Sub-Distribution Board Design and Sizing Guide

Sub-distribution boards (SDBs) are low-voltage switchgear and controlgear assemblies that distribute power from a main distribution board (MDB) to final circuits or sub-circuits. In modern projects, they are designed and verified in accordance with the IEC 61439 series, which replaced IEC 60439 and introduced performance-based verification for temperature rise, short-circuit withstand, dielectric properties, and mechanical strength [1] [3].

For most building applications, SDBs are covered by IEC 61439-3 for boards intended to be operated by ordinary persons, and by IEC 61439-2 for power switchgear assemblies and more specialized installations [1] [2].

What an SDB Does

The SDB provides local distribution, isolation, and protection for grouped loads such as lighting, sockets, HVAC equipment, pumps, and small motors. It improves selectivity, simplifies maintenance, and reduces cable runs from the MDB to final loads [2] [3].

Step 1: Collect Project Data

Correct sizing starts with complete project data. Manufacturer guides for IEC 61439 assemblies recommend collecting the following before design begins: connected load, diversity assumptions, circuit count, protective device types, ambient temperature, installation location, enclosure IP rating, and available short-circuit level at the supply point [3] [4].

• Supply voltage: typically 230/400 V, 50 Hz in the Middle East.
• Ambient temperature: commonly 35°C design basis, with Gulf conditions often requiring derating at 40°C or higher [6].
• Ingress protection: indoor boards may use IP40 or IP41, while dusty or semi-exposed areas often require IP54 or better [3].
• Separation form: Form 2b, 3b, or 4 may be selected depending on maintainability and risk level [2].

Step 2: Determine Design Current

The design current is based on the expected demand, not simply the connected load. For multiple final circuits, a rated diversity factor (RDF) is commonly applied so that the board is not oversized for loads that do not operate simultaneously [4].

The approximate three-phase current is:

$$ I = \frac{P}{\sqrt{3}\,V\,\cos\varphi} $$

where \(P\) is the active power in watts, \(V\) is the line-to-line voltage, and \(\cos\varphi\) is the power factor.

For grouped loads, the diversified demand can be estimated as:

$$ P_{\text{demand}} = \sum P_i \times \text{RDF} $$

In practice, RDF values for final circuits are often in the range of 70% to 90%, depending on occupancy and load type [4].

Step 3: Size the Main Protective Device

The incomer device, usually a MCCB or switch-disconnector with fuses, must be selected to carry the design current while coordinating with downstream protection. Under IEC 61439, the assembly designer must verify that the rated current of the assembly \(I_n\) is not exceeded under the declared installation conditions [3] [2].

A practical selection rule is:

$$ I_b \leq I_n \leq I_z $$

where \(I_b\) is the design current, \(I_n\) is the protective device rating, and \(I_z\) is the current-carrying capacity of the associated conductors.

Step 4: Verify Temperature Rise

Temperature rise is one of the most important verification points in IEC 61439. The standard uses performance-based verification methods: testing, comparison with a tested reference design, or calculation where permitted [4] [3].

For terminals, the commonly referenced temperature rise limit is 70 K above ambient, subject to the specific component and standard requirements [3] [7].

This matters especially in the Middle East, where high ambient temperatures reduce thermal margin. Boards installed in plant rooms, rooftops, car parks, or outdoor service areas should be derated and ventilated appropriately. Poor ventilation can create significant overtemperature even when the nominal current appears acceptable [4] [6].

Step 5: Verify Short-Circuit Withstand

The assembly must withstand the prospective short-circuit current available at the installation point. IEC 61439 requires verification of the short-circuit withstand strength of the busbars, connections, and protective arrangement, including the coordination with the upstream short-circuit protective device (SCPD) [3] [7].

A common design check is:

$$ I_{cw,\text{board}} \geq I_{k,\text{prospective}} $$

where \(I_{cw,\text{board}}\) is the board short-time withstand current and \(I_{k,\text{prospective}}\) is the prospective fault current.

In many commercial projects, 25 kA to 50 kA ratings are common, but the actual requirement must be based on the fault level at the MDB or feeder point [4].

Step 6: Size the Busbars

Busbar sizing must account for continuous current, temperature rise, spacing, and fault withstand. IEC 61439 does not prescribe a single universal current density; instead, the busbar arrangement must be verified for the declared assembly conditions [3] [2].

For preliminary engineering, copper busbars are often estimated using a current density range, but the final design must be verified thermally and mechanically. A simple sizing estimate is:

$$ A = \frac{I}{J} $$

where \(A\) is the cross-sectional area, \(I\) is the design current, and \(J\) is the assumed current density.

For compact boards, side or vertical busbar arrangements are often used to improve modularity and circuit distribution, while higher-current assemblies may use larger or multiple parallel copper bars [2] [4].

Step 7: Select Outgoing Circuits and Protection

Outgoing ways are typically populated with MCBs, RCBOs, RCDs, or fused switches depending on the load type and local utility requirements. In many Middle East jurisdictions, RCD protection is mandatory for final circuits, particularly socket and wet-area circuits [5] [6].

A typical SDB may include:

• Main incomer MCCB or switch-disconnector
• Surge protection device where required by project specification
• Lighting MCBs, socket MCBs, and dedicated motor or HVAC feeders
• RCBOs for selected or all final circuits
• Neutral and earth bars sized for the actual harmonic and fault conditions

For nonlinear loads, the neutral conductor may need to be oversized. Harmonic-rich office and IT loads can increase neutral current beyond phase current in some cases, so neutral sizing should not be assumed equal to phase size without analysis [4].

Step 8: Choose the Enclosure and Form of Separation

The enclosure must suit the environment, maintenance strategy, and safety requirements. For indoor commercial installations, Form 2b or Form 3b separation is often used to improve maintainability and limit disruption during servicing. Higher-risk installations may require Form 4 separation [2] [3].

In hot, dusty climates, enclosure material, gasket quality, anti-corrosion coating, and internal ventilation become especially important. For outdoor or semi-outdoor applications, UV resistance and higher IP ratings should be considered as part of the specification [6].

Practical Example: 400 A Office Floor SDB in Dubai

Consider a commercial office floor with the following diversified loads:

• Lighting: 20 kW
• HVAC: 50 kW
• Office equipment: 10 kW

Assuming an RDF of 0.8:

$$ P_{\text{demand}} = (20 + 50 + 10) \times 0.8 = 64 \, \text{kW} $$

With a 400 V, three-phase supply and a power factor of 0.9:

$$ I = \frac{64\,000}{\sqrt{3}\times 400 \times 0.9} \approx 102.6 \, \text{A} $$

In this case, a 125 A or 160 A incomer may be sufficient depending on spare capacity, future expansion, and diversity assumptions. If the project requires a larger board for future ways or a higher fault level, a 250 A or 400 A assembly may still be selected, provided the thermal and short-circuit verification is completed in accordance with IEC 61439 [3] [4].

For preliminary busbar estimation