Busbar riser design: current, derating and voltage drop
A busbar riser feeds loads at several levels of a building. The load current, installation conditions, voltage drop and fault withstand must describe the same proposed system. A nominal current rating by itself does not establish that a riser is suitable.
Build the load basis first
Separate connected load from coincident demand. Record the demand assumptions, supply voltage and operating power factor. For a balanced three-phase load, input current is I = P / (√3 × V × PF), using watts and line-to-line volts. If P is mechanical output, efficiency must also be included. Do not apply motor efficiency again when P already represents electrical input.
Worked example: 500 kW riser
Assume 500 kW electrical demand, 400 V and 0.90 power factor. Current = 500,000 / (1.732 × 400 × 0.90) = 801.9 A. For this training example only, suppose the selected manufacturer's combined installation correction is 0.80. A 1,000 A nominal system would then provide 800 A, slightly below the calculated demand. A 1,250 A nominal system would provide 1,000 A under that assumed correction.
The 0.80 factor is illustrative, not a universal busbar value. Obtain the actual correction for ambient temperature, orientation, enclosure, joints and installation from the proposed manufacturer. A product may already be rated at the project ambient temperature; applying another correction without checking can double-count it.
Voltage drop follows the load distribution
For each balanced three-phase segment, approximate ΔV = √3 × I × L × (R × cosφ + X × sinφ). If R and X are in ohms per kilometre, L must be in kilometres. Use impedance at the relevant operating temperature. Calculate every segment using the load downstream of it, then add the drops to the destination being checked.
A riser carrying the full demand for its entire length is different from one with regular floor tap-offs. Modelling all loads at the far end can overestimate normal distributed-load drop, but it should not replace the actual floor schedule.
Checks before procurement
- Match the fault level and clearing time to the offered assembly's tested withstand data.
- Coordinate tap-off protection, neutral loading, protective conductor continuity and upstream protection.
- Confirm joint access, supports, fire barriers at penetrations and the manufacturer's expansion provisions.
Use the lab to compare assumptions, then record the selected manufacturer's data in the project calculation. This worked example does not select a construction product.
Further reference
Schneider Electric: steady-load voltage drop equations. External guidance supports the topic; the illustrative inputs and worked examples above are prepared for this guide and are not manufacturer product selections.
Verify assumptions and results before project use. These worked examples are educational and do not provide authority approval. Report a correction.