Riser cable substitution on a 22-storey apartment block
You are the professional engineer certifying the electrical design of a 22-storey serviced apartment block supplied at 400 V, 50 Hz, TN-S from a 1500 kVA consumer substation. The rising main to the apartment floors is designed for 400 A per phase over a 160 m route. The contract sets a maximum voltage drop of 2.0% across the riser, with 1.0% allocated to final circuits and 1.0% to the supply and transformer. The specification calls for 4-core 300 mm² XLPE copper, installation method C, in a riser containing four bunched circuits at a design ambient of 40 °C. During inspection you find 4-core 185 mm² copper installed instead. Its tabulated rating is 410 A, and the contractor argues that this comfortably covers the 400 A load, so there is no non-conformance. The riser is now boxed in and fire-stopped, the first eight floors are due for energisation next week, and the developer asks you to accept the installation 'with a note' on the completion certificate.
1. Determine whether the installed 185 mm² cable complies. Show the derating and voltage-drop checks and identify which parameter governs.
Read the worked answer
Derated current-carrying capacity
- The 410 A tabulated value applies to a single circuit at 30 °C ambient with no grouping. It cannot be compared directly with 400 A.
- Ambient correction for 40 °C (XLPE, 30 °C base): 0.91.
- Grouping correction for four bunched circuits in a riser: 0.75.
- Combined factor 0.91 × 0.75 = 0.68, so Iz = 410 × 0.68 ≈ 279 A.
- Design current is 400 A and the protective device is 400 A; the cable is overloaded by roughly 43%. The condition Ib ≤ In ≤ Iz fails and the device cannot protect the cable. This is the governing safety failure.
Voltage drop
- 185 mm² four-core copper at operating temperature: approximately 0.147 mV/A/m (three-phase).
- ΔU = 0.147 × 400 × 160 / 1000 ≈ 9.4 V, i.e. about 2.35% of 400 V.
- The riser allowance is 2.0%, so it also fails on performance; 300 mm² would give about 1.7%.
Conclusion: reject. The contractor's argument fails because it uses an un-derated tabulated rating. The voltage-drop excess is a design/performance issue that the client could in principle waive, but the derating failure is a safety issue that no waiver can cure.
2. The contractor offers a signed waiver and the developer asks you to accept with a note. Set out your response and your immediate actions.
Read the worked answer
A completion certificate is a declaration that the installation complies with the design, MS IEC 60364 and the Electricity Regulations 1994. A note cannot convert a non-compliant cable into a compliant one, and a client waiver does not transfer my statutory and professional duty.
Actions:
- Issue a formal non-conformance report the same day, copied to the developer and main contractor, referencing the derating and voltage-drop calculations.
- Do not authorise energisation of the riser at 400 A.
- If occupation cannot wait, consider an engineered interim arrangement only: a reduced-rating device (for example a 250 A MCCB with locked settings) so that Iz ≥ In, a documented load restriction, and a written temporary-supply procedure. This serves the reduced load only; it cannot serve the 400 A design load.
- Require the riser to be opened and the cable replaced with 300 mm², or an engineered alternative such as a busbar riser, with revised calculations and an updated cable schedule.
- After replacement, require inspection and test to MS IEC 60364-6 (insulation resistance, continuity of protective conductors, polarity, Zs at each board, phase rotation), witnessed by me.
- If pressure continues, decline to certify in writing and escalate internally. I will not sign a certificate I know to be untrue.
3. The same contractor submitted cable schedules for three other projects in your office. What checks would you apply to catch this error elsewhere?
Read the worked answer
I would not rely on a spot check of one cable; I would review the method behind every schedule:
- Confirm every cable has Ib, In and Iz shown explicitly, with all applicable correction factors (ambient, grouping, thermal insulation, soil resistivity and depth for buried runs, enclosure).
- Check the voltage-drop budget is allocated by section (supply, riser, final circuit) rather than only totalled at the end, so a single section cannot consume the whole allowance.
- Cross-reference the cable schedule against the single-line diagram and the actual protective device ratings, not the designer's intended ratings.
- Check the adiabatic short-circuit withstand (k²S² ≥ I²t) at each reduced cable, since a smaller conductor may pass on current but fail the fault-energy check at 36 kA and 0.1 s clearing.
- Verify the stated input assumptions are project-specific: a 40 °C riser ambient and four bunched circuits must appear in the calculation, not just in the specification.
- Require a stamped calculation sheet and design-check record, and sample at least the riser, the longest submain and the largest motor feeder in each submission.