In short. When the society’s own set goes for repair or overhaul, residents expect lifts, water and flat backup to carry on as normal. A rental set sized to the actual evening peak, connected through the existing changeover, and checked for lift regeneration, keeps that promise. Plan it before the set is opened, and tell residents what to expect.
Who this page is for: For apartment associations and facility managers: keep residents’ backup unchanged while the community set is out of service.
Matching the rental to the existing set
| Check | Why |
|---|---|
| Actual evening peak load | Rental can often be smaller than the society’s set if loads are known |
| Voltage and phase rotation | Wrong rotation runs pumps and lifts backwards |
| Changeover type | Manual or AMF — the rental must suit it |
| Lift drives | Regeneration can trip a lightly loaded rental set |
| Standing area | Space, access, exhaust away from flats |
Worked example (hypothetical)
Hypothetical figures. A society’s 250 kVA set will be away for three weeks. Its logs show an evening peak of about 120 kW: 120 ÷ 0.8 ÷ 0.8 ≈ 188 kVA. A 200 kVA rental connected through the existing changeover carries the same backup.
Notice for residents (template)
Requirement form
| Item | Your site |
|---|---|
| Site address and access (road width, gate, crane access) | |
| Start date and expected duration | |
| Loads to be run, with kW and largest motor | |
| Voltage and phase required | |
| Connection point (existing panel, temporary board) | |
| Running pattern (standby only / daily hours / continuous) | |
| Fuel arrangement preferred | |
| Operator needed? (Y/N) | |
| Noise or placement constraints |
Lift regeneration, explained simply
When a fully loaded lift car comes down, or an empty car goes up, the lift motor acts like a generator and its drive may push energy back into the supply. On the grid, that energy is absorbed easily. A generator carrying only a light load at night cannot absorb it, so its speed or voltage rises and it may trip. Most lift drives can be fitted with braking resistors or set for generator operation; ask your lift maintenance company to confirm before the rental set takes over.
Overhaul timeline for the association
- Collect two weeks of load readings from the existing set.
- Agree rental size, dates and connection with the rental company.
- Inform residents with the notice template.
- Rental set delivered, connected and tested on load.
- Society’s set taken away for overhaul.
- Overhauled set returned, run-in and tested on load.
- Rental set disconnected and removed.
Who does what
| Task | Usually |
|---|---|
| Load readings and resident notice | Association / facility manager |
| Rental set, connection and testing | Rental company with the society’s electrician |
| Lift drive check | Lift maintenance company |
| Fuel and daily log | As agreed in the rental scope |
Temporary connection and changeover safety
The rental set is temporary; the electrical risks are not. Connecting it means working on the building’s live distribution, and the safest arrangements are the ones that change the existing system least.
Connection principles
- Use the existing changeover where possible. Landing the rental cable on the generator side of the society’s existing changeover keeps its mechanical or electrical interlock in service, so the rental set can never be connected to the grid supply.
- If a temporary changeover is unavoidable, it must be rated for the full current, interlocked, and switch the neutral where the earthing design requires it. A plain isolator without an interlock is not acceptable.
- Earthing. The rental set’s frame and neutral earthing must be made to the site’s earthing arrangement or to dedicated pits, and tested before the set takes load. A temporary set with a poor earth puts every metal enclosure in the building at risk during a fault.
- Isolation. The society’s own set must be isolated, locked off and its outgoing cable proved dead before it is disconnected. The work should be under a written permit, carried out by a licensed electrical contractor.
- No back-feeding. The rental set must never be connected through a flat’s socket, a sub-board or any point downstream of protection designed for incoming supply.
Cable sizing check
A rental set often has to stand further from the changeover panel than the society’s own set did — on an entry road, in a visitors’ car park, or beside a boundary wall away from flats. The extra length matters. Check three things before the cable is ordered:
Voltage drop (V) ≈ cable’s mV per A per metre × current × route length (m) ÷ 1000
Target: drop within about 3 % of 415 V (12.5 V) at full load for most sites; check the site’s design limit
Protect the temporary route from vehicles and foot traffic with ramps or covers, keep it clear of water logging, and label it at both ends. Our cable sizing and voltage drop screening calculator helps with the first estimate.
Connection-day handover checklist
Use this on the day the rental set takes over, and again in reverse when the society’s own set returns. Every item should be signed by the person responsible.
- Society’s set isolated, locked off and cable proved dead; permit issued
- Rental cable route protected, labelled and terminated with correct lugs and torque
- Rental set earthing connected and earth resistance tested; result recorded
- Phase sequence checked with a phase-sequence meter before any motor load is connected
- Voltage and frequency checked at the changeover panel, not only at the set
- Changeover interlock operated and proved: rental set cannot close onto grid supply
- Lift company has confirmed drives are set for generator supply; one loaded lift run down and one empty lift run up on the rental set
- Pumps started one at a time; rotation and pressure confirmed
- Rental set run on real load for at least an hour; voltage, current per phase and temperature logged
- Fuel level recorded at start; refuelling responsibility and log agreed
- Emergency stop location shown to the security staff; contact numbers posted at the set
- Security staff briefed: they report alarms, they do not operate switches or open panels
Timeline risks and how to absorb them
| Risk | What it does to the plan | How to absorb it |
|---|---|---|
| Overhaul finds more wear than expected | Parts must be ordered; weeks may be added | Agree from the outset that the rental can be extended on short notice; ask for the overhaul scope and parts list after strip-down |
| Returned set needs running-in | Set is back but not ready for full load | Keep the rental connected until the overhauled set passes its load test |
| Monsoon or festival period | Delivery, crane access and labour availability reduced | Schedule delivery and connection away from these weeks where possible |
| Changeover found to be faulty when opened | Connection delayed; building without backup | Inspect the changeover panel before the society’s set is removed |
| Fuel not arranged for the first long outage | Rental set stops mid-outage | Fill the rental set’s tank at handover; fix who orders the next fill |
A second worked example: the longer cable run
Illustrative case. A larger community’s 500 kVA set is going for overhaul. Its logs show an evening peak of about 250 kW. The DG room cannot take a second set, so the rental must stand on the entry road, with a cable route of about 75 m to the changeover panel.
| Step | Working | Result |
|---|---|---|
| Rental size | 250 ÷ 0.8 PF ÷ 0.8 loading = 390.6 kVA | 400 kVA set |
| Full-load current | 400 × 1000 ÷ (1.732 × 415) | about 557 A |
| Cable arrangement considered | Two parallel runs of aluminium armoured cable, about 278 A each | Each run’s derated rating must exceed 278 A |
| Voltage drop, illustrative cable value 0.26 mV/A/m | 0.26 × 278 × 75 ÷ 1000 | about 5.4 V, 1.3 % of 415 V |
| Same check with one run only | One run would carry 557 A — beyond its rating, and double the drop | Not acceptable |
The voltage drop is comfortable with two runs; the deciding check is the current rating after derating for cables lying side by side in the sun. The association learns this a week before delivery, not on the day, and the cable is ordered with the set. Temporary cables can be arranged through cable rental.
Common mistakes
- Booking the overhaul before the stand-in set and its connection are confirmed.
- Hiring a set sized only for lights, then finding it cannot start the lifts.
- Leaving the connection point and cable route until the day of delivery.
- Not telling residents about the planned switchover times.
What to send us
Send the existing set’s kVA, the overhaul dates, lift and pump details, where a rental set can stand and the cable distance to the changeover panel, and who will supply fuel during the hire. Use the generator rental enquiry, or WhatsApp the details and photographs.
Questions hirers ask
Can a rental generator be smaller than our society’s set?
Often, yes, if load records show the actual peak is well below the existing set’s rating. Size the rental for the real peak with margin.
Will our lifts work on a rental generator?
Yes, if the rental is sized correctly and the lift drives’ regeneration is accounted for.
How long does it take to connect a rental set to our changeover?
Usually a short planned job once the set is on site, provided the connection point and access are prepared.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.