How do you size an REBT elevator-room subpanel?

When a fault on one lift branch opens a shared upstream device, the panel can drop both lifts or room services; do not respond by upsizing that device. Identify what tripped and what lost power first, then check circuit separation, load data, cable sizing, and protective-device coordination in that order.

Read the tripped device and identify what went dark

Before resetting anything, record which protective device opened and which loads lost supply. That tells you whether to trace an individual circuit or work upstream toward a shared feeder.

Do not keep resetting a device that trips again. Determine whether the event was an overload, short circuit, earth leakage, or a normal operating transient from the device indication and the equipment diagnostics. Then follow the matching branch below.

Separate each lift before choosing a shared feeder

Do not size one common lift-room feeder from the sum of two nominal motor powers and treat separate downstream breakers as proof that a fault will stay on one lift. A downstream fault can also trip an upstream breaker unless the device pair has demonstrated coordination at the prospective fault current.

• Draw a separate protected supply path for each lift from the common-services distribution board. The design guidance for this arrangement calls for each lift to have its own automatic protective device and differential protection.
• Keep the two lift supplies independent so a fault on one does not unnecessarily stop both. Have the lift manufacturer or maintainer confirm where the lift controller’s internal protection starts and ends.
• Do not add a separate lift-control subpanel merely to duplicate protection that the supplied control cabinet already contains. Controller arrangements vary; check the actual equipment documents.
• Design the pump-room circuits around the pump motors and control equipment actually installed. Do not copy the lift feeder rating or assume that the pump room has the same branch arrangement.

A community common-services board can supply building services such as lifts and pumps, but ownership, metering, and any dedicated supply rules need a project-specific check. A separate garage supply or meter is a different design decision; do not combine it with the lift-feeder calculation without confirming the building’s metering and applicable requirements.

For the REBT review, verify the current applicable requirements rather than selecting a breaker from an informal “minimum subpanel” figure. The referenced design material points to for individual derivations and ITC-BT-47 for motor circuits; confirm the applicable edition, circuit classification, and any lift-specific rules for the actual project.

Trace lighting and service sockets to their actual source

Separate lift-machine power from lighting that must remain available when the machine supply is isolated. The lift-rule passage cited in the design material says electrical lighting for the car, shaft, and machine room must be independent of the machine supply. It describes either a separate line or a takeoff upstream of the lift’s specified main switch or switches in clause 13.4. Verify the current lift requirements and the approved lift wiring diagram before choosing between those arrangements.

• Trace car, shaft, and machine-room lighting separately from the lift motor feeder. A lighting fault should not remove lift machine power, and opening a lift machine disconnect should not extinguish lighting that must remain available.
• List emergency luminaires and their battery charging load in the load schedule if they are part of the installation. Do not use the battery supply as a substitute for identifying the normal lighting circuit.
• Protect room lighting and socket outlets as separate circuits where the design and applicable rules call for it. The design examples mention a 10 A lighting circuit and a 16 A socket circuit, but those are proposed ratings, not a universal minimum subpanel specification.
• Check the lift-specific requirements for shaft-light switching from the machine room and pit, and for a pit socket, when those spaces and equipment apply.

The source material gives two different suggestions for a machine-room service socket: one says the socket must remain available independently of lift-force and lighting cutoffs, while another places it downstream of the lighting differential with its own breaker. Those arrangements are not automatically equivalent. Show the required isolation boundaries on the one-line and confirm the socket supply with the lift maintainer and the applicable rules.

Read motor and controller data before calculating current

Take the supply voltage, motor nameplate current, rated input or output power, power factor, starting method, and drive details from the installed lift or pump documentation. Use the actual number of motors and simultaneous operating loads. A planning figure from a generic lift category is not a substitute for the selected machine’s data.

The worked example in the design discussion assumes two lifts rated at 4.5 kW each, a 400 V three-phase supply, and a power factor of 0.9. It also uses the label “ITA-1,” while the discussion disputes whether that category appears in the cited ITC-BT-10 material. Treat the example as arithmetic only: verify the applicable load schedule and the selected lift’s data.

• If 4.5 kW is electrical input power per lift, calculate the assumed line current as I = P / (√3 × VLL × PF). At 4,500 W, 400 V, and 0.9, the result is about 7.2 A per lift; two equal lifts sum to about 14.4 A before any applicable motor-circuit design factor.
• If 4.5 kW is mechanical shaft output, include motor efficiency: I = Pout / (√3 × VLL × PF × η). Better still, use the manufacturer’s rated current.
• For single-phase lighting or socket loads, use the single-phase load data and voltage. Do not put those loads into a three-phase formula as though they were balanced across all phases.
• For pumps, read each motor’s nameplate current and starter or drive information. Include the intended simultaneous operation and control sequence in the demand calculation.

Do not use a motor’s running current alone to decide whether a breaker will ride through starting. Check the manufacturer’s starting characteristics and the selected device’s trip curve, while retaining protection for the conductors and equipment.

Recalculate the motor feeder without mixing design factors

The design material quotes a multiple-motor rule from ITC-BT-47: size conductors feeding several motors for at least 125% of the full-load current of the largest motor plus the full-load current of the other motors. Confirm the rule’s applicability and wording in the current governing text.

For two equal motors at the example current of 7.2 A, that quoted calculation is Icalc = 1.25 × 7.2 A + 7.2 A ≈ 16.2 A. This is not the same as multiplying the sum of both currents by 1.3. The discussion also shows a proposed 30% increase applied to both lift loads and obtains about 18.8 A. Do not combine those approaches or present either as a universal breaker rating; apply the factor required for the actual circuit and use the actual motor currents.

For a three-phase load with stated line current, calculate real power as P = √3 × VLL × Iline × PF. On the example assumptions, a 16 A device corresponds to about 10.0 kW and a 20 A device to about 12.5 kW at 400 V and power factor 0.9. Those figures are power conversions, not proof that a chosen cable can carry that current or that the supply has adequate capacity.

Do not assign the proposed 20 A or 25 A head breaker from a rough power allowance such as “two lifts plus spare capacity.” Include the other connected loads, applicable motor factors, and diversity only where the project rules and equipment operation justify it. Then verify the feeder rating, voltage drop, fault protection, and device coordination independently.

Size every feeder from its route and installation conditions

A conductor cross-section alone does not establish its allowable current. The discussion cites 6 mm² as a minimum for an individual derivation under , but also notes that 6 mm² may be insufficient for a particular route or installation. Verify that minimum applies to the circuit being designed; do not treat it as a universal rating for lift or pump feeders.

1. Identify the full cable route and installation method, conductor type, grouping, ambient conditions, and terminal limits. Use the relevant cable data to calculate current-carrying capacity.
2. Check voltage drop using the actual one-way route length, conductor properties, load current, and supply arrangement. A lift on an upper floor can have a materially longer route than a nearby pump room; floor count alone is not a voltage-drop calculation.
3. Check short-circuit thermal withstand and confirm that the protective device can interrupt the prospective fault current at its installation point.
4. Coordinate breaker rating with conductor capacity and load characteristics. Do not infer that a 6 mm² cable is suitable for 25 A from an approximate rule of thumb; the installation method and cable data decide.
5. For each motor circuit, verify operating current and starting behavior against the selected cable and protection. For the feeder serving multiple motors, apply the applicable multiple-motor calculation.

The design conversation mentions 10–16 mm² as common for some individual derivations, but it does not make either size correct for this building. Select the size from calculated current capacity, voltage drop, fault withstand, and applicable minimums—not from what is “usual.”

Check short-circuit selectivity and board isolation

Do not assume that a 10 A branch breaker below a 20 A feeder breaker will isolate every branch short circuit. Devices in cascade can both trip when the fault current falls in the overlapping part of their characteristics. A nuisance trip on the upstream device can remove both lifts or all services connected to that feeder.

1. Find the prospective short-circuit current at the feeder origin and at each downstream board. A long feeder can reduce fault current at its far end, so check both locations.
2. Compare the exact device models, ratings, trip curves, and manufacturer coordination or selectivity data at those fault currents. A larger rating difference or a different C or D curve alone does not prove selectivity.
3. Check each device’s breaking capacity against the prospective short-circuit current where it is installed. Do not copy a breaking-capacity figure from a generic design example.
4. Show a means to isolate each board and each lift supply on the one-line diagram, and label the device so a technician can identify the correct circuit quickly.

The design reference points to a board-level head switch and a one-line diagram under ITC-BT-04. Confirm the current requirement and choose a device that performs the intended function. An isolator, circuit breaker, and residual-current device do not provide identical protection; a residual-current device alone does not provide overcurrent protection.

Match differential protection to each lift drive

Choose residual-current protection from the actual drive and lift manufacturer’s instructions, the earthing arrangement, and the applicable protection requirements. A variable-frequency drive can produce leakage current and waveforms that cause unsuitable differentials to trip; a more sensitive device is not automatically a better match.

• Keep differential protection independent for each lift branch where the design requires one lift fault not to interrupt the other.
• For a lift motor controlled by a drive, check the manufacturer’s specified RCD type and immunity requirements. The design discussion recommends a super-immunized device to reduce nuisance trips and mentions 300 mA for lift-force circuits, but neither value should be copied without confirming the equipment and applicable requirements.
• For room lighting and socket circuits, verify the required sensitivity and device ratings separately. The example of a 25 A, 30 mA differential is a design proposal, not a rule for every installation.
• Check the controller’s supplied protection before adding external devices. Some control cabinets include protection functions; the exact arrangement depends on the lift model.

If a differential trips, record which one opened and whether the drive was operating. Check the drive’s documented leakage-current behavior, insulation condition, wiring, and protective-device compatibility before replacing the device or increasing its trip threshold.

Draw the resolving layout and verify it before energizing

Close the design with a coordinated one-line drawing, verified load schedule, cable calculations, and protection schedule. The one-line should make it clear which source, breaker, conductor, differential, and disconnect serve each lift, the room services, and the pump equipment.

1. List the installed equipment and manufacturer data: each lift motor and controller, each pump motor and starter or drive, lighting, emergency-luminaire charging, and service outlets.
2. Draw separate lift feeders from the common-services distribution board, with separate lift protection. Draw the lighting and room-service supplies so their isolation relationship to each lift is explicit.
3. Calculate motor currents using manufacturer data. Apply the governing multiple-motor rule where it applies; document assumptions for power factor, efficiency, operating simultaneity, and any design factor.
4. Select feeder and branch conductors from current capacity, voltage drop, fault withstand, and applicable minimum cross-sections. Select protective devices from conductor limits, load behavior, fault current, and coordination data.
5. Have the lift maintainer confirm whether the control cabinet already contains relevant protection and approve the lift supply and isolation boundaries.
6. Before energizing, verify conductor identification and continuity, protective-earth connections, polarity and phase arrangement, protective-device ratings, and operation of each isolation device. Test each circuit separately and confirm that opening or tripping one lift branch does not remove the other lift’s supply or required lighting.
7. Record the as-built one-line and test results. If an upstream device trips during a downstream fault, stop and revise the coordination rather than repeatedly resetting or simply increasing the upstream rating.

Answer common lift-room subpanel questions