An electrical sub board is a second, smaller switchboard fed from your main switchboard by a cable called a submain. It has its own main switch, safety switches and circuit breakers, and controls the circuits in one area, such as a granny flat, detached garage, pool shed or unit. It shares out existing power rather than adding any.
What does a sub board actually do?
A sub board takes one large supply from the main switchboard and divides it into smaller, protected circuits close to where the power is used. Instead of running ten separate cables from the house to the back shed, the electrician runs one submain to a board in the shed and starts the lights, power points and equipment circuits from there.
The area gets its own main switch, so it can be isolated for work or in an emergency without turning off the whole property, and a tripped circuit in the granny flat is reset at the granny flat.
How is a sub board different from the main switchboard?
The main switchboard is where the supply from the street arrives. A sub board only ever receives power from another board on the same property. The two look alike inside, but they do different jobs and follow some different rules.
| Feature | Main switchboard | Sub board |
|---|---|---|
| Fed by | Consumer mains from the network | A submain from the main board |
| How many per property | One | None, one or several |
| Holds the meter and service fuse | Usually, or sits beside them | No |
| Neutral and earth link (MEN link) | Yes, made here | No, kept separate |
| Protected by | The service fuse | A breaker in the main board |
The earthing row matters most. In Australia the neutral and earth are joined at one point only, at the main switchboard, through the MEN link. A sub board keeps its neutral bar and earth bar separate and gets its earth through the submain cable. Joining them again at a sub board is a wiring fault.
The cable between the two boards has its own rules for sizing and ownership, which our guide to submains vs consumer mains covers in detail.
When do you need a sub board?
You need a sub board when a part of the property is far enough from the main switchboard, or busy enough, that running every circuit back to the main board stops making sense. The common cases on Sydney homes and businesses are:
- A granny flat or secondary dwelling. It is normally supplied through a submain to its own board, so the occupants can reset and isolate their own circuits.
- A detached garage, shed, studio or workshop. Especially once it has more than a light and a power point.
- Pool and spa equipment. Pumps, heaters, chlorinators and lighting are often grouped on a board in or near the pool shed.
- A second storey or large extension. When the main board is full or at the other end of the house, a sub board upstairs or in the new wing can be the cleaner option.
- Units and commercial tenancies. The board inside your apartment is almost always a sub board. The building's main switchboard sits in the meter room.
A single new circuit does not need a sub board. If you are only adding one load, such as an oven or an air conditioner, our guide to the dedicated circuit is the more relevant read.
Adding a granny flat, studio or pool shed?
DM Electrical works out whether the main switchboard and incoming supply can carry the new board before the job is priced. Based in Brookvale, Level 1 and Level 2 ASP accredited, across the Northern Beaches and greater Sydney.
Does a sub board give you more power?
No. A sub board shares out the power you already have. It cannot increase the total the property can draw from the street. That limit is set by the service fuse and the size of the consumer mains feeding the main switchboard, and every sub board on the property draws from inside it.
This catches out a lot of granny flat projects: the new board is installed correctly, yet the main switch or service fuse trips on a cold evening because the property now asks for more than the supply allows. Before a sub board goes in, the electrician works out the maximum demand of the new loads and checks it against the existing supply. If there is not enough headroom, the answer is load management, a consumer mains upgrade or, for heavier properties, three-phase power. Mains and network work is Level 2 ASP work, covered on our Level 2 ASP page.
What goes inside a sub board?
A sub board has the same building blocks as a main board: a main switch, safety switches, circuit breakers, and a neutral bar and an earth bar. In more detail:
- Main switch. Turns off everything the sub board feeds. It is an isolator, not the protection for the submain. That protection is the breaker back at the main board.
- Safety switches. New circuits in homes need RCD protection under the current Wiring Rules. Many sub boards now use RCBOs, which combine a breaker and safety switch for each circuit. Our RCBO vs RCD guide explains the trade-off.
- Labelling. Every circuit labelled, plus a label at the main board showing which breaker feeds the sub board, so the right one can be found quickly in a fault or an emergency.
What are the signs a sub board has a problem?
The giveaway is a fault that affects only the area the sub board feeds while the rest of the property is fine. Look for:
- The whole granny flat, shed or unit loses power, yet nothing has tripped on its own board. Check the submain breaker at the main switchboard first.
- The submain breaker at the main board trips under load, often when several big appliances run at once.
- Old rewireable fuses, no safety switches, or a board that feels warm or smells hot.
A warm board or a burning smell is not a reset-and-see job. Turn it off at the main switch if it is safe to do so and call an electrician. Old sub boards with fuses and no RCD protection are usually upgraded at the same time as the main board, as part of a switchboard upgrade.
Can I install a sub board myself?
No. Installing a sub board and its submain is licensed electrical work in NSW. The electrician sizes the submain, connects it to a correctly rated breaker at the main board, tests the work and issues a Certificate of Compliance for Electrical Work. A sub board alone does not need a Level 2 electrician, because it sits entirely inside your installation. It only becomes Level 2 work if the incoming supply has to be upgraded to carry it.
Frequently asked questions
Is a sub board the same as a distribution board?
Yes. Sub board, sub switchboard, distribution board and DB are different names for the same thing: a switchboard fed from another switchboard on the same property rather than directly from the street supply.
Does a shed need its own sub board?
Not always. A shed with one light and one power point can run on a single circuit from the main board. Once it has several circuits, heavy tools or equipment, or sits a long way from the house, a sub board fed by a submain is usually the safer and more practical setup.
Does a sub board increase my power supply?
No. It divides the power that already reaches the main switchboard. The total the property can draw is set by the service fuse and consumer mains, so a large new load may need a supply upgrade or load management as well as the sub board.
Does a sub board need a Level 2 electrician?
No. A sub board and its submain sit inside your own installation, so any licensed electrician can install them. Level 2 ASP accreditation is only needed if the job also involves the consumer mains, service fuse or connection to the network.
The bottom line
A sub board is a second switchboard that gives a granny flat, shed, pool area or unit its own main switch and protected circuits, fed by one submain from the main board. It keeps faults local but never adds supply capacity, so check the supply before planning a big new load. Get in touch and we will look at your main board and supply and tell you what a sub board would need.