What Changes Inside Your Switchboard Once Your Sydney Supply Goes Over 100 Amps?

What changes inside your switchboard once your Sydney supply goes over 100 amps? Once an installation's assessed load passes 100 amps per phase, the Service and Installation Rules of New South Wales move you into a different design category altogether: current transformer metering becomes mandatory, your designer has to submit a load schedule and single line diagram to the Metering Coordinator, the switchboard itself needs a documented fault current withstand rating, and the whole board has to be graded so your own protection trips before the network's upstream protection ever does.

⚡ Quick Answers: Switchboards Over 100 Amps

• Why does 100 amps matter so much? It is the threshold SIRNSW uses to decide when a standard whole current meter is no longer suitable and current transformer, or CT, metering becomes mandatory.
• What has to be submitted before the board is built? A load schedule, a single line schematic diagram, and power factor correction details if they apply, all assessed by the Metering Coordinator.
• What is a fault current withstand rating? A documented figure showing the installation can survive a prospective short circuit without damage, with boards over 400 amps needing to withstand that fault for a full second.
• What does grading or discrimination actually protect against? It makes sure your own main breakers or fuses trip before the network's upstream protection does, so a fault inside your building does not take out the street.
• What is an MEN link and why does its position matter in a large board? It is the earth neutral connection, and in a bigger switchboard it must be safely removable while live, clearly signposted and positioned within the customer's own section of the board.
• Does this apply to homes as well as businesses? It is far more common in commercial and industrial jobs, but any large home, such as one running a home business or extensive outbuildings, can trigger the same requirements once demand crosses 100 amps.

1. Current transformer metering becomes mandatory

A standard domestic meter is only rated to measure current directly up to a certain point. Once the assessed load of an installation, or a separately metered portion of it, exceeds the current rating of that meter, or 100 amps per phase, the Metering Coordinator requires current transformer metering instead. CT metering uses transformers to scale the actual current down to a level the meter can safely read, which is the only practical way to measure the much larger currents involved in a three phase upgrade for a sizeable commercial or industrial supply.

This is a different conversation to the straightforward three phase connection most homes go through. If you are comparing the two, our guides to going over 100 amps three phase with Ausgrid and the equivalent process with Endeavour Energy cover the network connection application itself, while this guide focuses on what changes inside the switchboard once that connection is approved.

2. The design submission needs real documentation, not a rough sketch

Getting a large service equipment design approved means submitting proper documentation, not a quick diagram scrawled on the back of an invoice. The submission to the network operator has to include the proposed load details, a single line schematic diagram showing how the installation is structured, and power factor correction details where they apply. A separate submission also has to go to the Metering Coordinator to assess the metering requirements, including how the CT enclosure itself will be designed.

This level of documentation is exactly why this kind of work is usually planned early in a power supply upgrade project, well before equipment is ordered, since a design that gets rejected for a missing diagram or an incorrect fault level figure can set a commercial fit out back by weeks.

3. The switchboard has to survive a real fault without damage

Every electrical installation has to be capable of withstanding, without damage, the prospective short circuit current it could realistically experience. For switchboards rated over 400 amps, SIRNSW requires them to be rated to survive that fault current for a full second, which is a meaningfully tougher standard than a smaller domestic board needs to meet. This rating has to be engineered into the switchboard's construction from the start, since retrofitting a higher fault rating into an undersized enclosure after the fact is rarely practical.

It is also one of the main reasons boards of this size cost noticeably more than a standard residential upgrade: heavier busbars, more substantial enclosures and properly rated switchgear all exist specifically to meet this withstand requirement.

4. Grading keeps a fault in your building from taking out the street

SIRNSW requires main breakers and fuses in a large installation to be graded, meaning they are selected and set so they trip fast enough that the network's own upstream protection never has to operate first. Without correct grading, a fault inside your building could trip the network's protection instead of your own, and only network crews can restore power after that happens, usually at a real cost and after a real delay.

Grading is a calculation exercise, comparing the time current curves of every protective device in the chain from your main switch back toward the network's own fuse or recloser, and it is one of the more technical pieces of a large switchboard design. Getting it wrong does not show up until there is an actual fault, which is exactly when you do not want to discover it.

5. Where the MEN link has to sit in a larger board

The main earthing neutral, or MEN, link is the connection that ties your installation's neutral and earth together at one defined point. In a larger, multi section switchboard, this connection has extra rules: it must be safely removable while the installation is live, so it can be tested without a full shutdown, it has to be clearly signposted, and it has to sit within the customer's own section of the board rather than somewhere that blurs the line between customer and network equipment. Boards made up of several separate assemblies have further specific requirements for where this link can go.

Getting MEN link placement wrong is a serious safety issue, not a paperwork technicality, since it affects how fault current returns to its source during an earth fault, which is the exact mechanism that trips a safety switch and protects people from electric shock.

6. CT metering design links back to switchboard access and labelling

None of the requirements above sit in isolation. A properly designed CT metering installation also needs adequate access, which ties back to the same enclosure and location rules covered in our guide to switchboard panel and enclosure installation rules, and it needs clear labelling so distributor staff and your own maintenance electricians can identify every section correctly. Larger commercial fit outs, including cafes, warehouses and multi tenant buildings, run into all of these requirements together rather than one at a time, which is why planning the whole design early with an experienced Level 2 electrician saves real time and cost down the track. Our sister site's guide to a cafe and restaurant fitout switchboard covers how this plays out in practice for a smaller commercial tenancy approaching the same threshold.

Why it's Level 2 ASP work

Everything covered in this guide, from the load schedule submission through to grading and MEN link placement, sits within the service equipment scope that only a Level 2 ASP can design and certify in New South Wales. This is also the point where a Level 3 design is often required for complex commercial switchboards, working alongside the Level 2 installation to make sure the whole system is engineered correctly from the connection point through to the final distribution board.

How High Demand Electrical helps

High Demand Electrical is a licensed local team with genuine experience designing and installing large, over 100 amp service equipment for commercial and industrial clients across Sydney. We prepare the full design submission, including the load schedule and single line diagram, coordinate directly with the Metering Coordinator and your network distributor, and make sure the finished switchboard meets its fault current withstand rating and grading requirements before it is ever energised. Servicing businesses across Sydney's West, South West and Sutherland Shire, we treat this kind of design work as a specialty, not a side job squeezed in between smaller residential upgrades.

Switchboards Over 100 Amps FAQs (Sydney)

How do I know if my business needs CT metering?

If your assessed load, or the load of a separately metered part of your installation, exceeds 100 amps per phase or the rating of a standard meter, CT metering becomes mandatory. See power supply upgrades for how we assess existing demand before recommending a path forward.

What documents do I need before I can order equipment?

At minimum, a load schedule, a single line schematic diagram and power factor correction details if applicable, all submitted to the network operator and Metering Coordinator for assessment.

Why is my fault current withstand rating higher than a neighbour's smaller board?

Boards rated over 400 amps must be able to withstand a prospective short circuit for a full second, a tougher standard than smaller domestic boards are required to meet, which also explains the higher cost of the switchgear involved.

What happens if grading is done incorrectly?

A fault inside your building could trip the network's own protection instead of yours, cutting power to a wider area and requiring network crews, rather than your own electrician, to restore supply.

Do I need a Level 3 design for a job like this?

Larger or more complex commercial switchboards often do. See Level 3 design for when this extra layer of engineering is required alongside standard Level 2 installation work.

Planning a commercial or industrial upgrade that will take your supply over 100 amps? Get a Power Supply Upgrade Quote from High Demand Electrical, or call 0402 559 777. A full site assessment and load review always comes first, since scope and pricing on a job this size depend entirely on your actual demand and existing infrastructure.