GENSET SIZING
Generator Set Sizing Calculations (Continued)
Step 2. Create a Generator Set Worksheet
a.When creating a worksheet, number a worksheet for each sequenced load step. The number block is in the upper right hand corner of the worksheet. Worksheet #1 will coincide with Load Step #1, Worksheet #2 will coincide with Load Step #2, and etc.. The step sequence guidelines will provide additional information to be followed here. The worksheets need not have load step numbers unless starting is sequential.
b.Enter the individually assigned load numbers (load schedule) onto the appropriate generator set sizing worksheet. That is, all the load numbers for load step #1 should be entered on worksheet #1, for load step #2 on worksheet #2, and etc.
c.For each load, enter the Load QTY marked on the load schedule in the column labeled QTY on the worksheet.
Figure 1 on page 19 is an example load calculation for an application involving a two-step load starting sequence. Because the application is a two-step load starting sequence, it requires two worksheets as shown. The entries are in italics.
Step Sequence Guidelines
Single Step, Simultaneous Starting — One commonly used approach is to assume that all connected loads will be started simultaneously in a single step, regardless of the number of transfer switches used. This approach assures that the genset is properly sized to meet the entire load demand and is the most conservative method.
Single Step, with Diversity Factor — This is similar to simultaneous starting in a single step, except that an estimated diversity factor, of perhaps 80 percent, is applied to reduce the starting kVA (SkVA) and starting kW (SkW) totals to account for whatever automatic starting controls may be provided with the load equipment.
Multiple Step Sequence — Sequenced starting of loads (where possible) will often permit the most precise load demand for selecting a generator.
A step sequenced start may be approximated, for example, by dividing the loads into blocks each served by a separate transfer switch and then using the standard time delay on transfer to stagger connection of each block onto the generator set. However, once all of the loads have been brought up on line with the genset, the load equipment may be frequently started and stopped by automatic controls. In such cases, the genset will have to be sized to start the largest motor last, with all other connected loads on line.
Consider the following when controls or delays are provided to step sequence the loads onto the generator set:
Start the largest motor first. Use only when on a manual starting system.
Load the UPS last. UPS equipment is typically fre- quency sensitive, especially to the rate of change of frequency. A
PAGE 18 — INDUSTRIAL GENERATOR SETS — APPLICATION & INSTALLATION MANUAL — REV. #4 (09/07/07)
