Shore Power Load Assessment for Vessel Retrofits

A vessel's generators are selected for operating cases that can include loads never seen at berth. Sizing a shore connection from total installed generating capacity can oversize the system; sizing it from one low-demand reading can undersize it. The design load should come from a representative maximum berth scenario, supported by measurements or the vessel's load analysis and recorded assumptions.

Start with the loads that run alongside

The assessment starts with the consumers that operate while the vessel is alongside, and with the operating scenario that puts the most of them on at once. The list differs by ship type and by trade, which is why a figure borrowed from a sister vessel on another route is a starting point only.

• Accommodation and hotel services: lighting, galley, ventilation and air conditioning, which follow the season and the climate of the ports
• Machinery kept running in port: pumps, compressors, engine-room ventilation and the auxiliaries of systems on standby
• Cargo-related load: reefer containers, cargo pumps, hatch and ramp hydraulics or cargo gear, depending on the ship
• Large intermittent consumers, such as ballast pumps during cargo operations

For each consumer there are three questions: does it run at berth, at what load, and which other consumers run at the same time. The last question is what turns a list of ratings into an operating case.

Simultaneous demand, not the sum of the nameplates

Adding up the rating of everything that can run at berth overstates the load, because consumers do not all run at full output together. Taking the average understates it, because the connection has to carry the peak. What sizes the system is the highest demand that occurs in a realistic berth scenario and lasts long enough to matter to the equipment: a hot afternoon with cargo operations under way, not a theoretical moment when every motor on board starts at once.

Short-duration peaks are checked separately from steady demand. A motor started direct-on-line can draw several times its running current for a few seconds, so the voltage dip, transformer and cable capability, and the shore-incomer protection settings all have to be checked for that starting case.

Measure where possible; reconstruct where necessary

Measured generator or bus load over representative port stays shows how the vessel actually operates alongside. Useful records may come from the alarm and monitoring system, the power management system or engine-room logs, provided they cover the seasons and cargo conditions that matter.

Where records are limited, the berth load can be reconstructed from the vessel's electrical load analysis and corrected for how she is actually operated in port. The result remains a model, so its assumptions are recorded and checked against any measurements that exist. If the programme allows, targeted measurements can then be taken before equipment ratings are fixed.

Margin is an engineering decision

Margin covers conditions that are not fully represented in the records: a hotter port, a heavier reefer manifest or equipment added later. Too little can constrain the vessel on a high-demand call; too much can drive unnecessary transformer, cable and space requirements. There is no universal percentage. The margin should be agreed against the vessel's expected trade and recorded with the assumptions behind it.

The result is stated in kilowatts and in kVA. Transformers and cables are rated by current, so the power factor of the berth load belongs in the assessment beside the kilowatt figure.

What the design load decides

• Transformer: rated for the design load in kVA, with impedance considered alongside voltage drop and available fault current
• Cable: the shore connection cable and the onboard feeder, sized for the load current, the installation conditions and the voltage drop over the run
• Breaker and switchboard: the shore incoming breaker rated for load current and the fault level at its location, with the main switchboard busbars checked for the shore-supply operating case

Design load is one input to whether a high- or low-voltage connection is appropriate. The published scope of IEC/IEEE 80005-1 says high-voltage connection is expected to be practicable for ships requiring 1 MVA or more at berth, or ships with a high-voltage main supply; IEC/IEEE 80005-3 covers low-voltage shore connection for ships requiring up to 1 MVA. Our guide to IEC/IEEE 80005-1 on the ship side explains what Part 1 covers.

How the supply then reaches the main switchboard is a separate piece of engineering, covered in shore power switchboard integration. The checks on the board itself are those described under switchboards and power distribution.

FuelEU's regulatory demand figure is not the design load

FuelEU Maritime defines an "established total electrical power demand at berth" for regulatory purposes: the highest value, in kilowatts, of the ship's total electrical demand at berth. That figure is used in the penalty calculation for a non-compliant port call. It describes the same operating condition as the engineering assessment, but it is a regulatory definition rather than the retrofit design basis. Our guide to the 2030 shore power rules covers the regulation.

What we ask for

• The electrical load analysis and the single-line diagram
• Generator load records from port stays, from the monitoring system or the logs
• The ports and berths the vessel calls at, and the season of each call
• Reefer capacity and typical reefer counts, where the vessel carries them
• Planned changes: new equipment, or a change of trade

These records usually narrow the likely load range before the onboard survey. The survey then confirms the vessel arrangement and the assumptions that matter, and the result feeds the wider retrofit scope set out in our guide to what an AMP retrofit involves.