Switchboards April 2026 · 7 min read
The main switchboard is the central point of an electrical installation. Everything downstream of it depends on how well it is designed, built, and maintained. This guide covers what a main switchboard is, what it must contain, how to size one correctly, and what to look for when specifying a replacement.
The main switchboard (MSB) is the primary point of connection between the electrical network supply and the internal distribution system of a building or facility. Everything in the installation draws power through it. Clive Wilson Switchboards, based in Invercargill, has been designing and manufacturing main switchboards for industrial and commercial facilities across New Zealand for over 55 years. Here is what you need to know.
The MSB performs four core functions:
On larger installations, the MSB also provides the interface for power factor correction equipment, metering systems, and protection relays that monitor the incomer and trip the main breaker on abnormal supply conditions.
| Feature | Main Switchboard | Distribution Board |
|---|---|---|
| Position in system | Receives the mains supply | Fed from the MSB or another DB |
| Fault level | High — typically 25 to 50+ kA | Lower — limited by upstream protection |
| Busbar rating | High — rated for full incomer current | Lower — matched to the feeder circuit |
| Typical standards | AS/NZS 61439-1 and -2 | AS/NZS 61439-1 and -3 |
| Metering | Incomer metering usual | Sub-metering may be included |
| Manufacturer | Specialist switchboard builder | Can be standard or custom assembled |
The main incomer protective device is typically a moulded case circuit breaker (MCCB) or air circuit breaker (ACB) rated to the full load current of the installation with a breaking capacity equal to or greater than the prospective short circuit current (PSCC) at the supply point. The PSCC must be confirmed from the network operator before the board is designed.
The main busbars carry the full installation current from the incomer to the outgoing circuits. They are sized to carry the maximum expected load current with an acceptable temperature rise, and must be rated to withstand the short circuit current for the duration it takes upstream protection to operate. Busbar sizing is calculated to AS/NZS 61439 requirements.
Each circuit leaving the MSB requires an appropriate protective device: MCCB for large feeders, MCB (miniature circuit breaker) for smaller final circuits, or fuse systems where specified. Each device must be rated correctly for its load current and fault level.
The main earth bar connects all protective earth (PE) conductors. On TN-C-S systems (the most common NZ supply arrangement), the neutral and earth are separated at the MSB. The neutral bar must be rated for the full neutral current including harmonic loading.
At minimum, a modern NZ main switchboard should include an energy meter at the incomer capable of logging kWh, kVArh, power factor, and maximum demand. This data is required for energy management, network compliance, and increasingly for sustainability reporting.
Correct sizing requires the following information:
| Input Required | Why It Matters |
|---|---|
| Prospective short circuit current (kA) from network operator | Determines breaking capacity of all protective devices |
| Maximum demand (kVA or kW + power factor) | Determines incomer rating and busbar sizing |
| Load list with circuit ratings | Determines outgoing circuit protection and distribution layout |
| Future load allowance (%) | Determines spare ways and busbar capacity margin |
| Installation environment (IP rating, form of segregation) | Determines enclosure specification |
| Protection relay requirements | Determines relay type, CT sizing, and protection settings |
Undersizing a main switchboard is a common and costly mistake. A board without adequate spare capacity requires replacement rather than expansion when loads increase. Clive Wilson Switchboards recommends a minimum 20 percent spare capacity on busbars and a minimum 25 percent spare circuit ways on any new MSB design.
The most common triggers for MSB replacement in NZ are:
Switchboard replacement on a live commercial or industrial site requires careful planning. At Clive Wilson, we manage the full process from design and manufacture through to installation coordination, including temporary supply arrangements where needed to minimise downtime.
Related reading:
Talk to us about your main switchboard design or replacement
📍 31-33 Clyde Street, Invercargill · 📞 +64 3-214 4264
