When a factory, commercial building or development needs more electrical capacity, the solution is not automatically “install an SSU.” The correct TNB supply arrangement depends on the proposed maximum demand, the existing network, site conditions, land and access, protection requirements, cable routing and the result of TNB’s system study. Depending on those factors, the project may involve an upgraded low-voltage supply, a new 11/0.4kV substation, a compact or indoor substation, an RMU, an SSU, or a combination of equipment and facilities.
This guide explains the roles of SSU and RMU, how they differ, the information needed for a TNB power-upgrade application and how factories can reduce avoidable redesign and delay.
Start with the required maximum demand
The first engineering question is the power the premises genuinely needs, not the name of the equipment to purchase. Maximum demand should be developed from the present load, new machinery, operating diversity, motor starting requirements and realistic expansion phases. The current TNB Electricity Supply Application Handbook describes different minimum supply arrangements according to the requested maximum demand, while reserving the final scheme for TNB’s system study.
As a general guide, supplies above 100A and up to approximately 350kVA may be served at 400V through a feeder pillar or a new or existing substation, subject to system conditions. Above approximately 350kVA and up to 1,000kVA, a new substation and direct 400V supply may be required. For approximately 1,000kVA to below 5,000kVA, an 11kV supply through a TNB switching arrangement may be considered. These thresholds should never be treated as an automatic approval formula: TNB may require a different arrangement because of local network capacity, location, reliability, security or economic considerations.
A staged expansion should also be declared properly. Understating the future demand can lead to a scheme that becomes inadequate shortly after commissioning, while an unsupported estimate may create unnecessary infrastructure and cost.
SSU and RMU are not the same thing
SSU refers to Stesen Suis Utama, or a main switching station. It is a facility within the distribution network used to serve a dedicated bulk consumer or to inject and transfer capacity from an upstream PMU or PPU towards a load centre. An SSU is therefore a station-level arrangement that may include several switchgear panels, protection systems, metering, control equipment, cable terminations and the necessary building or compound.
RMU means Ring Main Unit. It is compact medium-voltage switchgear—commonly used at 11kV and, where applicable, 22kV—for switching, isolation and protection within a ring or feeder arrangement. TNB’s approved-product listings include outdoor and motorised indoor RMU configurations, such as two-switch-and-one-fuse and multi-switch arrangements. The exact type and rating must follow the approved network design and current TNB specifications.
The key distinction is simple: an SSU is a switching-station facility, whereas an RMU is a piece of medium-voltage switchgear. An RMU does not automatically replace an SSU. A project may involve an RMU without requiring a standalone SSU, while an SSU may use a more extensive switchgear lineup depending on the required configuration. The terms should not be used interchangeably in budgets, drawings or discussions with TNB.
What supply arrangements may be considered?
Depending on the site and requested load, TNB may study one or more of the following:
- Upgrading the existing 400V service from the local low-voltage network.
- A direct low-voltage cable from an existing or new substation.
- A new compact 11/0.4kV substation.
- An indoor TNB or consumer substation within an approved room or building.
- An 11kV supply with RMU or other approved switchgear, metering and protection.
- A dedicated SSU or connection involving a PPU/PMU for larger developments or where the network requires it.
Compact substations may be suitable for certain commercial and industrial projects, subject to transformer capacity, metering-kiosk availability, access and system planning. Larger schemes may require indoor substations, dedicated switching facilities, land reservation or additional upstream works. The arrangement shown in a preliminary concept is not final until TNB completes the system study and issues or agrees to the technical proposal.
When should a factory consider a power upgrade?
A formal review should begin before the existing supply becomes a production constraint. Common triggers include:
- Installing a new production line, chiller plant, compressor, furnace or large motor load.
- Expanding the factory floor or adding a new building phase.
- Maximum demand approaching the practical capacity of the present supply.
- Repeated difficulty starting large equipment or maintaining acceptable operating conditions.
- A new tenant or change of use with a substantially different load profile.
- A requirement for 11kV supply because the requested capacity is no longer practical at low voltage.
Starting early matters because the electrical design can affect land allocation, transformer or switchgear rooms, fire separation, access for TNB, cable routes, road-opening permits and the overall construction programme.
Information and documents needed for an initial assessment
A useful application package normally begins with:
- Recent TNB bills, account details and the existing supply rating.
- The existing single-line diagram and available switchboard information.
- A detailed load schedule and proposed maximum-demand calculation.
- New equipment ratings, motor starting data and operating sequence.
- Site plan, floor plans, land or tenancy documents and project location.
- Proposed phasing and future demand, not only the first installation stage.
- Possible locations for a substation, RMU, SSU, metering panel and consumer switchgear.
- Preliminary cable route, access and authority or road-crossing constraints.
For applications above 100A, TNB’s handbook states that submission is made through a Professional Electrical Engineer registered with the Board of Engineers Malaysia. The consultant coordinates the technical documents, drawings and responses required for TNB’s evaluation.
Typical TNB power-upgrade process
- Load and site assessment. Confirm the present supply, operating load, proposed equipment and realistic maximum demand.
- Preliminary scheme. Prepare or update the single-line diagram, load schedule, site arrangement and possible substation or switching locations.
- Application submission. The appointed electrical consultant submits the application and supporting documents through the relevant TNB channel.
- TNB system study. TNB assesses network capacity and determines the appropriate connection concept.
- Joint technical coordination. Parties agree on substation details, cable or overhead-line route, metering arrangement, access and other requirements.
- Offer and implementation. After accepting the technical and commercial requirements, the applicant completes the site, building, cable, switchgear, protection and related authority works.
- Testing and energisation. Required inspections, protection tests, metering work, commissioning documents and TNB procedures are completed before supply is energised.
Common causes of delay and extra cost
Assuming an RMU or SSU before the system study. Purchasing equipment or fixing the room layout too early can create redesign when TNB issues a different scheme.
Reserving the site too late. Substation access, setbacks, ventilation, fire requirements and cable entry routes can conflict with completed architectural or civil works.
Incomplete maximum-demand planning. Missing future machinery or unrealistic diversity may result in repeated submissions or an undersized arrangement.
Confusing equipment with the facility. An RMU quotation does not cover every requirement of an SSU, substation, transformer, protection system, metering arrangement or civil structure.
Ignoring external cable and permit work. Road opening, HDD or open-cut works, Majlis/JKR requirements and utility coordination can control the project timeline even when the electrical design is ready.
Incomplete protection and metering details. CT ratios, relay settings, switchgear ratings and interfaces must be coordinated for the approved scheme rather than selected in isolation.
How YL Engineering can help
YL Engineering assists factories, commercial premises and developments with initial load and site reviews, maximum-demand calculations, single-line diagrams, TNB applications, Electrical PE coordination, and the planning of SSU, RMU, substation, metering and cable-route requirements. We can also coordinate related authority submissions, testing and commissioning activities within the agreed project scope.
View our TNB Power Upgrade & SSU Application service or WhatsApp us for an initial power-upgrade scope review. Please include the current supply, proposed maximum demand, location and any available single-line diagram.
Official reference
This guide is based on the current TNB Electricity Supply Application Handbook and TNB’s published approved-product information. Requirements, network conditions and approved products may change, so the latest TNB instructions must be confirmed for each project.
YL Engineering provides application and engineering coordination services. The final supply scheme, technical acceptance and approval remain subject to TNB and other relevant authorities.