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. Get it right and the rest of the system has a stable base. Get it wrong and the rest of the system inherits the problem.
Quick answer
A main switchboard (MSB) is the primary low-voltage assembly that takes the utility supply, provides isolation and metering, and distributes feeders to every sub-board, MCC and large load on the site. Built and verified to AS/NZS 61439, it carries the highest current and fault rating in the installation. CWS builds MSBs up to 8,000A on the Logstrup, Simotrol and Schneider Prisma Plus G platforms.
In this article
This guide covers what a main switchboard is, what it must contain, how to size one correctly, and what to plan for when specifying a replacement. 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.
The MSB performs four core functions in any installation. Each one matters and each one drives design decisions.
The MSB is designed and verified to AS/NZS 61439 (parts 1 and 2 for assemblies and power assemblies), installed under AS/NZS 3000, with cables sized to AS/NZS 3008.
The main switchboard and the distribution board sit at different points in the supply chain.
For sites that need both, see our lighting and power distribution capability, or the comparison guide MSB vs MCC, when do you need which? if the question is between an MSB and a motor control centre.
A main switchboard is more than a row of breakers. Every MSB has a set of core components that must be present, properly sized and properly coordinated.
An air circuit breaker (ACB) or moulded case circuit breaker (MCCB) sized for the incoming supply, with the right Icw and Ipk to handle the prospective fault current at the site.
Copper or aluminium busbars rated to the full incoming current, with bracing sized to the assembly’s peak withstand. Busbar capacity, fault withstand and segregation are all type-tested as a system, not added up component by component.
A protective device on every outgoing feeder, coordinated for discrimination with the main breaker and the downstream board. Sized for the downstream cable and the maximum prospective fault.
Dedicated bars for earth and neutral conductors, sized for the assembly’s full prospective earth-fault current and ready to accept every downstream conductor that lands in the board.
Revenue or check metering at the incoming, plus the instrumentation the operations team needs to see what the plant is doing: ammeters, kWh meters, power-factor displays, alarms.
The form of internal separation defines how separated the live parts of each functional unit are from busbars and from each other. Form 4a is the most common segregation on CWS Simotrol MSBs for industrial sites, with Form 4b specified where the highest level of segregation is required. See our forms of segregation guide for the full detail.
Sizing an MSB starts with a complete loading schedule and works back through diversity, busbar current, fault withstand, form of internal separation, IP rating and platform choice.
A row-by-row list of every feeder out of the board, the connected load it supplies, the diversity factor that applies, and the design current that drives the protective device selection. Without a loading schedule, the rest of the sizing exercise stops.
The prospective short-circuit current at the board, from the upstream transformer impedance, upstream protection settings and the cable run from the supply. Our NZ Transformer Fault Current Calculator gives a first-pass estimate. For the full treatment, see our switchboard fault rating guide.
CWS builds MSBs on three type-tested platforms:
The loading schedule and the fault level are the two documents that drive every other MSB sizing decision. Get them confirmed at enquiry and engineering starts the same day. Leave them open and the programme slips.
Replacing a live main switchboard is a different exercise to a greenfield build. The site often cannot stop running while the swap happens, the existing cable terminations need to land back in the new assembly, and the new MSB needs to fit the same footprint and cable entry points.
For a longer treatment of MSB replacement planning, see our guide on switchboard upgrade and replacement in NZ. For typical lead times, see our switchboard lead time guide.
An MSB sits at the supply intake, carries the highest current and fault rating on site, and distributes feeders to MCCs, sub-mains, distribution boards and large loads. A distribution board sits downstream and distributes power to final circuits and small loads. The MSB is the parent; distribution boards are the children.
Up to 8,000A on the Logstrup platform. Smaller MSBs run on the Simotrol or Schneider Prisma Plus G platforms. The right rating comes from the loading schedule and the fault level, not from a generic specification.
A loading schedule, a single-line diagram and a confirmed fault level (PSCC) at the point of installation. With those three documents we can scope, platform and quote. See our switchboard specification guide for the full information list.
Form 4a is the most common segregation on CWS Simotrol MSBs for industrial sites. It separates busbars, functional units and terminals from each other, with each functional unit’s terminals housed in the same compartment as the unit itself. Form 4b (terminals in a separate compartment) is specified where the highest level of segregation is required. Lower forms (Form 2 or 3) suit smaller commercial boards.
Partial-shutdown changeovers are possible on multi-section MSBs where one section can be isolated and replaced while the rest of the board stays in service. A full one-for-one swap usually needs a planned outage. The strategy depends on the existing board’s architecture and the operations team’s tolerance for downtime.
Smaller commercial MSBs can be turned around in three weeks with complete enquiry information. Mid-size industrial MSBs run four to six weeks. Larger Logstrup MSBs up to 8,000A can run 16 to 20 weeks depending on long-lead component availability. See our switchboard lead time guide for what drives the timeline.
Reviewed by Chris Wilson, Co-Director, Clive Wilson Switchboards. Registered electrician, 15+ years in LV switchboards. Updated May 2026.
