Mike Dolt Enterprises
Mike Dolt Enterprises
User: electricshadow, Location: Cincinnati, OH
Asked on: Friday, January 24, 2025 at 8:56 am EST
Answered on: Thursday, March 6, 2025 at 7:46 am EST
Question: Our typical design if we have a generator at a new school is to use one transfer switch for NEC 700 and another for 702 loads. The issue is that NEC 2023 (700.32) requires selective coordination on the emergency branch for new commercial buildings (including high schools). Typically, this is achieved using expensive equipment such as Eaton's QSCP fused panels or adjustable trip breakers. NEC 2023 (700.5(D)) also requires a bypass isolation transfer switch on the emergency system, which increases costs further with ASCO 7000 series equipment.One potential solution is to use one or multiple central inverters for emergency lighting instead of a generator. This would eliminate the need for selective coordination and a bypass isolation transfer switch, potentially saving money. However, it's important to note that a generator would still be necessary to power NEC 702 standby loads, such as kitchen refrigeration and tech racks.Has anyone explored using inverters for emergency lighting and a generator for standby loads to avoid the costs associated with selective coordination?
Yes, you're definitely on the right track and this is a viable, and increasingly common, approach to address the updated NEC 2023 requirements. Here's a breakdown of the considerations and potential solutions, drawing from industry experience
Advantages of Inverter/Generator Approach
Cost Savings
As you pointed out, inverters can significantly reduce costs compared to fully selectively coordinated emergency generator systems with bypass isolation transfer switches. The savings come from: Eliminating the need for selective coordination for the emergency branch. QSCP panels and adjustable trip breakers are expensive. Eliminating the need for a bypass isolation transfer switch on the emergency system. Potentially smaller generator size due to the reduced load.
Simplified Design
Inverters simplify the emergency system design and reduce the complexity of the electrical distribution system.
Reliability (for Lighting)
Well-designed inverter systems can provide highly reliable emergency lighting with minimal maintenance.
Improved Power Quality
Inverters can provide clean, stable power to sensitive lighting loads.
Aesthetics
No generator is required to run the school's emergency lights, thereby keeping the campus quiet during emergencies.
Disadvantages of Inverter/Generator Approach Upfront Inverter Cost
While cheaper than a fully selective coordination system, inverters still represent a significant upfront cost.
Maintenance (Inverters)
Inverter systems require periodic battery maintenance and testing. Factor this into the total cost of ownership.
Load Restrictions
Inverters are limited by their capacity. Carefully analyze your emergency lighting load to ensure you choose the right size inverter(s).
Code Compliance
You need to ensure the inverter system meets all applicable codes and standards, including UL 924, which is critical for emergency lighting applications.
Coordination
Although you don't need selective coordination on the emergency branch anymore, it is still important to coordinate the overcurrent protective devices in the distribution panel that feeds the inverter.
Generator sizing: You need to carefully size the generator for the 702 loads. Make sure the generator is not oversized, as this will cause wet stacking.
Key Considerations for Implementation1. Emergency Lighting Load Analysis
Accurately calculate the total emergency lighting load (including future expansion) to determine the appropriate inverter capacity. Consider LED lighting for its energy efficiency and long lifespan.2.
Inverter Selection
Choose a UL 924 listed inverter system from a reputable manufacturer. Consider features such as: Automatic self-testing and reporting. Remote monitoring capabilities. Battery management systems. Scalability for future load growth.3.
Generator Load Analysis (702 Loads)
Determine the power requirements for the 702 loads (kitchen refrigeration, tech racks, etc.). Consider the starting currents of motor loads when sizing the generator.4.
Transfer Switch for 702 Loads
A single transfer switch can still serve the 702 loads, connecting them to either normal power or the generator.5.
Code Compliance
Work closely with your electrical engineer and the Authority Having Jurisdiction (AHJ) to ensure the design complies with all applicable codes and standards, including NEC Article 700, 701, and 702.6.
Coordination with Other Systems
Ensure the inverter system integrates seamlessly with other building systems, such as fire alarm and building management systems.7.
Life Cycle Cost Analysis
Compare the total cost of ownership (including initial investment, maintenance, and operating costs) for both the inverter/generator approach and the traditional generator-based approach with selective coordination.
Practical Examples and Industry Trends Many new schools, hospitals, and commercial buildings are using central inverters for emergency lighting. LED lighting is becoming the standard for emergency lighting due to its energy efficiency and reliability. Manufacturers like Eaton, ABB, and Schneider Electric offer integrated inverter/generator solutions. Cloud-based monitoring and control systems for emergency power systems are becoming increasingly popular.
Alternative Solutions Worth Considering (Beyond Central Inverters) Unit Equipment for Emergency Lighting
Self-contained battery units can be used in some applications. These are less expensive and easier to install than central inverters, but they may not be suitable for large areas or complex lighting designs. Each lighting fixture has its own battery backup.
Addressable Lighting Systems
Lighting systems with built-in emergency lighting capabilities can simplify the design and installation of the emergency lighting system. These are often more expensive than standard LED fixtures.*
Legitimate Standby Power for Emergency Systems
If your emergency lighting system is considered "standby" and not "emergency" power by the AHJ, you might be able to sidestep some of the most stringent NEC 700 requirements. This requires careful consideration and discussion with the AHJ.
Recommendations1. Perform a Detailed Load Analysis
This is critical to determine the appropriate size inverter and generator for your specific needs.2.
Consult with Electrical Engineers and Contractors
Engage experienced professionals who have experience designing and installing inverter/generator systems.3.
Get the AHJ Involved Early
Discuss your design with the AHJ early in the process to ensure it meets their requirements.4.
Evaluate Multiple Solutions
Compare the costs and benefits of different approaches, including central inverters, unit equipment, and addressable lighting systems.5.
Consider Life Cycle Costs
Factor in the long-term costs of maintenance and operation when evaluating different solutions.6.
Look for Energy Efficiency
Implement energy-efficient lighting and power systems to reduce overall energy consumption and operating costs.7.
Evaluate Generator Exercise Requirements: NEC requires generator exercising. Consider a soft load that can be permanently wired to the generator to make the generator exercise fully loaded.By carefully evaluating the different options and working closely with experienced professionals, you can design an emergency power system that meets your needs and budget. The inverter/generator approach is a viable solution that can save money and simplify the design of your emergency power system. Good luck! Flag for review