Selecting the right generator for a building is essential for ensuring uninterrupted power during outages. But where do you start? Understanding power requirements, verifying site conditions/metered data, and following key electrical codes make the process smoother. This guide will walk you through a few critical steps necessary to confidently size a generator that meets your building’s needs.
Proper generator sizing isn’t just about picking the biggest or most commanding unit available. Oversizing a generator can lead to inefficiencies, increased costs, and unnecessary fuel consumption. On the other hand, an undersized generator may fail to support essential building functions during an outage. The goal is to strike the right balance by carefully evaluating power demands, adhering to relevant codes/standards, and ensuring additional load capacity to accommodate future expansion.
Key Steps in Generator Sizing
Assessing Power Needs
Establishing the power needs of a building is the first and most critical step in generator selection. Reviewing metered data from the building provides valuable insights into its energy consumption patterns. This data helps estimate peak demand, daily usage, and emergency power needs. However, metered data alone may not always provide the full picture. A deeper analysis is vital to identify critical loads and ensure that no essential equipment is overlooked. For example, if the recorded metered data took place during the summer and you are trying to assess the heating demand of HVAC systems, just looking at the metered data would be inaccurate. Life safety systems, elevators, fire pumps, and essential HVAC equipment must be factored into the load calculations if applicable to the building. To verify power needs, site visits are necessary to allow engineers to confirm the operation of existing equipment, assess electrical panel configurations, and address any potential issues that could affect generator performance.
Future growth should also be considered when sizing a generator. Many buildings expand their operations over time, adding new equipment and increasing power demands. Selecting a generator with some additional capacity ensures that it remains adequate even as building requirements evolve. To accurately determine future needs, discussions with the building owner and facility managers are essential. Understanding planned expansions or anticipated equipment upgrades can provide valuable insight into future power demands. Factoring in these elements during the generator selection process allows for a more adaptable system, reducing the risk of costly modifications down the line.
Calculating the Total Load
Once power needs are assessed, the next step is calculating the total electrical load that the generator must support. This involves listing all automatic transfer switches (ATS) and connected emergency power loads within the building. Once, each piece of equipment’s power consumption is summed up to determine the total demand, a load factor—typically set at around 70%—is applied to show realistic operating conditions. This helps prevent overloading the generator while ensuring that it remains efficient under normal circumstances. For example, if the total running load for a building is 530kW, selecting a 750kW or 800kW standby-rated generator provides additional capacity for reliability and future growth.
When calculating total load, it is important to consider load variety and sequencing. Not all electrical loads operate at full capacity simultaneously. Some equipment, like HVAC units and elevators, cycle on and off, meaning their actual power draw may be lower than their nameplate ratings suggest. Factoring in these variations helps avoid unnecessary generator oversizing, leading to cost and efficiency benefits. Additionally, power factors should be considered, as they impact the true power drawn by inductive loads such as motors. Engineers should also account for starting inrush currents, especially for motor-driven equipment, which can cause significant, but short, power spikes. Including soft starters or variable frequency drives (VFDs) in your calculations can help relieve these spikes and improve generator performance.
Electrical Codes and Regulations
To ensure safety and compliance, generator selection must align with relevant National Electrical Code (NEC) standards. Some of these important compliances include, but are not limited to, NEC 700 – Emergency Systems, NEC 701 – Legally Required Standby Systems, and NEC 702 – Optional Standby Systems. However, there are other codes that should not be overlooked, such as the building code applicable to the building being assessed. For example, per Section 3008.1.1 of the International Building Code (IBC), 'Not less than one elevator in each bank shall be designated for occupant evacuation. Not less than two shall be provided in each occupant evacuation elevator lobby where more than one elevator opens into the lobby.' Failing to account for these requirements could result in unnecessary elevator loads being placed on the generator, potentially leading to an oversized or inefficient selection. Furthermore, NEC 517 outlines specific generator load requirements for healthcare facilities. This code also outlines the separation of essential electrical systems into different branches, ensuring that critical loads receive priority during a power outage. This ensures that essential medical equipment and life-safety systems remain operational during outages.
In addition to the listed requirements above, NFPA 110 should also be considered, as it establishes performance and maintenance standards for emergency and standby power systems to ensure reliability. It categorizes power systems into different levels based on the impact of power loss, with level 1 systems required for applications where loss of power could result in serious injury or worse.
Adhering to these regulations are critical in designing a reliable and legally compliant emergency power system. Engineers must also stay updated with local and state requirements, as codes can vary by region. Keeping up with changes in NFPA and IBC codes ensures that generator sizing and installation align with industry best practices and legal obligations.
Additional Considerations
Generator sizing tools can greatly simplify the selection process by automating calculations based on input parameters. Most generator manufacturers offer dedicated software that allows engineers to enter building load data and receive generator recommendations based on industry standards. These tools take factors like load type and startup characteristics into account. However, while these programs are useful, it is important to use the specific software provided by the manufacturer being specified for the most accurate sizing results. Additionally, all results should be verified with real-world assessments and calculations to ensure accuracy.
Beyond sizing and compliance, several other factors should be evaluated before finalizing a generator selection. Startup time is particularly important for emergency systems, which often require a generator to begin supplying power within a certain amount of time in the event of an outage. Ensuring that the selected unit can meet this requirement is essential for uninterrupted safety and operations. Emissions regulations can also be considered. Many jurisdictions have strict guidelines regarding air pollution from standby generators. Choosing a unit that complies with local emissions standards helps reduce environmental impact and avoids potential fines.
Operational costs, including fuel expenses and maintenance, play a big role in generator selection as well. Oversized generators tend to consume more fuel than necessary, leading to increased costs over time. Similarly, a unit that operates at too low of a load for extended periods can suffer from wet stacking, a condition where unburned fuel accumulates in the exhaust system, reducing efficiency and increasing maintenance requirements.
Generator sizing doesn’t have to be complicated. By following a structured approach, facility managers and engineers can ensure reliable backup power while optimizing costs and efficiency. The key to proper generator selection lies in accurately assessing power needs, investigating metered data, and conducting thorough site visits to verify actual load requirements. Calculating total electrical demand with appropriate load factors helps prevent over-sizing or under-sizing issues that could lead to inefficiencies. Utilizing generator sizing software can aid in the selection process, but real-world verification is crucial. Compliance with NFPA codes and local regulations ensures safety and legality, while additional factors such as emissions standards and operational costs should not be overlooked. Proper planning, analysis, and verification at each stage result in a well-optimized generator that meets the building’s needs both now and in the future. With the right approach, you can select a generator with confidence, ensuring uninterrupted power and long-term reliability.
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Written By
Logan Cabral
Electrical Designer
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