Busduct Sizing Calculator

Inputs

Example Data Table

Examples are indicative. Validate against catalogs and project standards.

Formula Used

• Three-phase (kW): I = (kW × 1000) / (√3 × V × PF × Eff)
• Single-phase (kW): I = (kW × 1000) / (V × PF × Eff)
• Three-phase (kVA): I = (kVA × 1000) / (√3 × V)
• Single-phase (kVA): I = (kVA × 1000) / V

Design current build-up:

1. I_demand = I_base × Demand / Diversity
2. I_cont = I_demand × 1.25 (only if continuous load is selected)
3. I_spare = I_cont × (1 + Spare%/100)
4. I_required = I_spare / (Temp × Installation × Ventilation × Grouping)
5. Select the next higher standard rating ≥ I_required

Use manufacturer derating curves and applicable codes for final sizing.

Busduct Sizing Guide

Busduct systems provide a compact, high-capacity method to distribute power across large buildings, plants, and campuses. Correct sizing is not only about matching the connected load; it also ensures safe temperature rise, acceptable voltage drop, and reliable performance over the service life. A well-sized busduct supports future expansion, reduces overheating at joints, and improves maintainability during shutdowns. This calculator is designed for preliminary design and tender-level checks, helping you arrive at a practical standard rating quickly.

The sizing workflow begins by converting the load to current. When your load is entered as kW, the current depends on system voltage, power factor, and efficiency. For kVA inputs, power factor is already included in the apparent power, so the conversion is straightforward. After the base current is found, demand and diversity factors adjust the coincident loading. Demand reflects typical utilization of the connected equipment, while diversity accounts for the fact that not every load peaks at the same moment.

Next, the calculator applies an optional continuous-load factor and a spare-capacity margin. Continuous loading commonly requires additional headroom because sustained current increases conductor temperature. Spare capacity is practical for future machines, temporary construction loads, or measurement uncertainty. Finally, derating factors account for ambient temperature, installation exposure, ventilation, and grouping. These factors reduce usable ampacity, so the required busduct rating increases to compensate. Always confirm the final selection using the manufacturer’s published tables for the exact busduct family and enclosure.

Example using the table: For “Workshop MCC,” assume three-phase 400 V with 150 kW, PF 0.90, and efficiency 0.95. The calculated base current is then adjusted using demand 0.85 and diversity 1.10, followed by 10% spare and the ambient-temperature factor at 40°C. The outcome is a required rating that rounds up to the next standard size, producing a 320 A recommendation. Use the voltage-drop option to validate long runs; if the percentage is high, a larger rating or shorter route may be justified.

Beyond ampacity, project-ready selection should also verify short-circuit withstand (kA/1s), IP protection, fire-stopping requirements, expansion joints for thermal movement, and the tap-off arrangement for downstream distribution. Coordinating the upstream protective device and checking selective tripping completes a robust design. With these checks, busduct becomes a reliable backbone for modern electrical installations.

FAQs

1) What is the difference between busduct and cable feeders?

Busduct uses enclosed busbars with standardized joints and tap-offs, often simplifying installation and upgrades. Cable feeders are flexible and common for smaller currents, but can require larger trays, more terminations, and heavier pulling effort at high ratings.

2) Why does a higher power factor reduce current for kW loads?

For a fixed real power (kW), a higher power factor means less apparent power is needed. That reduces line current, which lowers losses and temperature rise. It can also improve voltage drop on long runs.

3) When should I apply the continuous-load factor?

Apply it when the calculated load is expected to run for long durations without meaningful cooldown, typically beyond three hours. This adds headroom to limit temperature rise and reflects common design practice for sustained loading.

4) Are the derating factors in this tool final for construction?

No. They are typical preliminary factors to guide sizing. Final design should use the busduct manufacturer’s derating curves for the exact enclosure, conductor material, joint type, and installation method used on site.

5) How do I choose R and X values for voltage drop?

Use manufacturer impedance data if available because it varies by rating and construction. If you lack catalog values, keep the estimate conservative and treat the result as a screening check, not a certified calculation.

6) What utilization range is generally comfortable?

Many designers aim for moderate utilization to accommodate heating, future growth, and minor loading changes. If utilization is very high, consider selecting the next standard rating, especially for harsh ambient conditions or limited ventilation.

7) Does this calculator cover short-circuit withstand sizing?

No. Ampacity sizing alone is not sufficient. You must verify the busduct short-circuit withstand rating against the calculated fault level and protection clearing time, and ensure joints and supports meet the required mechanical strength.

How to Use This Calculator

1. Choose phase and set the correct line voltage.
2. Select the load basis and enter the load value.
3. Enter power factor and efficiency when using kW.
4. Apply demand and diversity factors for the load profile.
5. Enable continuous load only for duty above three hours.
6. Set ambient temperature and environment for preliminary derating.
7. Optionally enable voltage drop and enter run length plus impedance.
8. Press Calculate to see results above the form.
9. Download CSV or PDF for design documentation.