ElecAS

Switchboard Sizing Calculator: LV Switchboard Dimensions, Bays & Form of Separation to AS/NZS 61439

Free, browser-based switchboard sizing calculator for Australian and New Zealand electrical engineers, switchboard designers and builders. It estimates the physical footprint of a free-standing low-voltage switchboard assembly (overall width, height and depth in millimetres and the number of bays) directly from your functional sections. Enter the switchboard rating, the incoming and metering arrangement (main switch / incomer, CT metering chamber, MEN link, surge protection), the outgoing distribution and feeder sections, a spare-capacity allowance, the IP rating and cable-entry arrangement, and the Form of separation (Form 1 through Form 4b to AS/NZS 61439.2 / IEC 61439-2). The calculator builds the width by stacking the functional sections into standard bays (incoming switchgear, metering, distribution chassis, outgoing feeder cubicles, spare capacity and any optional bays) takes the height from the rating envelope or the height you enter, capped at the 2200 mm operator-reach limit, and takes the depth as the greatest of the rating-band envelope, the actual switchgear compartment depth and the IP-rating depth floor, plus a rear alley where the board is rear-connected. The Form of separation is recorded as a verified-assembly constraint rather than a generic width or depth adder, because its external dimensional effect depends on the manufacturer's tested internal arrangement. It returns a live board width, bay count and elevation sketch you can use for early space planning, switchroom and riser allocation, tender allowances and coordination: before the manufacturer confirms the verified assembly. Outputs are preliminary functional-section estimates for early design, not a final enclosure specification: AS/NZS 61439 does not prescribe a universal enclosure-size formula, so final busbar, terminal, thermal, clearance and enclosure dimensions must come from the manufacturer's verified assembly system before construction.

Overview

How wide is the switchboard going to be? This calculator estimates the physical footprint of a free-standing low-voltage assembly, the overall width, height and depth in millimetres and the number of bays, from the functional sections the job actually needs. Enter the rating, the incoming and metering arrangement, the outgoing distribution and feeder sections, a spare-capacity allowance, the IP rating and cable entry, and the Form of separation to AS/NZS 61439.2, and it stacks those sections into standard bays and splits into further bays when a section exceeds the stack height.

Key facts

  • AS/NZS 61439 has no universal enclosure-size formula; final switchboard dimensions must come from the manufacturer's verified assembly.
  • The calculator builds a preliminary width from functional sections: incomer, metering, distribution chassis, feeders and spare capacity.
  • Higher Forms of separation (Form 3b, Form 4b) need more internal space in practice, but the increase is specific to the manufacturer's tested system, so the calculator records the Form as a constraint instead of applying a generic dimensional adder.
  • Board depth follows the greatest of the rating envelope, the switchgear compartment depth and the IP-rating depth floor, plus a rear alley for rear-connected boards.
  • A 20-25% spare-capacity allowance is the common convention for new switchboards.

Who this page is for

Electrical engineers, switchboard designers, switchboard builders and manufacturers, electrical contractors and estimators who need an early-stage switchboard footprint (width, height, depth and bays) for space allocation, switchroom and riser planning, tender allowances and coordination on Australian and New Zealand low-voltage installations.

Relevant standards

  • AS/NZS 61439.1:2016 (Low-voltage switchgear and controlgear assemblies: General rules)
  • AS/NZS 61439.2:2016 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies, including Forms of internal separation 1–4)
  • IEC 61439-2 (Low-voltage switchgear and controlgear assemblies: Power switchgear and controlgear assemblies)
  • AS/NZS 3000:2018 (Wiring Rules: switchboard construction, clearances and access requirements)

What this tool helps with

  • Estimate the overall width, height and depth of a free-standing LV switchboard in millimetres from your functional sections, for early space planning and tender allowances.
  • Build the width from the incoming switchgear, metering (CT chamber, MEN link, surge protection), distribution chassis, outgoing feeder sections and spare-capacity allowance.
  • Record the AS/NZS 61439.2 / IEC 61439-2 Form of separation (Form 1, 2a/2b, 3a/3b, 4a/4b) against the estimate as a verified-assembly constraint: no generic width or depth adder is invented, because the external dimensional effect of a Form depends on the manufacturer's tested internal arrangement.
  • Size the depth from the greatest of the rating-band envelope, the actual switchgear compartment depth and the IP-rating depth floor (IP40 through IP66), with a rear alley added automatically for rear-connected boards.
  • Add a spare-capacity percentage that expands both the chassis design ways and the reserved feeder space so the board has room to grow, plus discrete spare bays for future sections.
  • See a live bay count and elevation sketch that updates as you change the rating, sections and Form.
  • Understand up front that the result is a preliminary functional-section estimate for concept design: AS/NZS 61439 has no universal enclosure-size formula, so final dimensions must come from the manufacturer's verified assembly.

How to estimate a switchboard size with the ElecAS Switchboard Sizing Calculator

  1. Set the switchboard rating: Choose the main switchboard rating (in amps) for the free-standing 3-phase assembly. The incoming main switch / incomer device is inferred from the rating, and the rating also selects the preliminary height and depth envelope the board is planned inside.
  2. Choose the Form of separation: Select the AS/NZS 61439.2 / IEC 61439-2 Form of internal separation (Form 1, 2a/2b, 3a/3b or 4a/4b). The selection is recorded against every section and in the design notes as a verified-assembly constraint; it does not apply a generic width or depth increment, because the space a Form costs is specific to the manufacturer's tested cubicle system.
  3. Add the incoming and metering items: Pick the incoming and metering sections that apply: CT metering chamber, MEN link section, surge protection. Each is added to the board width as its own functional section.
  4. Enter the distribution and feeder sections: Add the outgoing distribution sections and their items. Final subcircuits are converted into chassis ways; larger outgoing ways are given dedicated feeder space.
  5. Set the spare capacity: Choose a spare-capacity percentage (commonly around 20–25%). It is applied to both the chassis ways and the feeder space so the footprint already includes room to grow.
  6. Read the dimensions, bays and elevation: Review the live overall width, the conservative height and depth, the bay count and the elevation sketch. Use them to reserve switchroom space, size the riser and set a tender allowance.
  7. Confirm with the manufacturer before construction: Treat the result as a preliminary estimate. Before construction, confirm the final busbar, terminal, thermal, clearance and enclosure dimensions with the switchboard builder's verified assembly system to AS/NZS 61439.

How switchboard sizing works: estimating LV switchboard dimensions to AS/NZS 61439

What the switchboard sizing calculator actually estimates

The ElecAS Switchboard Sizing Calculator produces a preliminary physical footprint for a free-standing low-voltage switchboard assembly: the overall width, height and depth in millimetres, the number of bays, and the layout implied by the chosen Form of separation. It is a concept-stage space-planning tool: you use it to reserve floor area in a switchroom, size a riser, set a tender allowance and coordinate the board with the architecture long before the switchboard builder confirms the final assembly.

What it deliberately does not do is invent a precise enclosure. AS/NZS 61439 is a performance and verification standard, not a dimensional catalogue: it does not publish a universal formula that turns a load schedule into an exact enclosure size. The real dimensions come out of a manufacturer's verified assembly system, where busbar bracing, terminal space, thermal rise, minimum clearances and the chosen switchgear range all interact. This calculator gives you a defensible early number; the verified assembly gives you the built one.

How the width is built up from functional sections

Switchboard width is the dimension that actually scales with the design, so it is built up section by section. The calculator starts with the incoming switchgear (the main switch or incomer inferred from the switchboard rating), adds the metering arrangement you select (a CT metering chamber, the MEN link section and any surge-protection allowance) then adds the distribution chassis that carries the outgoing final subcircuits, and finally the dedicated feeder sections for larger outgoing ways.

Outgoing final subcircuits are converted into chassis ways, and larger outgoing items are given feeder space. The total width is the sum of these functional sections, packed into standard bays, which is why the result is reported both as an overall millimetre width and as a bay count with an elevation sketch. Because the width tracks the sections you enter rather than circuit names, adding metering, more feeders or a higher spare allowance visibly widens the board.

Form of separation and how the calculator treats it

The Form of separation describes how far the internal barriers go in segregating busbars, functional units and terminals from one another: Form 1 (no internal separation), Form 2a/2b (busbars separated, 2b including terminals), Form 3a/3b (functional units separated as well) and Form 4a/4b (every unit and its terminals fully separated). AS/NZS 61439.2 / IEC 61439-2 define these Forms; the higher the Form, the more internal metalwork, glanding space and terminal segregation the assembly needs.

Higher Forms do cost space in practice: but how much depends entirely on the manufacturer's tested cubicle system, because the barriers are achieved inside a verified assembly rather than by a universal external increment. Rather than invent millimetres it cannot justify, the calculator records the selected Form as a verified-assembly constraint on the estimate and reports it on every section and in the design notes, without applying a generic width or depth adder. Choose the Form early (often Form 3b or Form 4b for main switchboards where maintenance on one circuit must be possible without isolating the whole board), and confirm its dimensional effect against the switchboard builder's range, because that is the only place the real number exists.

How the height and depth are derived

Unlike width, the height and depth of a switchboard do not scale smoothly with the number of circuits: they step with the switchgear range, the busbar system and the cable-entry strategy. Height is therefore an envelope rather than a sum: the calculator starts from the preliminary assembly height for the selected main rating (or the board height you enter), caps it at the 2200 mm operator-reach limit, and packs the functional sections into the usable stack height inside that envelope. When a column of sections will not fit the usable height, the board grows sideways into another bay instead of growing taller.

Depth is taken as the greatest of four requirements rather than a single figure: the depth band for the selected main rating, the actual depth of the incoming switchgear compartment (an ACB board is deeper than an MCCB or switch-fuse board of the same rating), the depth floor implied by the selected IP rating, and any minimum depth you specify yourself. A rear-connected board then adds a rear alley on top of that front-connected depth, and the result is rounded up to the nearest 50 mm. Treat the returned height and depth as a safe box to plan around, then refine them against the manufacturer's selected range once the design firms up.

Spare capacity: designing room to grow

Most switchboards are specified with spare capacity so future circuits can be added without a new board. The calculator takes a spare-capacity percentage and applies it to both the chassis ways (spare pole positions on the distribution chassis) and the feeder space (room for additional outgoing feeder sections), so the width you see already includes the growth allowance.

A common convention is around 20–25% spare, but the right figure depends on the client brief and how likely the installation is to expand. Setting it explicitly means the early footprint you reserve in the switchroom already carries the spare, rather than discovering at construction that there is nowhere to add the next circuit.

Why this is a preliminary estimate, not a verified assembly

AS/NZS 61439.1 and 61439.2 require that a low-voltage assembly be demonstrated to comply through design verification (by testing, by calculation/comparison with a tested reference design, or by satisfying design rules) and routine verification of each built assembly. Dimensions are an outcome of that process, not an input you can compute in isolation: the same load schedule can fit different enclosure sizes depending on the manufacturer's tested system, busbar ratings and temperature-rise results.

That is why every result in this tool is labelled a preliminary functional-section estimate for early design, and why the final busbar, terminal, thermal, clearance and enclosure dimensions must come from the manufacturer's verified assembly system before construction. Use the calculator to plan space and price a tender with confidence; confirm the built board with the switchboard builder.

Standards reference set and how the tool uses them

AS/NZS 61439.1:2016 sets the general rules for low-voltage switchgear and controlgear assemblies: ratings, temperature rise, short-circuit withstand and the verification framework. AS/NZS 61439.2:2016 (aligned with IEC 61439-2) covers power switchgear and controlgear assemblies specifically and defines the Forms of internal separation (Form 1 to Form 4) that drive the internal space allowances in this calculator.

AS/NZS 3000:2018 (the Wiring Rules) governs how the switchboard is installed and accessed (construction, clearances, working space and main-switch/MEN requirements), which is why the calculator handles the incomer, MEN link and metering as distinct functional sections. The switchboard rating and outgoing schedule you enter typically come from an AS/NZS 3000 maximum-demand assessment and AS/NZS 3008.1.1 cable sizing, so the ElecAS Maximum Demand and Cable Size calculators feed naturally into this one.

Key terms

Functional unit

A part of an assembly that performs one function (an incomer, an outgoing feeder or a final-subcircuit way) together with the switchgear and terminals belonging to it. AS/NZS 61439 sizes and separates an assembly by its functional units, which is why this calculator builds the board width from functional sections rather than from a circuit count.

Form of internal separation

The degree to which an assembly's busbars, functional units and terminals are segregated by internal barriers, defined in AS/NZS 61439.2 / IEC 61439-2 as Form 1 (none), Form 2a/2b (busbars separated), Form 3a/3b (functional units separated) and Form 4a/4b (units and their terminals separated). A Form is achieved inside a manufacturer's verified cubicle system, so it has no universal external dimension increment.

Bay (section)

One vertical cubicle of a floor-standing switchboard, bolted alongside its neighbours to form the assembly. The calculator packs the functional sections into bays of standard width and reports the bay count alongside the overall width, because switchrooms and transport access are planned bay by bay.

Distribution chassis

The DIN-rail assembly inside a section that carries the small outgoing final subcircuits. Circuits are converted into chassis "ways" on an 18 mm planning pitch, banded into a standard chassis width; ways beyond one rail row add height rather than width.

Way

One pole position on a distribution chassis. A 1P device occupies one way, a 2P or 1P+N device two, and a 3P+N or 4P device four, so the way count (not the circuit count) is what drives chassis width.

Incomer

The incoming main switch or main protective device at the origin of the switchboard, sized from the switchboard rating. Whether it is a switch-fuse unit, a moulded-case circuit breaker (MCCB) or an air circuit breaker (ACB) sets its compartment depth, which in turn can govern the depth of the whole board.

MEN link

The multiple earthed neutral link that bonds the neutral bar to the earth bar at the main switchboard, required by AS/NZS 3000:2018 for a MEN system. It needs an accessible zone in the board, so the calculator allows for it as its own functional section with a height that scales with the main rating.

CT metering chamber

A dedicated compartment housing current transformers and metering for supplies too large to meter directly. Its width allowance depends on the metering purpose (EMS/BMS monitoring, private metering, tenant sub-metering or revenue-grade metering), and above a threshold number of metered feeders it becomes a shared board-level section rather than CTs inside each feeder.

Design verification

The AS/NZS 61439.1 process by which a manufacturer demonstrates that an assembly design complies: by testing, by calculation or comparison with a tested reference design, or by satisfying design rules. Enclosure dimensions are an outcome of design verification, not an input that can be computed independently of the tested system.

Verified assembly

A switchboard built within a manufacturer's design-verified system, so its busbar bracing, temperature rise, short-circuit withstand, clearances and enclosure dimensions are all covered by that verification. The final dimensions of a real board come from the verified assembly; a functional-section estimate like this one is a planning figure that precedes it.

Spare capacity

The allowance, usually expressed as a percentage and commonly 20-25%, for circuits added after handover. In this calculator it expands the chassis design ways and reserves additional feeder space, and discrete spare bays can be added on top for whole future sections.

IP rating

The AS 60529 ingress protection classification of the enclosure (for example IP40 indoors, IP56 or IP66 outdoors). A higher IP rating requires deeper gland and sealing zones, so it sets a depth floor that the calculator applies when it exceeds the depth implied by the rating band and the switchgear compartment.

Reviewed by

Wisam Tozah: Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). See how these calculations are verified. LinkedIn.

Frequently asked questions

Does AS/NZS 61439 tell me how big a switchboard should be?

No. AS/NZS 61439 is a performance and verification standard, not a dimensional catalogue: it has no universal formula that turns a load schedule into an enclosure size. The ElecAS calculator builds a preliminary footprint from your functional sections (incomer, metering, chassis, feeders, spare) for early space planning; the final dimensions must come from the switchboard manufacturer's verified assembly system.

What Form of separation should I specify for a main switchboard?

Form 3b or Form 4b is common for main switchboards where one circuit must be workable without isolating the whole board; Form 1 or Form 2a/2b suits smaller distribution boards. Higher Forms add internal barriers and terminal segregation, so the same electrical content does need more space in practice: but the amount is specific to the manufacturer's tested cubicle system, so the ElecAS calculator records your selected Form against the estimate as a verified-assembly constraint rather than applying an invented universal width or depth adder. Confirm the dimensional effect with the switchboard builder for the range you intend to use.

How much spare capacity should a switchboard have?

A common convention is 20-25% spare, applied to both the chassis ways and the feeder space. The right figure depends on the client brief and how likely the installation is to grow. The calculator takes the percentage as an input and includes it in the reported width, so the footprint you reserve already carries the growth allowance.

Can I use the result for tender pricing and switchroom planning?

Yes, that is exactly what it is for: reserving floor area in a switchroom, sizing a riser and setting a tender allowance at concept stage. Treat the output as a preliminary functional-section estimate, then confirm the final busbar, terminal, thermal, clearance and enclosure dimensions with the switchboard builder's verified assembly before construction.

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