Load Growth in a Tailoring Unit Supply
A tailoring unit in a rented building takes a single-phase 230 V supply through a 25 m run of PVC conduit from the meter board to a small distribution board. Connected load: six sewing machines at 0.37 kW each, two 1 kW steam irons (both may be used at once), eight 18 W LED fittings and three 75 W fans. The existing circuit is 2.5 mm² copper with a 20 A MCB and no RCCB. The owner now wants a 3 kW water heater added for the workers' washroom. The sanctioned load on the bill is 5 kW. The conduit already carries four other circuits and is filled to about 45 % of its cross-section.
1. Calculate the full-load and diversified design currents, then decide whether the existing 2.5 mm²/20 A circuit is adequate.
Read the worked answer
Full-load current: (6 × 0.37 + 2 × 1.0 + 8 × 0.018 + 3 × 0.075) kW = 2.22 + 2.0 + 0.144 + 0.225 = 4.589 kW; at 230 V, I = 4589/230 = 20.0 A.
Diversified current, allowing 60 % for the machines (not all run at once), 75 % for the irons and 100 % for lighting/fans: (2.22 × 0.6) + (2.0 × 0.75) + 0.369 = 1.332 + 1.5 + 0.369 = 3.201 kW → 13.9 A.
Check the existing circuit:
- 2.5 mm² copper in conduit (two loaded conductors): about 18–19 A at 30 °C; at 40 °C apply 0.87 → about 16 A. That is below the 20 A full-load current.
- Voltage drop for 2.5 mm² is about 18 mV/A/m: 20 × 25 × 18 mV = 9.0 V = 3.9 % of 230 V, above the 3 % sub-circuit guide.
- In = 20 A is also greater than the derated Iz ≈ 16 A - a direct violation of the In ≤ Iz rule.
Conclusion: replace the run with 6 mm² copper (Iz in conduit ≈ 32 × 0.87 ≈ 27.8 A) and a 25 A MCB. Voltage drop at 20 A: 20 × 25 × 7.3 mV = 3.65 V = 1.6 %, acceptable. Use a 6 mm² earth continuity conductor.
2. Can the 3 kW heater be added to the same circuit? Evaluate the alternatives.
Read the worked answer
A 3 kW heater draws 3000/230 = 13.0 A. Added to the full load, 20.0 + 13.0 = 33.0 A; diversified total about 26.9 A. A 6 mm² cable derated to 27.8 A leaves no margin, and voltage drop becomes 33 × 25 × 7.3 mV = 6.0 V = 2.6 %, still under 3 % but tight.
Two bigger problems:
- Sanctioned load is 5 kW, while connected load becomes 7.59 kW. The meter, service cable and main switch are the utility's responsibility; running above sanction risks penalty and nuisance tripping.
- The conduit is already at 45 % fill, above the 40 % limit in IS 732, so no more circuits can be pulled in.
Options: A. Separate 4 mm² circuit for the heater from the DB with a 16 A MCB and 30 mA RCCB - best for selectivity; a heater trip will not shut down the tailoring unit. B. Upgrade the existing run to 10 mm² and a 32 A MCB - more copper and a better voltage-drop margin, but the conduit-fill and sanctioned-load issues remain. C. Run the heater only when the irons are off - an operational fix, not an engineering one, and unreliable.
Recommend A, plus an application to enhance sanctioned load to 8 kW and a new 25 mm conduit for the heater circuit.
3. List the wiring-practice and protection deficiencies you would correct before issuing a completion certificate.
Read the worked answer
- Protection: a 30 mA RCCB for all socket outlets and the wet-area heater circuit; the MCB rating must satisfy Ib ≤ In ≤ Iz. The present 20 A MCB on a derated 2.5 mm² cable fails this.
- Earthing: verify earth continuity from every socket and machine frame to the earth bar; earth conductor sized to the phase conductor (6 mm² for 6 mm² phase); measure earth resistance and record it.
- Conduit: fill ≤ 40 %, no joints inside conduit, adequate saddles, no sharp bends, separate power and signal routes; the existing 45 % fill must be relieved.
- DB and accessories: DB at an accessible height (about 1.5 m), each circuit identified, no daisy-chained sockets, correct polarity (phase on the right of the socket), separate neutral per circuit, no borrowed neutrals.
- Testing: insulation resistance ≥ 1 MΩ at 500 V (record the actual value), polarity, RCCB trip test, earth continuity, load test.
- Documentation: test sheet and single-line diagram; the owner should be told in writing that adding the heater exceeds the 5 kW sanction until enhancement is approved.
The inspection must be done before energizing the new heater, not after the first shock.