What Is Starting Current?

Starting current is a short-term increase in power consumption when a motor, pump or compressor starts. It may be several times higher than the normal operating value.

This surge does not last long, but the generator must be capable of handling it. Otherwise, the voltage may drop, the generator may shut down due to overload protection, connected equipment may malfunction or the motor may fail to start.

Simplified Starting Load Calculation

The following formula can be used for a preliminary estimate:

Starting power = running power × starting coefficient

For example, suppose a pump has a running power of 1.5 kW. For this example, we will use a starting coefficient of 3:

1.5 kW × 3 = 4.5 kW

This means that the pump may briefly require approximately 4.5 kW during startup.

The actual starting coefficient depends on the equipment design, the mechanical load on the motor and the starting method. It should be obtained from the manufacturer’s technical documentation or determined through measurements.

A generator usually supplies more than just the pump or compressor being started. Lighting, refrigerators, cash registers, automation systems and other equipment may already be operating at the site.

A simplified preliminary calculation therefore uses the following formula:

Peak load = running background load + starting power of the most demanding equipment being started

The background load includes equipment that is already operating before the motor starts. The equipment being started should not be included in the background load, otherwise its power will be counted twice.

An appropriate power reserve is then added. The required margin depends on the type of load, the operating mode and the characteristics of the specific generator.

Important: this simplified calculation helps estimate the approximate required capacity, but it does not replace a complete engineering assessment. When electric motors are involved, engineers must also consider apparent power in kVA, power factor, the number of phases, permissible voltage drop and the generator’s transient response.

Which Equipment Requires Special Attention?

The basic rule is simple: if the equipment contains a motor, pump, compressor or fan, its startup characteristics should be checked separately.

Equipment that commonly produces significant starting currents includes:

• refrigerators and freezers;
• refrigerated display cases;
• air conditioners;
• pumps;
• compressors;
• ventilation systems;
• machine tools;
• concrete mixers;
• professional power tools.

Heaters, electric kettles, electric cookers, boilers with heating elements and standard lighting generally do not produce a significant startup surge. Their power consumption reaches the normal operating level almost immediately.

Sensitive equipment also requires special attention. This includes servers, UPS systems, medical equipment, automation systems, welding inverters, variable-frequency drives and electronic control units.

Such equipment may not create a high starting current itself, but it may react poorly to a voltage drop caused by a nearby pump or compressor starting.

Preliminary Calculation Procedure

1. Determine Which Equipment Must Operate from the Generator

It is not always necessary to connect the entire facility to the backup power system. Some non-essential loads can be disconnected during a power outage.

For example, decorative lighting, some power outlets, water heaters and other non-critical equipment can be excluded from the backup circuit.

This reduces the required generator capacity and helps avoid paying for unnecessary kilowatts.

2. Record the Running Power of Each Device

The running power can usually be found:

• on the equipment nameplate;
• in the technical data sheet;
• in the manufacturer’s manual;
• in the electrical design documentation.

If only the current in amperes is specified, the voltage, number of phases and power factor will also be required for the calculation.

3. Identify Equipment with Demanding Startup Characteristics

Separately identify pumps, compressors, refrigeration systems, ventilation equipment, machine tools and other equipment containing motors.

For each device, determine the starting current or use an appropriate starting coefficient for a preliminary estimate.

4. Identify the Most Demanding Startup Scenario

Do not automatically add together the starting power of every device at the facility.

If equipment starts sequentially, all motors will not produce a starting surge at the same time. Instead, identify the most demanding realistic operating scenario:

• which equipment is already operating;
• which device starts at that moment;
• whether several motors can start simultaneously;
• how the equipment is started.

This scenario determines the actual peak load placed on the generator.

Calculation Example for a Cafe

Consider a small cafe. During a power outage, the lighting, cash registers, router, refrigerators and a pump must continue operating.

Suppose the equipment already operating before the pump starts consumes 4 kW. The pump itself is not included in this 4 kW background load.

The pump has a running power of 1.5 kW. For this example, we will use a starting coefficient of 3:

1.5 kW × 3 = 4.5 kW

Calculate the peak load:

4 kW + 4.5 kW = 8.5 kW

For a preliminary estimate, add a conditional 20% power margin:

8.5 kW × 1.2 = 10.2 kW

Therefore, this scenario would preliminarily require a generator rated at approximately 10–10.5 kW.

If the generator were selected only according to the 4–5 kW running load, it might be unable to handle the pump startup.

The starting coefficient and power margin in this example are illustrative only. Accurate generator selection requires checking the specifications of the pump and the specific generator model.

Calculation Example for a Construction Site

Suppose lighting, a site cabin, a pump and power tools are already operating at a construction site. Their combined background load is 7 kW.

A compressor with a running power of 5 kW is then started. For this example, we will use a starting coefficient of 4:

5 kW × 4 = 20 kW

Calculate the peak load:

7 kW + 20 kW = 27 kW

Add a conditional 20% power margin:

27 kW × 1.2 = 32.4 kW

This scenario would preliminarily require a generator rated at approximately 32–35 kW.

A 15–20 kW generator may be able to operate some of the equipment under steady-state conditions, but it may not be capable of handling the compressor startup.

In this situation, the options are to select a generator with a higher capacity or change the starting arrangement by starting equipment sequentially, using a soft starter or separating the loads.

What Else Does an Engineer Consider?

A calculation based only on kilowatts is suitable for a preliminary estimate. Accurate generator sizing requires several additional parameters.

kW and kVA

kW represents active power — the energy directly used by equipment to perform useful work.

kVA represents apparent power, including the reactive component. It is especially important when supplying motors, pumps, compressors, transformers and other equipment containing windings.

A generator may therefore appear suitable based on its active power in kW but still be unable to handle the full apparent load in kVA.

Phase Configuration and Load Distribution

A high-power single-phase load should not be connected to a three-phase generator without checking how the load is distributed between the phases.

A significant phase imbalance may cause voltage drops, overheating and unstable generator operation.

Permissible Voltage Drop

When a motor starts, the voltage temporarily decreases.

A small voltage drop may be acceptable for ordinary power tools. However, servers, medical equipment, UPS systems and automation equipment may respond by shutting down or generating an error.

Starting Method

Direct-on-line motor starting produces a higher initial surge. Soft starters and variable-frequency drives can reduce it, but they must also be properly matched to the generator.

Simultaneous Startup

At sites with automatic control systems, several loads may start almost simultaneously after power is restored. This scenario must be assessed separately.

How to Reduce the Starting Load

It is not always necessary to purchase a generator several times larger than the normal running load. In some cases, the peak load can be reduced by changing the operating sequence.

Start Equipment Sequentially

Lighting and automation systems can be started first, followed by refrigerators and ventilation equipment. The most demanding loads — such as pumps and compressors — can then be started separately.

This reduces the risk of several startup surges occurring at the same time.

Use a Soft Starter

A soft starter allows a motor to accelerate gradually and reduces the initial load placed on the generator.

This solution is often used for pumps, ventilation systems and other motors with demanding startup characteristics.

Use a Variable-Frequency Drive

A variable-frequency drive controls motor acceleration and helps reduce starting current.

However, the generator must be properly selected for operation with the drive, as a variable-frequency drive represents a complex nonlinear load.

Separate Essential and Non-Essential Loads

Not all equipment must continue operating during a utility power outage.

Separating the loads allows only critical systems to remain connected to backup power and reduces the required generator capacity.

How Much Power Reserve Is Required?

There is no single reserve percentage suitable for every facility.

The required margin depends on:

• whether the generator operates as a standby or prime power source;
• the type and composition of the load;
• the number of motors;
• their starting methods;
• the permissible voltage drop;
• the generator’s transient characteristics;
• planned future expansion of the facility.

An insufficient reserve may cause the generator to become overloaded and shut down.

Selecting a generator with an excessively large reserve is not always advisable either. Prolonged operation of a diesel generator at a very low load may adversely affect the condition of the engine.

For this reason, the generator should not simply be selected on the basis that “bigger is better.” It should be matched to the actual load profile of the facility.

Common Generator Sizing Mistakes

Considering Only the Running Power

The generator must handle not only the normal power consumption of the equipment but also the short-term surge produced when motors start.

Ignoring the Background Load

When a pump or compressor starts, lighting, refrigerators, cash registers, automation systems and other equipment may already be operating. Their power must also be included in the calculation.

Adding All Starting Loads Together

If equipment starts sequentially, there is no need to add the starting power of every motor.

Instead, determine the most demanding realistic startup scenario.

Treating Average Coefficients as Exact Values

Coefficients such as 3, 4 or 5 may be used for a preliminary estimate. However, the actual starting demand depends on the specific motor, mechanical load and starting method.

Ignoring the Phase Configuration

Even when the total capacity is sufficient, incorrect distribution of single-phase loads between the phases may cause phase imbalance, voltage drops and unstable generator operation.

What Information Should Be Prepared Before Generator Selection?

To allow an engineer to calculate the required generator capacity accurately, prepare the following information:

• a list of equipment that must operate from the generator;
• the running power of each device;
• voltage and phase configuration;
• starting current data or the motor starting method;
• information about which devices may start simultaneously;
• a list of sensitive equipment;
• the generator operating mode — standby or prime;
• the generator starting method — manual or automatic through an ATS.

The more accurate the initial information, the lower the risk of selecting an undersized generator that cannot start the equipment or an oversized generator that costs more than necessary.

Instead of selecting a generator using average starting coefficients, entrust the load assessment to ADD GROUP engineers. We will examine the equipment and its operating scenarios, perform instrumental measurements of the starting load when required and select a generator that can start the equipment without a critical voltage drop or a protective shutdown.