Power Quality and Generators - Part 10: Generators and the 2005 NEC
This is the tenth and final installment of an ongoing series that covers basic engineering and code issues for standby generators.
The 2005 National Electrical Code (NEC) has some major changes that could significantly transform the way emergency distribution systems are designed and built in the near future.
Here, I will identify some changes regarding standby generator systems and provide some insight into the effect that these changes will have on the everyday design and implementation of emergency, standby and optional generator electrical distribution systems. The 2005 NEC section changes that affect generator systems will be quoted, followed by my analysis of the specific change.
Alternate power sources
2005 NEC Section 700.5 (B): Selective Load Pickup, Load Shedding and Peak Load Shaving. The alternate power source shall be permitted to supply emergency, legally required standby and optional standby system loads where the source has adequate capacity or where automatic selective load pickup and load shedding is provided as needed to provide power to (1) the emergency circuits, (2) the legally required standby circuits and (3) the optional standby circuits, in that order of priority.
Analysis: "Where the source has adequate capacity" was added in the revised 2005 NEC. This text brings more clarity to the fact that load shedding is only required if the generator does not have the capacity to feed all of the proposed connected loads. I also believe this amplifies the need to accurately simulate the electrical load profiles to determine if the standby generator can provide reliable power to the proposed load.
A generator system load analysis utilizing either hand calculations or sophisticated simulation software is required to determine if the proposed connected load can be served by a standby generator. This analysis should consider the worst-case scenario of all potential motor loads starting at the same time. In addition, future anticipated loads should be analyzed.
Load shedding can be achieved by providing a separate automatic transfer switch (ATS) for each of the three branches of loads (life safety, legally required standby, optional). The legally required standby and optional ATS must have a center off position and a load-shed relay. When the generator is critically close to an overload condition, it will send a signal to the load-shed relays to remove the non-life safety ATS and non-life safety loads and ensure that the life-safety loads remain on emergency generator power. The first load to be removed under an overload condition would be the optional load, followed by the legally required standby loads.
In the event that it can be calculated that all of the connected loads at the initial build-out do not exceed the capacity of the standby generator, I think it is a good idea to implement the load-shed relay system. Future build-out design teams may not adhere to the good design practice of ensuring that future optional loads do not over load the capacity of the standby generator. Without a load shedding scenario, future optional loads could potentially exceed the capacity of the standby generator and take out the entire standby system, including the life-safety loads.
It is common for a system's starting kVA or starting kW and maximum allowable transient voltage drop to determine the size of the standby generator. Motors can draw six or more times the full-load amps during startup. The starting of motors can dramatically affect the inrush current and associated starting kVA (sKVA) and the starting kW (sKW) required and may exceed the maximum sKVA or the sKW of a generator set that would otherwise be large enough to serve the steady-state load. This could require an oversized generator set based solely on the motor starting requirements of the electrical system.
If the standby generator sizing simulation indicates that all loads cannot be transferred at the same time during startup, stepping the sequence of the loads within the requirements of NEC Article 700 can greatly reduce the sizing of the generator set. Startup load stepping can also be achieved with the use of different ATS for the life safety, legally required and optional standby loads. Since larger generators are often required because of the peak starting kW or kVA on the system, stepping the loads long enough for the inrush of motors not to be simultaneous can reduce the ultimate size of the generator set required to feed the critical loads.
Fuel cell systems
2005 NEC Section 700.12 (E): Fuel Cell System. Fuel cell systems used as a source of power for emergency systems shall be of suitable rating and capacity to supply and maintain the total load for not less than 2 hours of full demand operation.
Installation of a fuel cell system shall meet the requirements of Parts II through VIII of Article 692.
Where a single fuel cell system serves as the normal supply for the building or group of buildings concerned, it shall not serve as the sole source of power for the emergency standby system.
Analysis: The two hours of full demand operation is similar to the requirement for generator sets as defined in NEC 700.12 (B) (2) Internal Combustion as Prime Movers. A separate article for fuel cells was first introduced into the 2002 edition of the NEC, which specifically identified fuel cells as a potential source for standby emergency power.
A fuel cell provides direct-current voltage, and current technology allows a proton exchange membrane fuel cell the size of a piece of luggage to power a car. Fuel cells for homes and commercial use are just starting to become available. This technology can utilize methanol, natural gas and propane as the fuel source. Issues such as hydrogen infrastructure and life of the fuel cell need to be addressed and improved before this technology becomes more widespread.
2005 - NEC Section 700.27: Coordination - Emergency system(s) overcurrent devices shall be selectively coordinated with all supply-side overcurrent protective devices.
Coordination (Selective) is the localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings and settings. Overcurrent is defined as any current in excess of the rated current of the equipment or ampacity of the conductor. It may result from overload, short circuit or ground fault.
Analysis: For clarity it is important to include the NEC definition (Article 100) of overcurrent and the fine print notes defining emergency systems and legally required standby loads in Article 700 and Article 701 below:
NEC Section 700.1 FPN no. 3: Emergency systems are generally installed in places of assembly where artificial illumination is required for safe exit and panic control. Emergency systems may also provide power for such functions as ventilation. Fire detection and alarm systems, elevators, fire pumps, public safety communication and industrial processes.
NEC Section 701.2 FPN: Legally Required Standby systems are typically installed to serve loads, such as heating and refrigeration systems, communication systems, sewage disposal, lighting systems and industrial processes, that, when stopped during any interruption of normal electrical supply, could create hazard or hamper rescue or fire fighter operations.
Analysis: Take note of the definition of overcurrent above. The fact is that it indicates that an overcurrent can be the result of a short circuit and an overcurrent can be in the region where many breakers overlap.
Two normal thermal magnetic breakers-one fed downstream of the other-are typically coordinated in the overload region but would not be considered to be selectively coordinated in the short circuit region as per the definition of over load in the 2005 NEC, as in the diagram to the right.
Fuses can be completely coordinated. The melting time of a fuse is significantly faster than the mechanical operation of a breaker and will allow the downstream device to clear the fault without causing the upstream device to open.
Fused electrical distribution systems will take up considerably more space than a typical panel board that uses thermal magnetic breakers. Additionally, many maintenance and facility personnel currently benefit from the ease of resetting a breaker, instead of ensuring that spare fuses are available and replacing spent fuses in the event of a short circuit or overload.
A lack of good maintenance protocols can actually reduce site availability and uptime if spare fuses are not available after an overload trip or short circuit. Another issue with utilizing fused distribution includes the possibility of single phasing motor loads from the loss of one of the three-phase conductors.
Additional options for selective coordination
Other options to provide complete selective coordination include the use of ANSI rated power breakers and switchgear construction with the instantaneous portion of the time current curve turned "off" and the use of zone interlock
This change, if fully enforced, could require the emergency or legally required standby electrical distribution system to incorporate expensive ANSI-rated electronic power breakers with the instantaneous portion of the time current curve turned off and switchgear construction, the use of zone interlocking or fused distribution. This course of action could add a lot of expense, especially to smaller, less critical electrical distribution systems.
2005 - NEC Section 701.6: Capacity and Rating- The legally required standby alternate power source shall be permitted to supply both legally required standby and optional standby system loads under either of the following conditions:
- Where the alternate source has adequate capacity to handle all connected loads.
- Where automatic selective, load pickup and load shedding is provided that will ensure adequate power to the legally required standby circuits.
Analysis: This section in the 2002 NEC only indicated that load shedding and pickup would be provided as needed to ensure that the power source has the capacity for the legally required circuits. This section for legally required standby systems, similar to Section 701.5 (B) above for emergency systems, clarifies that a load shedding and load pickup system is only required if the alternative source does not have the capacity to pickup all of the connected loads. When determining if a load shedding scenario is required, I think it is prudent to evaluate potential future loads.
Fuel cell systems
2005 - NEC Section 701.11: Fuel-Cell System. Fuel-cell systems used as a source of power for legally required systems shall be of suitable rating and capacity to supply and maintain the total load for not less than 2 hours of full demand operation.
Installation of a fuel cell system shall meet the requirements of Parts II through VIII of Article 692.
Where a single fuel cell system serves as the normal supply for the building or group of buildings concerned, it shall not serve as the sole source of power for the legally required standby system.
Analysis: This section allows for the use of fuel-cell technology for the legally required standby power source. This section is essentially identical to the requirements stated above for Section 700.12 (E).
2005 - NEC Section 701.18: Coordination - Legally required standby system(s) overcurrent devices shall be selectively coordinated with all supply side over current protective devices.
Analysis: This section indicates that the same issues involved and described under Section 700.27 above, apply to this section for legally required standby loads.
Transfer equipment and temporary connections
2005 - NEC Section 702.6: Transfer Equipment. Exception: Temporary connection of a portable generator without transfer equipment shall be permitted where conditions of maintenance and supervision ensure that only qualified persons service the installation and where the normal supply is physically isolated by a lockable disconnect means or by disconnection of the normal supply conductors.
Analysis: Transfer equipment is intended to prevent an inadvertent connection between the normal and alternate standby power source. All transfer equipment shall meet the requirements of Article 705. This new exemption will allow temporary installation under certain provisions to be connected without this critical piece of equipment. As with all exemption, this would have to be approved by the Authority Having Jurisdiction prior to connecting a temporary generator without a transfer switch. This exemption must also be used with caution; an inadvertent connection of two out of phase sources could be very hazardous.
Audible and visual signals
2005 - NEC Section 702.7: Signals - Audible and visual signal devices shall be provided where applicable, for the following purposes.
- Derangement. To indicate derangement of the optional standby source.
- Carrying Load. To indicate that the optional standby source is carrying load.
Exemption: Signals shall not be required for portable standby power sources.
Analysis: This exemption was added to the 2005 NEC and removes this requirement of audible and visible signaling devices for portable generators used for optional standby loads.
Coordination of electrical systems in medical facilities
2005 - NEC Section 517.26: Application of Other Articles - The essential electrical system shall meet the requirements of Article 700, except as amended by Article 517
Analysis: This notation in the 2005 NEC indicates that the electrical systems for the essential loads as defined in 517 for medical facilities will have to be selectively coordinated as described above for Article 700. The same issues involved in the expense and the size of the equipment as noted above will potentially apply to essential electrical systems in medical facilities.
Connecting to alternate power sources
2005 NEC Section 517.34 (A): Equipment System Connection to Alternate Power Source- The equipment system shall be installed and connected to the alternate power source such that the equipment described in 517.34 (A) is automatically restored to operation at appropriate time lag intervals following the energizing of the emergency system. Its arrangement shall also provide for the subsequent connection of equipment described in 517.34 (B).
(6) Supply, return and exhaust ventilation systems for airborne infectious/ isolation rooms, protective environment rooms, exhaust fans for laboratory fume hoods, nuclear medicine areas where radioactive material is used, ethylene oxide evacuation and anesthesia evacuation. Where delayed automatic connection is not appropriate, such ventilation systems shall be permitted to be placed on the critical branch.
Analysis: These ventilation systems were noted in the 2002 edition of the National Electrical Code as part of 517.34 (B) Equipment for Delayed Automatic or Manual Connection. The significant change as defined is this text is that in the 2005 National Electrical Code, these ventilation systems are now not allowed to be transferred manually and require an automatic connection to the standby power source.
It is the electrical engineer or electrical distribution systems designers job to stay abreast of all applicable codes. There is a continual changing of the guard as new building codes, electrical codes, fire codes and energy codes are continuously revised. The revised sections in the 2005 NEC noted above could have some dramatic effects on the design and implementation of standby generator systems and their associated electrical system distribution. Not understanding the impacts of these codes or how the local authority having jurisdiction will implement these codes could be quite costly.
Articles in This Series
- Part 1: Sizing and Code Issues
- Part 2: The Effects of Harmonics
- Part 3: Complying with the Codes and Controlling Noise
- Part 4: Fuel Configurations for Standby Gensets
- Part 5: Paralleling Generators in Critical Applications
- Part 6: Generator Sizing and UPS
- Part 7: Commissioning, Training and Long-Term O&M Programs
- Part 8: Basic Calculations for Sizing Generators and the Impacts of Certain Loads
- Part 9: Design Criteria for Grounding
- Part 10: Generators and the 2005 NEC