Electrical Supply for Temporary Air Packages

Reddy Compressors

A temporary electric compressor is ready for service only when its complete package can start, run and recover on the available electrical supply while delivering the required air. A spare feeder or an apparently adequate generator rating does not establish that result. Cable installation, starting behaviour, protection, earthing, treatment auxiliaries and the response to a power interruption must work together.

This guide helps plant engineers, electrical teams, rental coordinators and procurement managers define the electrical interface for temporary compressed-air packages. It develops the installation principles in the supplied Reddy Compressors material into a practical specification, calculation and acceptance framework. It covers planning rather than instructions for live electrical work. Electrical design, connection, testing and energisation require competent authorised personnel applying the equipment instructions and requirements applicable to the site. All numerical examples are hypothetical and are not equipment selections.

01 Define the air service that electricity must support

Begin with the required air duty: net delivered flow, operating pressure, quality, hours, peak events and permitted interruption. Electrical demand follows the actual compressor and treatment arrangement selected for this duty. A package operating at a higher discharge pressure or through a different loading profile may draw different power even though its motor nameplate remains unchanged.

Identify whether temporary air supports production, construction tools, a planned shutdown or essential instrument users. A short interruption acceptable for a workshop may be unacceptable for a control system. The electrical arrangement must therefore support the agreed process response, including safe shutdown where continuous operation cannot be assured.

Define the measurement boundary. State whether quoted power includes the compressor enclosure alone, external cooling, dryer, regeneration heater, drains, pumps and distribution accessories. Compare alternatives at the same useful-air boundary. A smaller compressor input figure can be misleading when substantial treatment power or purge air is supplied outside that figure.

02 Obtain the exact package electrical information

Request the selected package model, serial identification where available, electrical drawing, rated voltage and frequency, permissible supply variation, full-load input current and maximum input under the specified operating conditions. Obtain starting method, starting-current profile, acceleration requirements, permitted starts, control supply details and manufacturer protection requirements. An earlier machine of similar capacity is not an adequate substitute.

Separate motor shaft output from electrical input. A motor marked 110 kW normally identifies its rated mechanical output; it does not automatically identify the total electrical input of the compressor package. Conversely, if a supplier already provides measured or declared package input, do not divide it by motor efficiency again. That would count losses twice.

List every external load and its connection requirement. A dryer may need a separate supply, while a compressor enclosure may internally feed its fans and controller. Heater regeneration can create a different peak from normal compression. Confirm whether any auxiliary must remain energised before starting or after stopping, and how its loss is detected by the operating team.

03 Establish what the site can actually supply

Survey the proposed supply point with the site electrical authority. Record voltage, frequency, phase arrangement, earthing system, feeder capacity, protective devices, source impedance or available fault information, upstream loading and planned outages. Check the physical connection arrangement, access, isolation responsibility and route to the package.

Available capacity is the permissible additional loading under relevant site conditions, not the transformer rating minus a casual meter reading. Production peaks, temperature, existing commitments and upstream constraints may change the usable margin. Obtain representative demand evidence or a conservative approved operating scenario where records are incomplete.

Examine the weakest operating state. A feeder supported by utility power today may be transferred to a smaller generator tomorrow. The same temporary compressor can behave acceptably on one source and fail to accelerate or recover on another. Record the intended source for normal operation, maintenance, emergency operation and restoration, with any operating restrictions attached to each state.

04 Build a load schedule with clear boundaries

Prepare a schedule covering running duty and the events that govern starting or recovery. Avoid applying the same diversity factor to every load. Some auxiliaries follow compressor loading, some run continuously, and others cycle independently. A standby compressor may still carry heaters or control power and may start while another machine is stopping.

Use separate columns in the project schedule for declared data, measured data and assumptions. A provisional value remains visible until replaced by supplier or site evidence. Record the revision used for the design so that substituting another rental machine triggers a specific review of the affected electrical requirements.

05 Calculate running current without confusing kW and kVA

For a balanced three-phase supply, apparent power S in kVA is approximately √3 × line-to-line voltage V × line current I ÷ 1,000. Real input power P in kW equals S multiplied by true power factor. Therefore I = 1,000P ÷ (√3 × V × true power factor). With significant distortion, use suitable RMS measurements and the actual true power factor; displacement power factor alone does not capture all effects.

For illustration, a package takes 120 kW electrical input at 415 V with a true power factor of 0.90. Its apparent power is 133.3 kVA and the corresponding balanced line current is approximately 185.5 A. These values describe the assumed running condition. They do not establish cable size, breaker rating or starting capability.

If instead the only information is a 110 kW shaft load and an assumed motor efficiency of 95%, the estimated motor electrical input is 115.8 kW before separately supplied auxiliaries. The efficiency must match the relevant operating condition. Neither example authorises replacing the manufacturer's current data with a generic conversion factor.

For mixed loads, combine real and reactive components where that method is appropriate, or use a validated load model. Simply adding kW and dividing by a convenient assumed power factor can conceal important differences between motors, heaters and electronic drives.

06 Treat starting as a separate operating condition

A source that carries steady running demand may not support acceleration. Obtain starting current, duration, torque requirements and starting method for the actual compressor. Direct-on-line, star-delta, soft-starter and variable-speed-drive arrangements have different electrical and mechanical behaviour. The compressor's unloading and restart provisions also influence the event.

Suppose the supplier specifies 180 A running current and a starting profile reaching 900 A for the proposed operating condition. At 415 V, 900 A corresponds to approximately 647 kVA of instantaneous apparent demand if voltage remains at that value. It does not mean that a 647 kVA generator is automatically required or sufficient: source response, voltage dip, duration, existing loads and acceleration must be evaluated together.

Lower starting current is not useful if available torque becomes inadequate and acceleration takes too long. Repeated unsuccessful starts can create thermal stress and extend the air outage. Specify permitted restart intervals and investigation responsibilities rather than expecting operators to keep resetting a trip.

Coordinate the start with other site events. Simultaneous compressor, dryer and production-motor starts can create a much larger disturbance than any individual event. A defined start order can reduce the peak, provided its delay remains compatible with the required air service.

07 Assess generator compatibility beyond its headline rating

For generator supply, obtain the operating rating appropriate to the duration and load profile, ambient and altitude corrections, alternator characteristics, transient response and manufacturer sizing assessment. Check both engine real-power capability and alternator electrical capability. A nominal kVA figure alone cannot resolve motor starting, harmonic loading or voltage recovery.

Provide the generator supplier with the actual compressor and auxiliary load schedule, starting sequence and acceptable voltage and frequency performance at the equipment. Include existing loads already on the generator and loads expected to reconnect after an outage. A generic percentage allowance cannot replace this information.

Specify the outcome at the compressor terminals and receiving air system. The engine may recover speed while a dryer remains tripped, or the compressor may restart while process pressure falls below its minimum. Electrical recovery and useful-air recovery are related but different acceptance points.

If the proposed generator cannot meet the duty, options may include a different generator, a revised approved starting arrangement, staged loads or a different rental-air configuration. Evaluate the revised arrangement as a whole. Do not bypass protection or change compressor controls merely to make an unsuitable supply appear workable.

08 Size cables for their actual installation

Cable selection must address current-carrying capacity, installation corrections, voltage drop, short-circuit thermal withstand, protective-conductor requirements and mechanical suitability. Relevant conditions include ambient temperature, grouping, ventilation, containment, exposure, route length, terminations and whether the cable is flexible temporary equipment or part of a fixed installation.

For a simple screening example, a candidate cable has a tabulated capacity of 250 A under its stated reference conditions. If justified correction factors are 0.85 for temperature and 0.80 for grouping, corrected capacity becomes 170 A. That is below the 185.5 A example running current. The nominal 250 A entry therefore does not demonstrate adequacy for this installation.

Actual correction factors and their permitted combination must come from the applicable cable design method. Do not mix unrelated tables or assume all temporary cables can use the same factors. Coordinate cable capacity with the protective arrangement, allowing for the particular requirements of motor circuits and equipment instructions.

Do not infer allowable current from conductor area alone. Insulation, conductor material, construction, connectors and installation can alter the result. Parallel cable arrangements need deliberate design for current sharing, routing, terminations and protection; adding an unmatched extra cable is not a routine capacity fix.

09 Evaluate voltage at the equipment terminals

The compressor responds to its terminal voltage, not the reading at a distant switchboard. Include the upstream system and temporary feeder in the assessment. Calculate steady operation and evaluate starting separately using the appropriate current, power factor and source response. Agree acceptable limits using the equipment requirements and applicable installation criteria.

For a balanced sinusoidal three-phase screening calculation, cable voltage drop is approximately ΔV = √3 × I × L × (R cosφ + X sinφ). Here I is amperes, L is one-way route length in kilometres, and R and X are conductor resistance and reactance in ohms per kilometre. Use resistance at the relevant temperature and verified cable data. This simplified expression is not a complete harmonic or transient model.

Assume 185 A, 80 m, R = 0.25 Ω/km, X = 0.08 Ω/km and cosφ = 0.90. The calculated cable drop is approximately 6.66 V, or 1.60% of 415 V. If upstream voltage under that running condition is already 405 V, estimated equipment voltage becomes about 398.3 V. Assess that actual terminal condition against the package requirements; a small cable percentage alone is insufficient.

For a hypothetical 900 A start at cosφ = 0.35, the same cable-only approximation gives about 20.3 V drop. The upstream source also changes during the event. This result is neither the complete starting dip nor proof that the motor will accelerate. It identifies why the start deserves its own assessment.

10 Coordinate protection with the source and package

Define which devices provide overload protection, short-circuit protection, earth-fault protection and isolation. Confirm their ratings and settings against the source, conductors and package documentation. A device large enough to avoid starting trips may fail to provide the intended protection if selected without coordination.

Check maximum prospective fault conditions against equipment breaking and withstand ratings. Also assess the minimum fault conditions needed for the protective arrangement to operate as intended. Generator-fed systems can have different fault behaviour from a utility-fed system; settings acceptable for one source may not provide the same result on the other.

Evaluate selectivity where loss of the upstream board would interrupt other essential services. State whether full or partial selectivity is required and what the supporting study establishes. Do not claim that the compressor feeder is independent merely because it has a separate breaker.

Document authorised settings and retain a record of changes. Repeated nuisance trips require investigation of the fault record, starting behaviour, connections, source and load. Increasing a setting without that assessment can remove an important indication of an underlying problem.

11 Define earthing and residual current protection together

The temporary installation must have an intentional earthing and bonding arrangement compatible with the source and connected equipment. Record how protective conductors, exposed conductive parts, distribution equipment and generator arrangements are addressed. A ground rod on its own does not demonstrate an effective protective system.

Where residual current protection is required, its type, sensitivity, time characteristics and compatibility with electronic drives must be selected for the actual system. Drives and filters can influence leakage currents and protective-device behaviour. Do not apply one generic residual-current device to every compressor package or remove a device to cure unexplained trips.

Confirm neutral requirements for controls and single-phase auxiliaries. A three-phase motor connection does not prove that the complete station can operate without a neutral. Neutral and earthing arrangements must remain correct during any source transfer; they are not details to improvise during connection.

The electrical authority should define inspection and test evidence for the completed installation. This may include protective-conductor continuity and verification that the selected protection achieves its intended function. The appropriate tests depend on the installed arrangement and must be performed safely by qualified personnel.

12 Review variable speed drives and power quality

A variable-speed compressor can have useful operating and starting characteristics, but its supply requirements remain equipment-specific. Obtain maximum input current, harmonic information, protective-device guidance, EMC requirements and allowable source conditions. Do not assume that a drive with low acceleration current is automatically compatible with every generator.

Distinguish the incoming supply cable from the drive-to-motor cable inside or associated with the package. Motor-side cable length, shielding, filters and connection requirements are manufacturer matters. A temporary rental connection should not involve casually extending or altering the internal drive circuit.

Check whether existing power-factor-correction equipment and the temporary load interact adversely. Capacitors, harmonics and switching events need an appropriate assessment; adding capacitors near a drive is not a universal cure for poor power factor. Where power quality is suspected, measure the relevant parameters instead of drawing conclusions from one display value.

Record voltage imbalance, phase currents, frequency and significant disturbances under representative duty. Define the instruments and limits appropriate to the package. A balanced average voltage can conceal unequal phase conditions, while a steady handheld reading may miss a short dip that resets the controller.

13 Protect the temporary distribution physically

Route cables to avoid crushing, sharp edges, hot surfaces, standing water and uncontrolled vehicle crossings. Provide suitable support, protection and segregation for the site environment. Account for movement of equipment, forklift routes and access needed for inspection or emergency isolation.

Select enclosures, connectors and entries suitable for exposure, contamination and handling. Protect against tension at terminals and unauthorised access. The same attention is needed at intermediate distribution boards as at the compressor itself; a protected package does not make an exposed connection acceptable.

Cable drums and bundled surplus length can have different thermal conditions from freely installed cable. Follow the manufacturer's permitted use and rating for the actual arrangement. Do not apply a freely laid cable rating to a tightly coiled run without evidence.

Mark the source, destination and isolation point so the next shift can identify the supply. Keep electrical routes coordinated with air hoses, condensate drainage and access paths. Any relocation that changes length, exposure, grouping or connection should prompt a focused reassessment before re-energisation.

14 Plan isolation and commissioning responsibilities

Assign the site electrical authority, installation contractor, equipment supplier and operating team their specific responsibilities. Identify who approves the connection design, makes the connection, verifies the installation, authorises energisation and accepts the operating results. Equipment delivery does not transfer every electrical responsibility to the rental supplier.

The isolation plan must address every relevant energy source and possible re-energisation path. Compressor controls, remote starts, alternative electrical supplies, drive stored energy and pneumatic pressure can remain relevant after an ordinary stop command. Manufacturer discharge times and the site's safe-isolation procedure govern work on the equipment.

An emergency stop is an operating protective function, not proof of electrical isolation. Similarly, an isolated feeder does not by itself establish that all mechanical or pneumatic hazards are controlled. HSE guidance supports planning work around all sources of energy; its UK guidance is a reference, not a declaration of compliance for an Indian installation.

Schedule the connection so that authorised personnel, drawings, test instruments and operating representatives are available together. An installation completed hurriedly at the end of a shift often leaves unresolved responsibilities exactly when first starting requires coordinated observation.

15 Verify operation under representative demand

Acceptance should progress from documented installation checks to controlled energisation, starting, loaded operation and the agreed recovery tests. The exact procedure belongs to the competent commissioning team and equipment instructions. Do not perform intrusive tests or deliberately provoke unsafe faults to demonstrate a point.

Observe the loads that govern the design. Testing at low air demand may not demonstrate full-load current, regeneration overlap or maximum site voltage drop. If production conditions cannot be reproduced, state the remaining limitation and define the controlled operating envelope established by the test.

16 Coordinate restart with compressed air continuity

After power returns, the station may need control recovery, permissive checks, dryer stabilisation and compressor acceleration before useful air is restored. Measure or obtain credible evidence for that sequence. A generator transfer time alone does not describe the air interruption.

Consider an idealised 2 m³ receiver allowed to fall from 8 bara to 7 bara, with reference pressure 1 bara and equal-temperature assumptions. The corresponding usable reference-air volume is approximately 2 × (8 − 7) ÷ 1 = 2 m³. At an essential demand of 6 reference m³/min and no incoming air, theoretical reserve is 20 seconds.

If useful-air recovery takes 45 seconds, that receiver example cannot bridge the interruption. Even the 20-second estimate omits dynamic temperature effects, distribution losses and other practical constraints. Address the mismatch through a validated supply or process strategy; do not label the arrangement uninterrupted merely because backup electricity is installed.

Staged restart can reduce electrical demand while extending air recovery time. Evaluate both effects together. The authorised sequence must also avoid automatically restarting machinery into an unsafe plant condition after personnel have begun fault investigation.

17 Detect deterioration during the hire period

Establish an operating baseline after commissioning, including source state, load, current, voltage, alarms and air delivery. Compare later readings under similar conditions. An unexplained current increase may reflect operating pressure, mechanical loading, electrical conditions or measurement differences; it is not a diagnosis by itself.

Inspect the temporary installation at intervals appropriate to exposure and use, and after relocation, damage, severe weather or source changes. Keep operators focused on observable signs such as damaged cables, enclosure ingress, unusual noise, repeated trips or loss of cooling. Suspected unsafe conditions require withdrawal and electrical assessment, not repeated operation to see whether they disappear.

Provide a fault record linking electrical and air-system events by time. A voltage disturbance, drive alarm, dryer restart and pressure dip recorded separately can look unrelated. Synchronised records make investigation more useful and reduce unnecessary replacement of a compressor that is responding to a supply problem.

18 Compare electrical costs on the actual duty profile

Estimate energy from input power over time: E = Σ(P × hours). For illustration, 120 kW for 10 hours plus 35 kW for 2 hours uses 1,270 kWh. At an assumed ₹9/kWh, the energy component is ₹11,430 for that day. This is an example, not a tariff quote or total rental cost.

Add applicable demand charges, installation, distribution hire, testing, generator costs where relevant and removal. Distinguish fixed costs from operating costs. A cheaper electrical connection that imposes restrictive starting windows may create production costs that exceed its apparent saving.

Suppose a verified alternative reduces average input by 8 kW for 12 comparable hours each day. At the same assumed tariff, gross energy benefit is ₹864/day. With ₹500/day additional hire, the remaining benefit is ₹364/day before other differences. An extra ₹20,000 installation cost would then take approximately 55 operating days to recover. This conclusion only holds if air service, hours and measured power differences are comparable.

Exclude speculative savings and avoided downtime from project conclusions unless the supporting evidence and valuation are stated. Separate estimates used for planning from results established after operation.

19 Prepare a complete electrical enquiry

Send the air duty, intended package, operating hours, site location, proposed source and connection date. Include the supply survey, single-line information, existing loading, route length, environment, permissible interruptions and planned source transfers. Identify whether the quote must include distribution boards, cables, installation, testing, generator supply or only equipment terminals.

Ask suppliers to state exclusions and assumptions explicitly. Confirm responsibility for permits, access, shutdown coordination, electrical labour, protective settings, commissioning records and removal. A clear boundary prevents the compressor arriving before the site has a usable connection.

Reddy Compressors enquiries can be framed around the required compressed-air service and its electrical interface, with Secunderabad and Hyderabad as the operating base and Telangana and Andhra Pradesh as priority regions. Wider project support and specialist electrical arrangements remain subject to scope, site conditions and confirmed resources. No generic page should promise immediate availability or a guaranteed restoration time.

20 Frequently asked questions

Can an electric rental compressor use any spare feeder

No. The feeder must be assessed for the package current, starting behaviour, protective arrangement, fault conditions, earthing and cable installation. Upstream capacity and other connected loads also matter. A spare outgoing way confirms physical availability, not electrical suitability.

Is the motor kW enough to choose a generator

No. Motor shaft output does not capture package input, treatment auxiliaries, starting demand or generator transient performance. Supply the complete load schedule and operating sequence to the generator supplier for an equipment-specific assessment.

Does a soft starter remove the need for starting calculations

No. It changes the start, but achievable torque, acceleration time, current limits and source response still need evaluation. The compressor manufacturer's approved configuration and actual operating conditions govern the result.

Why can a compressor run on utility power but trip on a generator

The sources may differ in voltage and frequency response, impedance, fault behaviour and compatibility with electronic loads. Existing generator loads also affect the event. Investigate recorded conditions and alarms before attributing the problem to compressor capacity or changing protection settings.

Can a cable be selected from its ampere rating alone

No. The rating must match the installation conditions, and the design must also address voltage drop, fault withstand, protection, terminations and mechanical suitability. Route changes or grouping can invalidate an earlier selection.

Does backup electricity guarantee continuous compressed air

No. Useful air resumes only after the complete package and treatment system recover. Compare that interval with validated air reserve and the process interruption limit. Where they do not match, revise the continuity or safe-shutdown strategy.