Switchgear Busbar: Types, Sizing & Current Ratings

Last Updated: 2026-10-17 · Written by the NAIJI Electric engineering team. We design busbar systems for 12–40.5 kV metal-clad and metal-enclosed switchgear panels shipped to 30+ countries. This guide covers what we specify internally.

What Is a Switchgear Busbar?

A busbar is the common conductor inside a switchgear assembly that collects current from the incoming feeder and distributes it to every outgoing circuit breaker panel. In a typical switchgear lineup, three-phase busbars run horizontally through the top or rear of each panel, connecting all bays into one electrical unit.

The busbar is arguably the most critical passive component in the switchgear. Every amp flowing through the switchgear passes through it. A busbar failure — from overheating, loose joints, or short-circuit forces — is a catastrophic event that takes the entire switchgear out of service and can cause an arc-flash incident with energy levels exceeding 30 cal/cm².

This guide covers the three main busbar types used in medium-voltage switchgear, sizing calculations, current rating tables, and material selection — the engineering decisions that NAIJI Electric makes on every 12–40.5 kV switchgear project.

Three Busbar Types in MV Switchgear

1. Copper Flat Bar (Rectangular Bar)

The workhorse of MV switchgear. Rectangular copper bars — typically 30×5 mm up to 120×10 mm — are bolted to epoxy-resin post insulators and run through dedicated busbar compartments. For current ratings above 1,600 A, two or three bars per phase are stacked with 6–10 mm air gaps between them to allow convection cooling.

Advantages: Easy to fabricate (cut, bend, drill, bolt), excellent joint reliability with torque-controlled bolted connections, proven in millions of switchgear panels worldwide. Standard tin or silver plating on contact surfaces reduces joint resistance by 30–40%.

Typical application: Indoor metal-clad and metal-enclosed switchgear from 630 A to 4,000 A. NAIJI Electric uses ETP copper (C11000, 99.9% Cu) flat bar for all standard MV panels.

2. Tubular (Hollow Round) Busbar

Large-diameter copper or aluminum tubes — typically 60–150 mm outer diameter — used in high-current applications above 3,150 A. The hollow cross-section maximizes the skin-effect benefit: at 50/60 Hz, current concentrates near the conductor surface, so the hollow center contributes little to current capacity but adds weight and cost.

Advantages: Superior current-to-weight ratio for ratings above 3,000 A, uniform current distribution (no proximity-effect hot spots between stacked bars), natural high mechanical strength against short-circuit forces due to the tubular section modulus.

Typical application: Generator switchgear (3,150–6,300 A), bus duct connections between transformer and switchgear, GIS bus sections. Less common in standard distribution switchgear where flat bar handles the required ratings.

3. Insulated (Shrink-Sleeve or Epoxy-Coated) Busbar

Bare copper or aluminum bar covered with heat-shrink tubing, epoxy powder coating, or cast-resin insulation. The insulation layer provides phase-to-phase and phase-to-ground dielectric clearance in a smaller space than bare-bar-plus-air-gap designs.

Advantages: Reduced phase spacing (35–50% smaller busbar compartment), elimination of vermin and contamination flashover risk, lower partial discharge in humid or polluted environments. Some designs achieve IP4X touch protection on the busbar without additional barriers.

Trade-offs: Insulation traps heat, so the conductor must be derated by 10–20% compared to bare bar at the same cross-section. Insulated busbars also cost 40–60% more per meter than bare bar due to the coating process.

Typical application: Compact switchgear for offshore, marine, mining, and tropical environments. Also used in ring main units where the entire busbar assembly is resin-encapsulated.

Busbar Sizing: The I = J × A Formula

Busbar sizing starts with a simple relationship:

The current density J is not a free variable. It is constrained by the temperature rise limit set in IEC 62271-1 (for MV switchgear) or IEC 61439-1 (for LV switchgear). Exceeding the temperature limit accelerates joint oxidation, insulator aging, and contact degradation — shortening switchgear life from 30 years to as little as 10.

Allowable Current Density by Material

Worked example: Design a main busbar for 2,500 A in a 12 kV metal-clad switchgear panel at 35°C ambient.

1. Choose copper flat bar, J = 1.6 A/mm² (conservative for enclosed panel with moderate ventilation).
2. Required area: A = I / J = 2,500 / 1.6 = 1,562 mm² per phase.
3. Select two bars per phase, each 100 × 8 mm = 800 mm². Total = 1,600 mm² > 1,562 mm². ✔
4. Verify short-circuit withstand for the selected cross-section (see section below).

Busbar Current Rating Table: Copper vs Aluminum

The following table shows continuous current ratings for common busbar sizes in medium-voltage switchgear. Values assume bare bar, horizontal mounting, natural convection cooling, 35°C ambient, and temperature rise per IEC 62271-1 (105 K for bare conductor). Single bar per phase unless noted.

Note: Multi-bar ratings are not simply N × single-bar values. The proximity effect between parallel bars creates uneven current distribution, typically reducing total capacity by 5–12% compared to N × single-bar. The values above include this derating. Dual bars with a 6–8 mm air gap perform better than bars bolted together with no gap.

Temperature Rise Limits per IEC 62271-1

The IEC 62271 series defines maximum allowable temperature rise for busbar components during a type test. These limits protect insulation materials, bolted connections, and plating from thermal degradation over the switchgear's 30-year service life.

* IEC 62271-1 Table 6 permits 90°C for bare conductors with a rise limit that depends on ambient: ΔT = 90 – T_ambient. At 35°C, that is 55 K. The 105 K figure applies when the manufacturer declares a maximum conductor temperature up to 140°C for short-duration overloads, but continuous design normally uses the 55–75 K range for conservative engineering.

Practical implication: If your switchgear room ambient is 50°C (common in tropical or enclosed rooftop installations), the allowable temperature rise drops to 40 K, requiring the busbar to be oversized by approximately 20–25% compared to the standard 35°C design.

Copper vs Aluminum: Material Comparison

Choosing between copper and aluminum busbars involves five factors beyond just conductivity. Here is the engineering comparison based on our switchgear production experience:

NAIJI Electric standard practice: We use copper flat bar as the default for all MV switchgear (12–40.5 kV). Aluminum is offered on customer request for LV bus duct and outdoor applications where the panel dimensions can accommodate the larger cross-section. For bolted aluminum joints, we specify Belleville spring washers (to compensate for creep and thermal cycling), anti-oxidant paste (Penetrox or equivalent), and torque values 15–20% lower than copper to avoid cold flow.

Short-Circuit Withstand: The Force That Governs Everything

Steady-state heating determines the busbar cross-section, but short-circuit forces determine the insulator strength, support spacing, and busbar thickness. The electromagnetic force between two parallel conductors carrying fault current is:

For a switchgear rated 25 kA (rms) with a 2.5 peak factor, the peak current is 62.5 kA. With 200 mm phase spacing and 600 mm support span, the force on each busbar is:

F = 0.2 × 62.5² × 0.6 / 0.2 = 2,344 N (239 kg-force) per support point

This force acts in under 10 milliseconds, and the busbar must not permanently deform. The busbar’s section modulus and the insulator’s cantilever strength must exceed this value with a safety factor of at least 2.5. For the full short-circuit rating methodology, see our dedicated calculation guide.

Thermal Short-Circuit Withstand

In addition to mechanical forces, the busbar must absorb the thermal energy of the fault current without exceeding the short-time temperature limit (250°C for copper, 200°C for aluminum). The minimum cross-section for thermal withstand is:

A_min = I_sc × √t / k

Where k = 143 for copper and k = 94 for aluminum (per IEC 60865-1). For 25 kA / 3 seconds:

• Copper: A_min = 25,000 × √3 / 143 = 303 mm²
• Aluminum: A_min = 25,000 × √3 / 94 = 460 mm²

A 60 × 6 mm copper bar (360 mm²) passes the thermal withstand check for 25 kA / 3 s, but an 80 × 8 mm bar (640 mm²) would be selected in practice because the steady-state current rating of the 60 × 6 bar is only 590 A — usually below the switchgear’s rated current.

Busbar Design Considerations for Switchgear Engineers

Skin Effect at 50/60 Hz

At power frequency, current concentrates near the conductor surface. The skin depth in copper at 50 Hz is approximately 9.3 mm. Bars thicker than 10 mm have underutilized core material. This is why flat bars are rarely thicker than 10 mm — beyond that, it is more efficient to use two thinner bars with an air gap than one thick bar.

Proximity Effect in Multi-Bar Configurations

When two bars per phase are mounted parallel with a small gap, the current distribution is not uniform. The facing surfaces carry more current than the outer surfaces, creating localized hot spots. Maintaining a 6–10 mm air gap between bars reduces this effect and allows convection cooling between the surfaces. For three bars per phase, computational analysis (FEA) is recommended to verify current sharing.

Joint Design

Bolted busbar joints are the single most common failure point in switchgear. Best practices:

• Overlap length: Minimum 2× bar width for copper, 2.5× for aluminum.
• Surface preparation: Machine-flat surfaces, cleaned with solvent. No file marks crossing the current path.
• Plating: Tin or silver plating on contact faces reduces joint resistance by 30–40% and prevents oxidation. Required for aluminum; strongly recommended for copper.
• Hardware: High-tensile bolts (Grade 8.8), flat washers, Belleville spring washers (mandatory for aluminum, recommended for copper), Nyloc nuts.
• Torque: Per manufacturer specification. Typical: M12 bolt on copper = 60–70 N·m; on aluminum = 50–60 N·m.

Busbar Compartment in Metal-Clad Switchgear

In metal-clad switchgear to IEEE C37.20.2 / IEC 62271-200, the busbar compartment is a separate grounded metal enclosure isolated from the circuit breaker compartment and cable compartment by grounded metal barriers. This compartmentation prevents an arc fault in one compartment from propagating to the busbar. The busbar compartment must pass the Internal Arc Classification (IAC) test — a controlled arc fault that verifies enclosure integrity and pressure-relief operation.

Busbar Selection: Step-by-Step

Here is the decision process our engineering team follows for every NAIJI Electric switchgear project:

1. Determine rated current. From the power system single-line diagram. Include 10–15% margin for future load growth.
2. Determine short-circuit level. From the short-circuit study. Both rms value (for thermal withstand) and peak value (for mechanical withstand).
3. Select material. Copper for MV switchgear (≤12 kV), aluminum only when specifically required by customer or application constraints.
4. Calculate minimum thermal area. A_min = I_sc × √t / k. This is the absolute floor.
5. Select bar size from current rating table. Choose a cross-section where the continuous current rating ≥ rated current × 1.1 (10% margin).
6. Verify mechanical withstand. Calculate electromagnetic force at peak short-circuit current. Check that busbar stress < yield strength / 2.5.
7. Set support spacing. Reduce span between insulators until mechanical stress is within limits. Typical: 300–600 mm for MV switchgear.
8. Verify temperature rise. Confirm the selected bar meets the IEC 62271-1 temperature rise limit at the declared ambient temperature. For non-standard ambients, apply derating factors.

Common Busbar Specification Mistakes

From our experience manufacturing switchgear for utility, industrial, and renewable energy projects, these are the errors we see most often in busbar specifications:

Mistake 1: Sizing only for current, ignoring short-circuit. A 50 × 6 mm copper bar carries 500 A comfortably, but at 31.5 kA fault level with 500 mm support spacing, the electromagnetic force can exceed the insulator’s cantilever strength. Always check both thermal and mechanical withstand before finalizing the bar size.

Mistake 2: Specifying aluminum busbars in MV without addressing joint degradation. Aluminum oxide is an insulator (unlike copper oxide, which is a semiconductor). Aluminum bolted joints will increase in resistance over time unless protected with anti-oxidant compound and Belleville washers. We have seen aluminum busbar joints reach 80–100°C after 5 years in service without proper protection — twice the temperature of adjacent copper joints.

Mistake 3: No derating for high ambient temperature. A switchgear room in the Middle East, tropical Africa, or an enclosed container reaches 50–55°C ambient. At 50°C, the allowable temperature rise drops from 55 K to 40 K, requiring 20–25% busbar oversizing. Failing to derate means the switchgear passes the factory type test (at 35°C) but overheats in the field.

Mistake 4: Ignoring skin effect with thick bars. We occasionally see specifications calling for 120 × 15 mm or 100 × 20 mm bars. Bars thicker than 10 mm waste copper in the core. Two 100 × 8 mm bars (1,600 mm², ~2,200 A) outperform one 100 × 16 mm bar (1,600 mm², ~1,900 A) despite identical cross-sections, because the dual-bar configuration has 60% more cooling surface and better current distribution.

NAIJI Electric Busbar Engineering

NAIJI Electric manufactures MV switchgear with copper busbar systems for 12–40.5 kV, rated up to 4,000 A and 50 kA short-circuit withstand. Our standard busbar practice includes:

• Material: ETP copper (C11000) per EN 13601, conductivity ≥99.9% IACS. Silver-plated contact surfaces on all bolted joints.
• Insulation: Cycloaliphatic epoxy post insulators, type-tested per IEC 62271-1 for mechanical withstand (including internal arc) and thermal cycling.
• Quality control: Every busbar joint is torqued with calibrated tools and verified with a digital torque audit. Main bus joints are infrared-scanned after the temperature rise type test to confirm uniform heating.
• Documentation: Busbar material certificates (EN 10204 Type 3.1), torque records, and IR scan images are included in the switchgear test report.

Our switchgear product range includes metal-clad, metal-enclosed, and ring main unit configurations with busbar systems engineered for your specific project requirements.

Need busbar sizing support or a switchgear quotation? Send your single-line diagram and project specifications. Contact NAIJI Electric →

Related guides: Switchgear Lineup Design Guide | Types of Electrical Switchgear | Metal-Clad vs Metal-Enclosed Switchgear | Short-Circuit Rating Calculation Guide