Electrical Load Schedule

Introduction

Figure 1. Example of an electrical load schedule

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 switchboard or occasionally by sub-facility / area.

Why do the calculation?

Preparing the load schedule is one of the earliest tasks that needs to be done as it is essentially a pre-requisite for some of the key electrical design activities (such as equipment sizing and power system studies).

When to do the calculation?

The electrical load schedule can typically be started with a preliminary key single line diagram (or at least an idea of the main voltage levels in the system) and any preliminary details of process / building / facility loads. It is recommended that the load schedule is started as soon as practically possible.

Calculation Methodology

There are no standards governing load schedules and therefore this calculation is based on generally accepted industry practice. 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 1: Collect a list of the expected electrical loads in the facility
Step 2: For each load, collect the electrical parameters, e.g. nominal / absorbed ratings, power factor, efficiency, etc
Step 3: Classify each of the loads in terms of switchboard location, load duty and load criticality
Step 4: For each load, calculate the expected consumed load
Step 5: For each switchboard and the overall system, calculate operating, peak and design load

Step 1: Collect list of loads

The first step is to gather a list of all the electrical loads that will be supplied by the power system affected by the load schedule. There are generally two types of loads that need to be collected:

Process loads - are the loads that are directly relevant to the facility. In factories and industrial plants, process loads are the motors, heaters, compressors, conveyors, etc that form the main business of the plant. Process loads can normally be found on either Mechanical Equipment Lists or Process and Instrumentation Diagrams (P&ID's).

Non-process loads - are the auxiliary loads that are necessary to run the facility, e.g. lighting, HVAC, utility systems (power and water), DCS/PLC control systems, fire safety systems, etc. These loads are usually taken from a number of sources, for example HVAC engineers, instruments, telecoms and control systems engineers, safety engineers, etc. Some loads such as lighting, UPS, power generation auxiliaries, etc need to be estimated by the electrical engineer.

Step 2: Collect electrical load parameters

A number of electrical load parameters are necessary to construct the load schedule:

Rated power is the full load or nameplate rating of the load and represents the maximum continuous power output of the load. For motor loads, the rated power corresponds to the standard motor size (e.g. 11kW, 37kW, 75kW, etc). For load items that contain sub-loads (e.g. distribution boards, package equipment, etc), the rated power is typically the maximum power output of the item (i.e. with all its sub-loads in service).

Absorbed power is the expected power that will be drawn by the load. Most loads will not operate at its rated capacity, but at a lower point. For example, absorbed motor loads are based on the mechanical power input to the shaft of the driven equipment at its duty point. The motor is typically sized so that the rated capacity of the motor exceeds the expected absorbed load by some conservative design margin. Where information regarding the absorbed loads is not available, then a load factor of between 0.8 and 0.9 is normally applied.

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. Where power factors are not readily available, then estimates can be used (typically 0.85 for motor loads >7.5kW, 1.0 for heater loads and 0.8 for all other loads).

Efficiency accounts for the losses incurred when converting electrical energy to mechanical energy (or whatever type of energy the load outputs). Some of the electrical power drawn by the load is lost, usually in the form of heat to the ambient environment. Where information regarding efficiencies is not available, then estimates of between 0.8 and 1 can be used (typically 0.85 or 0.9 is used when efficiencies are unknown).

Step 3: Classify the loads

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

Voltage Level

What voltage level and which switchboard should the load be located? 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. Typically, loads <150kW tend to be on the LV system (400V - 690V), loads between 150kW and 10MW tend to be on an intermediate MV system (3.3kV - 6.6kV) where available and loads >10MW are usually on the HV distribution system (11kV - 33kV). Some consideration should also be made for grouping the loads on a switchboard in terms of sub-facilities, areas or sub-systems (e.g. a switchboard for the compression train sub-system or the drying area).

Load duty

Loads are classified according to their duty as either continuous, intermittent and standby loads:

1) 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, etc

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

3) Standby loads are those that are on standby or rarely operate under normal conditions, e.g. standby loads, emergency systems, etc

Note that for redundant loads (e.g. 2 x 100% duty / standby motors), one is usually classified as continuous and the other classified as standby. This if purely for the purposes of the load schedule and does not reflect the actual operating conditions of the loads, i.e. both redundant loads will be equally used even though one is classified as a standby load.

Load criticality

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

1) 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, etc

2) 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, e.g. emergency lighting, key process loads that operate during emergency conditions, fire and safety systems, etc

3) Critical are those critical for the operation of safety systems and for facilitating or assisting evacuation from the plant, and would normally be supplied from a UPS or battery system, e.g. safety-critical shutdown systems, escape lighting, etc

Step 4: 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, derived as follows:

P_{l}={\frac {P_{abs}}{\eta }}\,

Q_{l}=P_{l}{\sqrt {{\frac {1}{\cos ^{2}\phi }}-1}}\,

Where $P_{l}\,$ is the consumed active load (kW)

Q_{l}\,

is the consumed reactive load (kVAr)

P_{abs}\,

is the absorbed load (kW)

\eta \,

is the load efficiency (pu)

\cos \phi \,

is the load power factor (pu)

Notice that the loads have been categorised into three columns depending on their load duty (continuous, intermittent or standby). This is done in order to make it visually easier to see the load duty and more importantly, to make it easier to sum the loads according to their duty (e.g. sum of all continuous loads), which is necessary to calculate the operating, peak and design loads.

Step 5: Calculate operating, peak and design loads

Many organisations / clients have their own distinct method for calculating operating, peak and design loads, but a generic method is presented as follows:

Operating load

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

OL=\sum L_{c}+0.5\times \sum L_{i}\,

Where $OL\,$ is the operating load (kW or kVAr)

\sum L_{c}\,

is the sum of all continuous loads (kW or kVAr)

\sum L_{i}\,

is the sum of all intermittent loads (kW or kVAr)

Peak 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 (e.g. for changeover of redundant machines, testing of safety equipment, etc). The peak load is calculated as the larger of either:

PL=\sum L_{c}+0.5\times \sum L_{i}+0.1\times \sum L_{s}\,

or

PL=\sum L_{c}+0.5\times \sum L_{i}+L_{s,max}\,

Where $PL\,$ is the peak load (kW or kVAr)

\sum L_{c}\,

is the sum of all continuous loads (kW or kVAr)

\sum L_{i}\,

is the sum of all intermittent loads (kW or kVAr)

\sum L_{s}\,

is the sum of all standby loads (kW or kVAr)

L_{s,max}\,

is the largest standby load (kW or kVAr)

Design load

The design load is the load to be used for the design for equipment sizing, electrical studies, etc. The design load is generically calculated as the larger of either:

DL=1.1\times OL+0.1\times \sum L_{s}\,

or

DL=1.1\times OL+L_{s,max}\,

Where $DL\,$ is the design load (kW or kVAr)

OL\,

is the operating load (kW or kVAr)

\sum L_{s}\,

is the sum of all standby loads (kW or kVAr)

L_{s,max}\,

is the largest standby load (kW or kVAr)

The design load includes a margin for any errors in load estimation, load growth or the addition of unforeseen loads that may appear after the design phase. The load schedule is thus more conservative and robust to errors. On the other hand however, equipment is often over-sized as a result. Sometimes the design load is not calculated and the peak load is used for design purposes.