### Generators Sizing Calculations – Part Eleven

Today, we will explain in detail the applicable procedures for Generators Sizing Calculations for Existing and New Installations.

 Fourth: Applicable Procedures For Generators Sizing Calculations

 Step#1: Determine the Required Generator(S) Set Rating

 Step#2: Assign the System Voltage and Phase

 Some factors needs to be considered when selecting a generator for an application like:   Number of Phases, Voltage Ranges.

 1- Number of Phases   Generators are available in single- or three-phase as follows:   1- Single-Phase Generators   For smaller single-phase loads, these Generators usually do not go above 40 kW. They are commonly used in residential environments and have a power factor of 1.0. 2- Three Phase Generators   Mainly for larger industrial power generation, these Generators can provide both single and 3-phase power for running industrial motors with higher horsepower, branch power out for separate lines, and are in general more flexible.  They are typically used in commercial environments and have a power factor of 0.8.   Note: The three-phase selection permits single-phase loads to be connected.

2- Voltage Ranges

To determine the size of the generator needed, we must assign the applicable voltage range. There are three voltage ranges; Low, medium, and high.

 Low voltage range Medium voltage range High voltage range Low voltage is the voltage on a local level or part of a site. Medium voltage is a low level distribution rating. This voltage is distributed to power residential sites and other campuses. High voltage runs over regions and is the voltage at a utility or the national grid. 600 V or less with 800 kVA for less than 250 V is typical. 601 V-5000 V with 5-10 MVA is the range for the medium rating. 5001 V-15,000 V used with MVA’s greater than 10 are considered High Voltage.

Figure.1 shows a voltage versus kVA chart, indicating low, medium and high voltage selections.

Fig.1: Voltage/KVA Chart

Common Voltages on Generator Sets:

 Single-Phase 120 240 120/240

 3-Phase 208 120/208 220 240 440 480 277/480 600 2400 3300 4160 6900 11500 13500

The most common voltages in North America are:
• 120/240V, 1 phase, 3W,
• 120/208V, 3 phase, 4W wye
• 277/480, 3 phase, 4W wye
• 120/240V 3 phase, 4W Delta

120/240V, 1 phase, 3W

This is the standard voltage for single phase systems. 120 V is common for office equipment and small single-phase motors. Motors are limited to 10HP maximum. Either 120 volts or 240 volts can be used to supply loads. 120 volt loads must be balanced across the generator L1 to N and L2 to N. Disadvantages are that single phase motors are more unreliable than three phase motors, especially capacitor start motors. The voltage drop is higher in single phase systems for a given load for the same wire size used in 3 phase systems.

120/208V, 3 phase, 4W- wye

This is a good choice for a three phase system because you can balance 120 volt loads around the wye to equally load the generator. 208V single phase and 208V three phase loads can be used as well as 120V single phase loads. Motors must be rated 200 volt operation; 240 volt resistance heating equipment used on a 208V generator will only produce 75% of its rated KW output.

277/480, 3 phase, 4W- wye

This voltage is usually used on large systems to reduce incoming service size, wire size and distribution equipment size. Fluorescent and other discharge lighting can be used at 277 volts. 480 volt single phase, 480V three phase for large air compressors, motors, chillers, and air handlers, and 277 volt single phase loads can be used on this system.
Also good for minimizing voltage drops on long runs. Disadvantages are that a step down transformer is required to get 120/208 or 120/240 volt power for lights and outlets. Motors should be started directly from the generator buss, not from a step down transformer to minimize voltage drop.

120/240V, 3 phase, 4 W Delta

This Voltage should not use on generator as it overloads 1 phase if there are large 120 volt loads.  This is the least desirable voltage to use if there is a large amount of 120 volt load. The generator cannot be balanced and may overheat the windings. Advantages are 240 volt motors and equipment are more common than 200 volts.

 Why analysis and categorization of generator loads is very important?   Loads have different electrical characteristics. When developing a load analysis, it is helpful to analyze and categorize generator set loads into groups with common characteristics to assure proper consideration of their power demand because A generator set is a limited power source, sometimes referred to as a “limited bus”. The limited bus does not have the reserve capability of a utility grid.   Note: There are no rigid standards for categorizing loads.

A- For New Constructions

After accumulation of load list it is necessary to segregate the loads in different application categories as listed below:

• Category-1: Linear loads like lighting, heating etc.,
• Category-2: Running highest motor or a group of motors to be started at a time or sum of auto accelerated motor,
• Category-3: Running motor loads except VFD and soft started,
• Category-5: Soft started motor.

 Category Application Required Data Category-1 Linear loads Rated Load in KW Power factor PF Category-2 Running highest motor or a group of motors to be started at a time Rated Load in KW Starting Load in KW Running PF Starting PF Starting Time based on load type (ts) Category-3 Running motor loads except VFD and soft started Rated Load in KW Overall PF Category-4 UPS loads Rated Load in KW Overall PF Category-5 Soft started motor Rated Load in KW Overall PF

B- For Existing Installations

After accumulation of load list it is necessary to segregate the loads in different application categories as listed below:

Enter running kW for all motor loads (except the largest) expected to run during peak load levels into Table-3. Refer to Table-1 for typical motor load sizes and electrical requirements.

 Table-3 Category-1: Motor Load Table (refer to Table 1) Device HP RA LRA kW Running (= HP) Starting kW

Notes:

For HP < 7.5; starting kW = HP x 3
For HP > 7.5; starting kW = HP x 2
Starting kW for loads with no listed HP; calculate HP based on running amps in Table-4 below.

 Table-4: How To Calculate kW for loads with no listed HP 120 V 1ø Amps x 120/1000 = kW 240 V 1ø Amps x 240/1000 = kW 208 V 3ø (Amps x 208 x 1.732 x PF) /1000 = kW 240 V 3ø (Amps x 240 x 1.732 x PF) /1000 = kW 480 V 3ø (Amps x 480 x 1.732 x PF) /1000 = kW PF is the load power factor - Typical application power factor is 0.95.

Enter kW for all non-motor loads expected to run during peak load levels into Table-5. Refer to Table-2 for typical residential loads and rules of thumb.

 Table-5 Category-2: Non-Motor Load Table (refer to Table-2) Device Amps Kw

 Step#4: Match the System to the Load Profile, Calculate the Required Number of Generator Sets and Paralleling Requirement

 This step is used to evaluate whether the standby power system should be composed of a single large generator set or several smaller generator sets operating in parallel.   To do this, we have to determine the facility’s load profile by grouping the loads according to their priority as follows: Essential Or Critical Loads: such as life-safety systems; security, computer and data systems; communications; or certain critical manufacturing processes. Important, But Not Critical Loads: such as ventilation systems, general lighting and less-vital manufacturing processes. Non-Essential Loads For Business Continuity: an example might be the HVAC system, unless it is needed for cooling major computer or data facilities. Standby power systems with multiple generator sets offer a number of advantages that include: Enhanced reliability and availability, Operational flexibility, Fuel savings, More convenient maintenance scheduling, Easier system expansion.     So, the owner and designer must decide one of the following two solutions:   Install a standby power system using a single large generator set or Install a system with multiple smaller, paralleled generator sets.   To select the best solution, the following factors must be taken into consideration:   The facility’s load profile and priorities, The impact of enhanced power system reliability and availability, The impact of enhanced operational flexibility on improved maintenance scheduling, The potential fuel cost savings and electricity rates for participation in utility interruptible-rate programs, The impact of load growth and reduction of future expansion costs.   However, the lowest cost should never be the sole consideration when implementing a standby power system. Care must be taken to review the total project requirements, capabilities, and cost tradeoffs as explained above.

In the next article, we will continue explaining the applicable procedures for Generators Sizing Calculations for Existing Installations. So, please keep following.

The previous and related articles are listed in the below table:

 Subject of Previous Article Article Glossary of Generators – Part One Glossary of Generators – Part Two First: Reasons for having on-site generators Second: Applicable performance standards for generator sets Third: Selection Factors Used For Generators Sizing Calculations Generator Power Ratings Application type Third: Selection Factors Used For Generators Sizing Calculations 3- Location Considerations, 4- Fuel Selection Considerations, 5- Site Considerations, Third: Selection Factors Used For Generators Sizing Calculations 6- Environmental Considerations, 7- System Voltage and Phase, Third: Selection Factors Used For Generators Sizing Calculations 8- Acceptable percent of voltage & frequency dip, 9- Acceptable duration of the voltage & frequency dip, Third: Selection Factors Used For Generators Sizing Calculations 10- Percent And Type Of Loads To Be Connected – Part One 10- Percent And Type Of Loads To Be Connected – Part Two Third: Selection Factors Used For Generators Sizing Calculations 11- Load step sequencing 12- Future needs Fourth: Applicable Procedures For Generators Sizing Calculations 1.1- Generator Load Factor 1.2- Load Demand Factor 1.3- Load Diversity Factor

 Back To