Office fit-out: six circuits bunched in one trunking at 40 degrees C
A contractor is installing final circuits for a refurbished office suite. Six 230 V single-phase circuits will share one 50 mm metal trunking along a ceiling route for 19 m. Each circuit is wired in 2.5 mm2 PVC-insulated single-core copper, bunched in the trunking (reference method B), and protected at the local distribution board by a 20 A type C MCB. The heaviest circuit, a 3.5 kW power circuit, has a design current of 15 A; the others are smaller. The measured ambient temperature in the ceiling void in summer is 40 degrees C. The installer's sheet lists the tabulated current-carrying capacity of 2.5 mm2 as 24 A, which exceeds the 20 A breaker, so he considers the design complete and intends to pull all six circuits into the trunking on the same day. The designer's voltage-drop limits are 3 percent for lighting and 5 percent for other circuits.
1. Check the design against the corrected current-carrying capacity and state whether it complies.
Read the worked answer
The tabulated 24 A is only valid for the reference conditions: 30 degrees C ambient, one circuit, no grouping. Two correction factors apply.
- Ambient temperature, PVC insulation at 40 degrees C: Ca = 0.87.
- Grouping, six loaded circuits bunched in an enclosed trunking: Cg = 0.57.
Corrected capacity: Iz = 24 x 0.87 x 0.57 = 11.9 A.
Compliance requires Ib <= In <= Iz, i.e. 15 A <= 20 A <= 11.9 A. The third condition fails badly: the cable would be asked to carry up to 20 A while its corrected capacity is under 12 A. The design is non-compliant and would lead to sustained conductor temperatures well above the 70 degrees C rating, PVC embrittlement at the trunking entries, and eventually insulation failure. The installer has applied tabulated capacity without correction factors, which is the classic error this situation tests. Note that the grouping factor, not the ambient temperature, is doing most of the damage; ventilating the void would not rescue the design.
2. Select a conductor size and protective device that satisfy the requirements, including a voltage-drop check for the 19 m run.
Read the worked answer
Hold the 20 A device and the 15 A design current, and search for a cable whose corrected capacity reaches 20 A. The combined factor is 0.87 x 0.57 = 0.496.
- 4 mm2: 32 x 0.496 = 15.9 A, below 20 A (and only just above Ib), so no.
- 6 mm2: 41 x 0.496 = 20.3 A, which satisfies In <= Iz. Ib = 15 A is also below In = 20 A.
Voltage drop, 6 mm2 single-phase:
Vd = mV/A/m x Ib x L / 1000 = 7.3 x 15 x 19 / 1000 = 2.08 V.
As a percentage of 230 V that is 0.9 percent, comfortably inside the 5 percent limit, so no uprating is needed for drop.
Provide the schedule note as: 6 mm2 PVC single-core, method B, 20 A type C MCB, Ca 0.87, Cg 0.57, Iz 20.3 A, Vd 0.9 percent. If the trunking fill or bend radius cannot take 6 mm2, the alternative is to change the circuit arrangement rather than the breaker alone: splitting the circuits into groups of two gives Cg = 0.80, and 2.5 mm2 then yields 24 x 0.87 x 0.80 = 16.7 A, permitting a 16 A MCB with the 15 A design current.
3. The client rejects the larger cable on cost. Describe two alternative design changes that would restore compliance, with the calculation that supports each.
Read the worked answer
Option 1 - split the trunking into three groups of two circuits. Grouping factor for two circuits bunched is 0.80. With 2.5 mm2: Iz = 24 x 0.87 x 0.80 = 16.7 A. Reduce the MCB to 16 A so that Ib (15 A) <= In (16 A) <= Iz (16.7 A). Compliance is restored with the original cable and breaker size, at the cost of an extra trunking or a segregation barrier and revised drawings. The trade-off is real: three smaller trunkings take more ceiling space and more supports.
Option 2 - reduce the ambient temperature. If the void can be ventilated to 30 degrees C, Ca becomes 1.00. Even then, 2.5 mm2 with six circuits gives 24 x 1.00 x 0.57 = 13.7 A, still below 16 A, so ambient alone is not enough; it must be combined with a grouping reduction. This is worth stating explicitly, because it demonstrates that grouping dominates.
Reject: simply fitting a 16 A MCB to 2.5 mm2 with six circuits. That gives 16 A > 13.7 A and still overloads the cable. Also reject reducing the design current on paper, since the 3.5 kW load is a real load. Whichever option is chosen, the revised Iz, factors and Vd must be recorded on the schedule of test results and the trunking fill recalculated against the space factor after the change.