should distinguish what is required before equipment manufacture from what may be verified at SAT after delivery.

A useful handover is a calculation workbook with a field-by-field input list, not just the final transformer kVA and a confidence-sounding sentence. The workbook should store source document, revision, project assumptions, what the electrical EPC has actually checked, which conditions remain untested, and who will accept each residual risk. QuangAnhcons can use this register to discuss a realistic site visit or design-coordination scope with the owner before committing to BOQ quantities.

Mini work programme and hold points

The process below is a relative-day planning aid, not a promise of manufacturing or utility approval dates. A day means one assumed working day, and tasks may overlap only when source inputs genuinely permit it.

ID	Work package	Illustrative effort	Dependency and hold point
M01	Receive OEM load list / SLD / supply contract	2–3 days	Open RFIs logged; no design release yet
M02	Confirm machine ratings and running cases	3–5 days	After M01; OEM acknowledges electrical input meanings
M03	Run independent P/Q, source and scenario checks	2–4 days	M02; owner agrees demand basis and reserve
M04	Develop feeder and transformer design study	Project-specific	M03; source/fault and installation inputs required
M05	Coordinate procurement/factory testing	Vendor-specific	No release before agreed design hold point
M06	SAT and energized commissioning	Project-specific	Installation, utility permission and safety records complete

A programme diagram with attractive dates is not evidence of permitting or equipment availability. Release the bill of quantities only after the interface boundaries and design inputs have been reviewed with the owner. A supplier’s early estimate may be suitable for budgeting without being adequate for construction.

References, editions and evidential limits

Source	Scope relevant to this article	Access and limit (10 October 2026)
IEC 60076-1:2011	Transformer general specification scope	Official IEC catalogue title and edition; not a clause-by-clause design audit
IEC 60909-0:2026	Calculation scope of LV/HV AC short circuits	Latest IEC edition catalogue confirmed; no short-circuit study in this example
IEC 60364-5-52:2009+A1:2024	LV wiring systems and conductor installation	Official scope/edition only; no rating table taken from a paid standard
Schneider Electric power metering	P, Q, S and power-factor measurement	Open manufacturer user-guide content; device-specific measurement concepts only
Vietnamese TCVN/QCVN and actual supply/distributor specifications	Legal, safety and connection applicability in Vietnam	Project site and current local source documents still required; no unverified clause cited

Related reading and next action

For ownership and responsibility boundaries, see Electrical design responsibility matrix for FDI factories. For the procurement programme after the line load is confirmed, see Long-lead electrical equipment in Vietnam. For construction interfaces, the industrial electrical construction overview explains where delivery packages meet. These links complement rather than replace the line-by-line schedule above.

QuangAnhcons’s factory substation energization video gives general installation and commissioning context. It does not depict this hypothetical imported machine line or verify any numeric assumption in the calculation.

To scope a load study or request an electrical EPC quotation, send the OEM load list, machine voltage/frequency, current SLD and operating sequence to projects@quanganhcons.com or phone +84 919 758 191. No transformer size, cable cross-section or protective setting should be released from the illustrative figures alone.

[contact-form-7 id="7239967" title="Contact form 1"]