A Proper Calculation Methodology for anProcess Equipment Loading Scheduleis Essential for a Successful Engineered Project

There are no standards governing Equipment Load Schedules and therefore aloading calculation methodology that is based on generally accepted industry practice, rather then nation codes or standards is needed. This blog captures the essential components of a functioning electrical loading schedule. You can calculate heater load with 3 phase heater load calculation tool.

Basically, the electrical load schedule is an estimate of the instantaneous electrical loads operating in a facility, in terms of active, reactive and apparent power (measured in kW, kVar and kVA respectively). The load schedule is usually categorised by switchboards (MCCs) or occasionally by sub-facilities. The following methodology assumes that the load schedule is being created for the first time and is also biased towards industrial plants. The basic steps for creating a load schedule are:

Step A:

Collect a list of the expected electrical loads in the facility. The list is gathered from Electrical Single line Diagrams, Mechanical Instrumentation and Piping Diagrams or Equipment Lists from clients

Step B:

For each load, collect the electrical parameters as follows:

Apparent power – VA – is measured in volt-amperes and it is the product of a circuit’s voltage and current.

Inductive Power – Q – is measured in volt-amperes reactive (VAR) and is the power stored in and discharged by the inductive motors, transformers or solenoids.

Reactive power required by inductive loads increases the amount of apparent power – measured in kilovolt amps (kVA) – in the distribution system. Increasing the reactive and apparent power causes the power factor – PF – to decrease.

Active (Real or True) Power is measured in watts (W) and is the power drawn by the electrical resistance of a system doing useful work.

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Consumable power is the expected power that will be drawn by the load. Most loads will not operate at its rated capacity, but rather at a lower point.

Power Factor is defined by IEEE and IEC as the ratio between the applied active (true) power – and the apparent power (va). Power factor is an important measurement in electrical AC systems because an overall power factor less than 1 indicates that the electricity supplier needs to provide more generating capacity than actually required.

A low power factor is the result of inductive loads such as transformers and electric motors. Unlike resistive loads creating heat by consuming kilowatts, inductive loads require a current flow to create magnetic fields to produce the desired workIf the power factor is close to 1 (purely resistive circuit) the supply system with transformers, cables, switchgear and UPS could be made considerably smaller

Any power factor less than 1 means that the circuits wiring must carry more current than what would be necessary with zero reactance in the circuit to deliver the same amount of (true) power to the resistive load.

Power factor of the load is necessary to determine the reactive components of the load schedule. Normally the load power factor at full load is used, but the power factor at the duty point can also be used for increased accuracy.

Power Factors are usually stated as leading or lagging to show the sign of the phase angle. With a purely resistive load, current and voltage changes polarity in step and the power factor will be 1. The electrical energy flows in a single direction across the network in each cycle.

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Inductive loads – transformers, motors and wound coils – consumes reactive power with current waveform lagging the voltage.

Capacitive loads – capacitor banks or buried cables – generates reactive power with current phase leading the voltage. Inductive and capacitive loads stores energy in magnetic or electric fields in the devices during parts of the AC cycles. The energy is returned to the power source during the rest of the cycles.

Efficiency accounts for the losses incurred when converting electrical energy to mechanical energy. Some of the electrical power drawn by the load is lost, usually in the form of heat to the ambient environment.

Step C

Classify each of the loads in terms of switchboard location, load duty and load criticality.

Once the loads have been identified, they need to be classified accordingly.

Large loads may need to be on MV or HV switchboards depending on the size of the load and how many voltage levels are available.

Continuous loads are those that normally operate continuously over a 24-hour period. e.g. process loads, control systems, lighting and small power distribution boards, UPS systems.

Intermittent loads that only operate a fraction of a 24 hour period, e.g. intermittent pumps and process loads, automatic doors and gates.

Standby loads are those that are on standby or rarely operate under normal conditions.

Step D:

Loads are typically classified as either normal, essential and critical.

Normal loads are those that run under normal operating conditions, e.g. main process loads, normal lighting and small power, ordinary office and workshop loads.

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Essential loads are those necessary under emergency conditions, when the main power supply is disconnected, and the system is being supported by an emergency generator.

Criticalloadsare those critical for the operation of safety systems and for facilitating or assisting evacuation from the plantand would normally be supplied from a UPS or battery system

Calculate consumed load: The consumed load is the quantity of electrical power that the load is expected to consume. For each load, calculate the consumed active and reactive loading.

Step E:

Calculating operating, peak and design loads.A generic method is presented asfollows:

Operating Load: The operating load is the expected load during normal operation. The operating load is calculated as follows:

Continuous Load + 0.5*Intermittent load.

The peak load is the expected maximum load during normal operation. Peak loading is typically infrequent and of short duration, occurring when standby loads are operated. The peak load is calculated as::

The sum of all continuous loads + sum of all intermittent Loads + 0.1* sum of all standby loads.

Design Load: The design load is used for equipment sizing, electrical studies. The design load is generically calculated as the larger of either the Peak or Operating loads.

Identifying and organizing the essential components of a functioning electrical loading schedule assures that the design of the electrical system will be a success.

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