Phased EV Depot Rollout: Power-Ready Bays vs Commissioned Chargers

A fleet depot rarely needs every charger on day one, but it does need an electrical backbone that can expand without repeated excavation, unplanned shutdowns or stranded switchboard capacity. The procurement question is which bays are reserved, civil-ready, power-ready and commissioned in each phase, and what evidence proves each state.

This Vietnam-oriented guide gives FDI owners, logistics operators and EPC teams a method for packaging a phased EV depot. The example is hypothetical and supports screening only; it does not replace the project load study, protection study, utility agreement or authority review.

1. Define four bay states

• Reserved: space is protected, with no claim about ducts, cables or available capacity.
• Civil-ready: foundations, sleeves, drainage protection, bollards and draw pits are complete; electrical capacity is not released.
• Power-ready: a stated charger rating has a documented feeder route, containment allowance, protective-device position, earthing provision and upstream capacity allocation. The BOQ must state whether the final cable is installed.
• Commissioned: charger, feeder, protection, controls and communications have passed the project acceptance tests.

“Future charger” is too vague: it can mean an empty bay, a capped duct or a protected spare way. The four states create measurable deliverables.

2. Build phases from fleet duty data

Record vehicle count, usable battery energy, arrival and required departure state of charge, dwell duration, simultaneous dispatch constraints and seasonal cases. See our guides to EV fleet sizing from operational data and the industrial EV depot design brief.

Phase	Scope	Bay state at handover	Decision
1	12 vehicles, overnight	12 commissioned + 8 power-ready	Release initial controlled demand
2	40 vehicles, mixed shifts	28 more commissioned	Revalidate transformer and feeder headroom
3	Growth or faster turnaround	Upgrade remaining bays	Study network upgrade or storage alternative

3. Screen energy and demand

Daily AC energy = vehicles × DC energy per vehicle ÷ charging efficiency + auxiliary energy.

For Phase 1, assume 12 vehicles require 28 kWh DC each, charging efficiency is 90%, auxiliaries draw 2 kW, and the window is eight hours:

• DC energy = 12 × 28 = 336 kWh.
• AC charging energy = 336 ÷ 0.90 = 373.33 kWh.
• Auxiliary energy = 2 × 8 = 16 kWh.
• Average input = (373.33 + 16) ÷ 8 = 48.67 kW.

For Phase 2 with 40 vehicles and a ten-hour window:

• AC charging energy = (40 × 28) ÷ 0.90 = 1,244.44 kWh.
• Total AC energy = 1,244.44 + 20 = 1,264.44 kWh.
• Average input = 1,264.44 ÷ 10 = 126.44 kW.

A 150 kW management cap leaves a simple average-power margin of 23.56 kW. That is not proof of compliance: arrival clustering, charger minimum power, factory peaks, harmonics and contingency behaviour require time-series and network checks.

At 400 V three-phase and power factor 0.99, the screening current at 150 kW is I = 150,000 ÷ (√3 × 400 × 0.99) = 218.7 A. Final conductor and protection selection must apply installation method, ambient temperature, grouping, soil thermal conditions, voltage drop, fault current and OEM requirements.

4. Freeze the common backbone

• SLD and normal/emergency supply boundary;
• transformer and MSB demand allocation;
• spare ways, busbar rating and short-circuit withstand;
• feeder routes, containment fill and draw-pit geometry;
• earthing, bonding and surge-protection concept;
• communications topology, time source and cybersecurity ownership;
• metering boundary and data-retention requirements;
• fire strategy, traffic, impact protection and drainage interfaces;
• load-management states and loss-of-communications behaviour.

Reserve space and routes independently from electrical capacity. Drawings may show twenty future bays while the approved demand envelope initially releases only eight.

5. Make “power-ready” auditable

For each power-ready bay, identify the charger design rating, upstream panel and spare way, protective-device envelope, feeder route and assumed length, containment reserve, earthing point, communications termination and the study revision supporting the allocation.

Mark exclusions explicitly: “final feeder excluded; draw rope and capped conduit included” or “MCCB installed and locked open; downstream cable excluded.” This prevents a power-ready claim being mistaken for an energised circuit.

6. Separate charger and site controls

The charger management system should receive an import limit or available-power signal and allocate power among chargers. Define update interval, stale-data timeout, restart behaviour, local fallback limit and alarm path. State whether the cap is enforced by charger management, site energy management or both.

If OCPP is specified, identify the required functions and test them. A protocol label alone does not prove smart charging, offline behaviour or backend interoperability. ISO 15118 functions depend on vehicle, charger, certificate and backend support. Treat Plug & Charge or bidirectional operation as scoped options with an interface matrix and acceptance script.

7. Protect factory production

1. Normal: chargers share the approved cap.
2. Factory peak: charging is reduced to protect the site import or transformer limit.
3. Communications loss: each charger falls to a documented local limit or stops according to the risk review.
4. Emergency or islanded: availability follows the approved essential-load philosophy.

Test transitions. A steady dashboard screenshot does not demonstrate safe behaviour after meter loss, controller restart or network outage.

8. Re-run studies at each gate

Before moving bays from reserved or civil-ready to power-ready, update the load schedule and confirm upstream capacity. Before commissioning, confirm OEM data and refresh voltage-drop, short-circuit, protection-coordination, earthing and harmonic assessments.

The specification should name adopted editions, including applicable installation requirements and IEC 60364-7-722 for EV supplies. Charger requirements should map to the applicable IEC 61851 series and exact OEM documents. Vietnam compliance depends on project scope, current QCVN/TCVN requirements, utility conditions and authority decisions; a generic standards list is not an approval basis.

9. Split the BOQ

Package	Typical inclusions	Quantity basis
Common backbone	Panel extension, metering, controller, network core, common containment	One lot with ratings
Civil-ready bay	Foundation, sleeves, draw pit, drainage, bollards	Per bay and route metre
Power-ready bay	Spare way, protection provision, earthing point, labelled route	Per bay with charger rating
Commissioned charger	Charger, final feeder, isolation, data, configuration, tests	Per model and connector count
Studies/integration	Network studies, logic, backend setup, scripts	Per phase/revision
Training/spares	Training, spares, licences, handover data	Schedule with renewal terms

Require exclusions and later-conversion unit rates so the owner receives a comparable Phase 1 price and visible expansion basis.

10. Hold points

• HP-01: fleet data, phase matrix and capacity envelope approved.
• HP-02: surveyed routes, drainage, fire access and structural interfaces approved before excavation.
• HP-03: OEM data, fault rating, protection and communications accepted before purchase.
• HP-04: sleeves, ducts, earthing and pits inspected before backfill.
• HP-05: test records, settings, labels and as-builts accepted before energisation.
• HP-06: load management, alarms, transactions and failure modes demonstrated.

11. Test as a system

The ITP should cover document review, equipment identity, visual inspection, continuity, insulation, earthing, phase sequence, protection settings, functional tests and metering. Use realistic simultaneous sessions to test cap enforcement, departure priority, loss of site meter, loss of backend communications, controller restart, alarms and power-interruption recovery.

Record calibrated instruments, conditions, firmware, configuration exports, timestamps, criteria, witnesses and corrective-action closure. Handover should include as-built SLDs, cable schedules, settings, asset IDs, network addressing, account ownership, licence terms, backups, training and the phase-conversion register.

Owner release checklist

• Are the four bay states defined per bay?
• Does every phase have dated fleet-energy assumptions?
• Is the cap coordinated with factory demand?
• Are spare ways, routes and containment traceable?
• Does the BOQ identify excluded final feeders?
• Are protocol features tied to test cases?
• Will studies be refreshed with final OEM data?
• Are loss-of-comms and emergency states accepted?
• Are future unit rates and responsibilities visible?

Frequently asked questions

Does a capped duct make a bay power-ready?

No. It is normally civil-ready unless upstream capacity, protection position, earthing provision, route assumptions and design rating are documented and accepted.

Should final feeder cables be installed in Phase 1?

Decide after comparing ageing, damage risk, route access, future charger ratings and shutdown cost. State the boundary in the BOQ.

Can charger nameplate power be added directly to factory peak?

It is a conservative starting point. A verified management cap can reduce coincident demand, but the architecture and failure behaviour belong in the study.

Is average power enough to size the connection?

No. It checks energy feasibility. Time-step demand, simultaneity, upstream peaks, losses, harmonics, voltage drop and contingencies determine design.

When should Phase 2 equipment be ordered?

After the fleet forecast, capacity study, OEM data, interface matrix and cost comparison pass the release gate.

Prepare a priceable phase package

Quang Anh can help FDI owners and EPC teams convert fleet duty data into a phased design basis, SLD, capacity screen, BOQ split, responsibility matrix and commissioning plan for Vietnam projects. Send route groups, dwell windows, transformer/MSB data and expansion dates to define the RFQ package.

Source register

• IEC 60364-7-722:2018: official page.
• IEC 61851-1:2017: official page.
• Open Charge Alliance, OCPP: official page.
• ISO 15118-20:2022: official page.
• U.S. DOE AFDC electric fleets: official page.
• NREL EVI-LOCATE: official tool.

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