Low Voltage Switchgear Types and Selection: A Calculation Guide

Low voltage switchgear is an assembly of switching, protection, measuring, and control devices rated at or below 1000 V AC that receives power from a source — typically a distribution transformer — and distributes it to feeders, motor circuits, and end loads while providing isolation and fault protection. This guide walks through how engineers actually size and choose between the main LV switchgear types: the governing formulas for rated and fault current, a fully worked numeric example, a comparison of construction types (fixed, withdrawable, motor control centers, forms of internal separation), and the practical selection pitfalls that show up in real projects. The same logic applies whether the project sits in the Middle East and Gulf (including Egypt and Saudi Arabia), North Africa, the CIS region, or Sub-Saharan Africa, though ambient temperature, altitude, and grounding practice will shift the final numbers.

Why LV switchgear selection matters

Choosing the wrong LV switchgear type is rarely a catastrophic single failure — it is usually a slow accumulation of derating, nuisance tripping, poor selectivity, or an assembly that cannot survive the actual prospective fault current at its installation point. Because LV switchgear sits directly downstream of the distribution transformer, it must be sized against two independent constraints: the continuous load current the busbars and devices must carry, and the prospective short-circuit current they must withstand or interrupt without damage. Get either one wrong and the assembly is either oversized (wasted capital) or, worse, unable to survive its first real fault.

The method: rated current and fault current formulas

Step 1 — Rated/continuous current. For a three-phase source such as a distribution transformer, the secondary full-load current is:

I (A) = S / (√3 × V)

where S is the transformer rated power in kVA and V is the secondary line voltage in kV.

Step 2 — Prospective short-circuit current. A simplified estimate for the fault current at the transformer terminals, ignoring upstream and cable impedance, uses the transformer’s percentage impedance:

I_sc (A) = I_rated × (100 / Z%)

This is a first-order approximation; full IEC 60909 short-circuit calculations also account for source impedance, cable/busbar impedance, and correction factors, but the transformer-only method is enough to illustrate the order of magnitude an LV switchgear assembly must withstand.

Step 3 — Busbar and device thermal check. Once I and I_sc are known, the busbar cross-section is chosen from manufacturer current-carrying tables (derated for enclosure temperature rise per IEC 61439), and the circuit breaker’s rated short-circuit breaking capacity must be equal to or greater than I_sc at that point.

Worked example: sizing LV switchgear downstream of a distribution transformer

Assume a 1000 kVA, 11/0.4 kV distribution transformer with an impedance of 6%, feeding an LV switchboard.

Rated current: I = S / (√3 × V) = 1,000,000 / (1.732 × 400) ≈ 1,443 A

For example, this tells the design engineer that the incoming LV switchgear section (and its main busbar) must be rated for at least 1600 A (the next standard frame size above the calculated 1,443 A) to leave margin for future load growth.

Prospective fault current at the transformer LV terminals: I_sc = I_rated × (100 / Z%) = 1,443 × (100 / 6) ≈ 24,050 A ≈ 24.1 kA

This means the incoming circuit breaker and the busbar bracing of the LV switchgear must be verified against a prospective short-circuit level in the region of 24 kA at that point (before adding cable impedance, which will reduce the value further downstream). In practice the design engineer would then select an air circuit breaker (ACB) or molded-case circuit breaker (MCCB) frame whose rated short-circuit breaking capacity comfortably exceeds this computed prospective value, and specify busbar bracing to match.

Selectivity check (qualitative): downstream feeder breakers should be graded — either by time delay or by current-limiting characteristic — so that a fault on an outgoing feeder is cleared by the feeder breaker alone, without tripping the upstream incomer. This is verified using manufacturer time-current curves, not by formula alone, and is one of the most commonly overlooked steps in LV switchgear design.

Comparison of low voltage switchgear types

Selection criteria and common pitfalls

• Match rated current with margin, not exactly. Sizing busbars and incomers to the calculated full-load current with zero margin leaves no room for load growth or harmonic-related derating.
• Verify short-circuit withstand at the actual installation point, not just at the transformer terminals — cable and busbar impedance downstream will reduce the fault level, and using the transformer-terminal value everywhere is conservative but can lead to unnecessary over-specification of downstream feeder breakers.
• Check selectivity (discrimination) between the incomer and feeders. Two breakers with adequate individual breaking capacity can still fail to coordinate if their time-current curves overlap.
• Choose the form of internal separation based on operational need, not by default. Higher separation forms increase safety for live maintenance but also increase panel width and cost — selecting the highest form “to be safe” everywhere is a frequent source of budget overrun.
• Consider ambient temperature and altitude derating. In hot-climate GEO regions, enclosure internal temperature rise can force a derating of both busbars and circuit breakers relative to standard test conditions — this must be checked against the manufacturer’s derating curves, not assumed.
• Do not confuse LV switchgear standards with MV switchgear standards. LV assemblies are governed by the IEC 61439 series (assemblies) and IEC 60947 series (individual switching devices such as circuit breakers and contactors), not by the medium-voltage IEC 62271 series.
• Coordinate with the upstream transformer and downstream protection relay settings so that the whole chain — transformer, LV switchgear, and outgoing feeders — is selective under both overload and short-circuit conditions. For background on how the source transformer and upstream MV switchgear interact with this LV board, see our transformer and switchgear hub pages, and for feeder protection coordination see our protection relay guide.

Standards governing low voltage switchgear

Low voltage switchgear and controlgear assemblies are covered by the IEC 61439 series (general rules in IEC 61439-1, and specific requirements for power switchgear and controlgear assemblies in IEC 61439-2), which defines rated characteristics, temperature-rise verification, and the forms of internal separation referenced above. Individual switching devices — circuit breakers, contactors, and switch-disconnectors — are covered by the IEC 60947 series. Short-circuit current calculation methodology at the system level is covered by IEC 60909. These are the correct governing standards for this topic; the IEC 62271 series applies to medium- and high-voltage switchgear and is not applicable here.

Related guides

See also our guides on gas-insulated switchgear, ring main units and metal-clad switchgear.