Sizing Switchgear Correctly: A Load Calculation Guide for South African Facilities

Switchgear sizing decisions get made early in a project — often before the building is fully designed — and they are expensive to get wrong in either direction. Undersized switchgear runs hot under normal load, trips on legitimate demand rather than faults, and forces a costly mid-life upgrade. Oversized switchgear is capital sunk into capacity the facility may never use, and it can actually work against protection coordination if breaker and relay settings are stretched to match equipment that's bigger than the load requires.

This article sets out the load calculation method plant engineers and facility managers can use to specify switchgear correctly the first time, and to sanity-check a sizing proposal from a contractor or supplier before signing off on it.

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Why Sizing Is Where Switchgear Problems Start

Most switchgear failures traced back to root cause are not manufacturing defects — they're sizing decisions made without a proper load study: a busbar rated for the connected load rather than the diversified demand, a breaker selected on rule-of-thumb rather than calculation, or a panel with zero spare capacity that gets a plant expansion two years later and no room to fit it. Getting the sizing calculation right at specification stage is the cheapest point in the equipment's life to fix it.

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Step 1: Build the Connected Load Schedule

Start with a complete schedule of every load the switchgear will supply: motors, lighting circuits, HVAC, process equipment, welding sets, battery chargers, and any single large load that dominates the profile (a compressor, a kiln, a large pump). For each load, record the rated current or kW, the power factor, and the duty cycle — continuous, intermittent, or standby.

This is the connected load — the theoretical maximum if every piece of equipment ran simultaneously at full rated output. In practice almost no facility ever reaches this figure, which is why the next step matters.

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Step 2: Apply Demand and Diversity Factors

Demand factor is the ratio of the maximum demand a load actually draws to its full connected rating — a motor rarely runs at 100% of nameplate current continuously, and lighting circuits are rarely all switched on at once. Diversity factor accounts for the fact that not all circuits on a board hit their individual maximum demand at the same moment.

SANS 10142-1 provides guidance figures for typical demand factors by load category (lighting, socket outlets, motors, water heating), and these should be the starting point rather than a generic industry rule of thumb, since they're the figures an inspector or approving authority will check the design against. Where a load's actual operating profile is well understood — a specific production line with known simultaneity — use the measured or specified figure over the tabulated default.

The output of this step is the design current, sometimes called maximum demand current: the current the switchgear must be sized to carry continuously under realistic operating conditions, not the sum of every connected load's full rating.

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Step 3: Determine the Design (Maximum Demand) Current

With demand and diversity factors applied, calculate the maximum demand current at the switchgear's supply point. This figure — not the connected load — is what drives busbar, incomer, and cable sizing from here on.

Two adjustments are worth making at this stage before moving on:

• Ambient temperature and enclosure derating. A switchgear panel in a hot plant room or direct sun-exposed enclosure needs its component ratings derated per the manufacturer's temperature correction tables — a breaker rated at 40°C ambient does not deliver the same continuous rating at 50°C.
• Harmonic loading. Facilities with significant VFD, UPS, or rectifier load should account for harmonic current when sizing neutral conductors and busbars — triplen harmonics add in the neutral rather than cancelling, which catches out sizing based on balanced-load assumptions alone.

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Step 4: Select Busbar and Incomer Ratings

Busbar and incomer ratings should sit comfortably above the calculated design current, not just clear it. A busbar sized to the exact calculated demand leaves no room for load growth, measurement uncertainty in the original load schedule, or future additions without a full panel replacement.

A commonly used approach is to select the next standard busbar/incomer rating above the design current, and separately confirm that this provides a sensible headroom margin for anticipated growth (see Step 6). "Next standard size up" and "adequate headroom for the facility's five-year plan" are two different checks — doing only the first without the second is one of the most common sizing mistakes we see.

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Step 5: Verify the Short-Circuit (Fault) Rating

Current rating is only half the specification. The switchgear's short-circuit withstand rating — its ability to survive the maximum prospective fault current at that point in the network without mechanical or thermal failure — must be verified against a fault level calculation for the specific point of installation, not assumed from a similar installation elsewhere.

This requires knowing the upstream fault level (from the utility connection or upstream transformer impedance) and confirming the switchgear's rated short-circuit breaking and making capacity, and busbar withstand rating, meet or exceed it — for both the prospective fault current magnitude and its duration. LV switchgear assemblies should be specified and verified against IEC 61439 type-tested ratings for the actual fault level at the installation point, not a generic catalogue figure for "similar" equipment. Undersizing this rating is invisible during normal operation and only shows up catastrophically at the first real fault — which is exactly why it needs to be calculated, not assumed.

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Step 6: Size and Coordinate the Outgoing Breakers

Each outgoing breaker should be sized to its specific downstream load's design current — using the same demand-factor logic as Step 2, applied at circuit level — and cable sized and protected to match, per SANS 10142-1 cable current-carrying capacity and derating tables for the actual installation method and grouping.

Coordinate outgoing breaker settings against the incomer and any upstream protection so that a fault on one outgoing circuit trips only that circuit's breaker, not the incomer — this is the discrimination/selectivity study, and it's a sizing question as much as a settings question, since undersized breaker frames can constrain the achievable time-current coordination.

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Step 7: Build In Headroom for Future Load

A switchgear panel sized to exactly today's demand, with zero spare ways and zero busbar headroom, guarantees a full panel replacement the first time the facility adds a production line, a compressor, or an EV charging bay. Standard practice is to size busbar and incomer capacity with room for a realistic future load increment — informed by the facility's actual expansion plans, not an arbitrary blanket percentage — and to specify spare breaker ways in the panel itself, since an empty way is far cheaper to fill later than a panel is to replace.

Where a facility's growth plans are genuinely unknown, err toward more spare ways rather than larger busbar headroom — additional breaker ways are a modest incremental cost at manufacture, while oversizing busbar and incomer capacity against a demand figure that may never materialise is a larger sunk cost.

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Common Sizing Mistakes We See on Site

• Sizing to connected load instead of design current — produces switchgear that is dramatically oversized for the actual diversified demand, at a real cost premium.
• Copying a sizing decision from a "similar" facility without redoing the load schedule and fault-level calculation for the specific site — no two sites have the same upstream fault level or load profile.
• Verifying current rating but not fault rating, or the reverse — both are independent checks and both must pass.
• Zero spare breaker ways, forcing a full panel replacement for the first future addition rather than a straightforward extension.
• Ignoring ambient and harmonic derating in plant rooms that run hot or carry significant VFD/UPS load, leading to nuisance thermal trips that get misdiagnosed as a protection setting problem rather than a sizing gap.

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Working With GNTech

GNTech specifies and supplies LV and MV switchgear — including Telergon LV isolation and protection equipment and Pronutec MV ring main units — sized against a proper load calculation and fault-level study rather than a generic catalogue match. Our technical team can review an existing load schedule, run the demand and fault-level calculations for a new installation, or sanity-check a sizing proposal from another supplier before you commit capital to it. Contact the GNTech team to discuss switchgear specification for your facility.