Your questions answered: Emergency, standby, and backup generators

Presenters from the critical power webcast on Dec. 11, 2018, answer questions left unaddressed during the live presentation.

By Danna Jensen, PE, LEED AP BD+C, Certus, Carrollton, Texas December 18, 2018

When designing generator systems, electrical engineers must ensure that generators and the building electrical systems that they support are appropriate for the specific application. Whether providing standby power for health care facilities or power for other facility types, engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria.

Presenters from the webcast on critical power: emergency, standby, and backup generators respond to questions not addressed during the live event.


  • Danna Jensen, PE, LEED AP BD+C, senior vice president, WSP USA, Dallas
  • Kenneth Kutsmeda, PE, LEED AP, engineering design principal, Jacobs, Philadelphia

Question: What are some paralleling challenges and solutions for large facilities?

Jensen: Large facilities have larger load demands. And when dealing with level 1 systems where a certain amount of the loads must be back up and running in a little as 10 seconds, it can be challenging to overcome this requirement with larger engines. It takes longer to parallel two larger engines as opposed to smaller ones, but the smaller ones my not support your load. One solution to this is to have multiple paralleling switchgear with a tie breaker in between. This allows for faster response times and larger loads.

Question: NFPA 110: Standard for Emergency and Standby Power Systems requires a load bank testing of generator systems. How do you avoid having the generator system overload if the normal power fails during the test?

Kutsmeda: Use what is called a “load bank dump signal.” Add an under-voltage relay on the incoming feeder that senses loss of utility power and sends a signal to dump the load bank load.

Question: Are there any special considerations to keep in mind if one would like to be able to have load-shedding capabilities? Do you need a certain type of generator control panel to have this capability?

Jensen: Load shedding can be done in a number of different ways. They can be accomplished via the paralleling switchgear controls directly to the automatic transfer switch (ATS), or by distribution breakers, or through another set of relays themselves. The most important aspect of load shedding is to fully test and commission the system to ensure it is programmed and operates as intended.

Question: What are some recommendations for generators feeding uninterruptible power supply (UPS) systems? Discuss UPS compatibility with generators and de-rating.

Kutsmeda: Older UPS system had very high harmonics that required you to oversize the generator to compensate. Newer UPS system have filters to minimize harmonics so that is no longer required. You’ll want to make sure the UPS has very low input current distortion (5% or lower). With that you can put about 75% UPS power onto the generator without having to oversize.

Another thing to avoid is large UPS systems with very low load. At low loads, the UPS put capacitance back onto the system, which drives the power factor to leading. On the UPS end, make sure the UPS ramping function is turned so that it soft load the generator. You also can reduce the battery charging current while the system is on generator to reduce load.

Question: How does the CAT scan momentary load play out sizing of the generator?

Jensen: Any imaging equipment has a high in-rush load and are very sensitive to voltage drop (typical manufacturers say their equipment must be fed from “clean” power with only +/-2% voltage drop. This could lead to oversizing of generators to be able to maintain such a strict tolerance. Another option is to include a UPS with power conditioning to ride through any of the start-up and voltage dips on a generator system. Use a manufacturer’s sizing software and plug in the appropriate parameters in order to get the best recommended system size.

Question: How does the reliability of a Tier 4 genset fare with critical facilities? Or is Tier 2 preferred in critical systems?

Kutsmeda: EPA Tier 2 and Tier 4 are determined by the emission and runtime requirements. If a generator is only used to provide power with the loss of utility power and it operates less than 100 hours a year for maintenance/testing, then it can be Tier 2. If the generator provides power for peak shaving, storm avoidance or maintenance and testing exceed 100 hours per year, then the generator must be a Tier 4.

There are, however, two types of Tier 4 generators. Tier 4 certified means the SRC are installed and certified in the factory with the engine. Tier 4 certified requires shutdown of the generator with loss of urea or issue with the SCR. Tier 4 compliant means the SCR system is installed in the field. Tier 4 compliant does not require the generator to be shutdown with loss of urea. Because of this Tier 4 compliant is more widely used for critical applications if emissions reduction is required.

Question: How do you determine how much spare capacity we have on a generator?

Jensen: Once a system is up in operation, an easy way to determine the remaining capacity is to meter the normal side of the transfer switches for an extended period. The codes require at last 30 days of metered load when using that to determine spare capacity. A good option is to include a power metering system on all transfer switches so this data is always available for the plant engineer to pull up and have a good handle on the overall capacity of the system.

Question: Referring to NFPA 70: National Electrical Code (NEC) Article 700: Emergency and 2 hours onsite fuel storage: How do you design for this if using natural gas? What about dual-fuel with propane?

Kutsmeda: If the natural gas source is reliable, some authorities having jurisdiction (AHJs) will allow natural gas because it could potentially be an indefinite source of fuel (more than 2 hours). Other options are using propane tanks or a dual fuel generator, which starts using diesel and then transfers to natural gas. The dual fuel would also have the 2 hours of fuel stored onsite.

Question: What ancillary or generator accessories require power, and what branch of the emergency power supply system (EPSS) should they be powered from?

Kutsmeda: Block heaters, jacket water heaters, battery charger, fuel pumps, and louvers. For critical facilities (NEC 708), those loads mush be connected to the critical power system. For emergency systems, they should be connected to the emergency distribution.

Question: Expand on segregation of emergency loads—is a separate room needed for transfers switches only?

Jensen: NPFA 110 requires the emergency power supply (EPS) to be in a separate dedicated 2-hour room for Level 1 systems. Other EPSS equipment is also permitted in this room (NFPA 110- NFPA 110 also requires the EPSS to be in a separate room from the normal power service entrance equipment when the equipment is rated over 150 V to ground and equal to or greater than 1,000 amps. So for systems of this size/voltage and greater, the transfer switches must be in a separate room from the normal power service equipment, but they can share the same room as other EPSS equipment, such as the paralleling switchgear and emergency power distribution boards.

Question: Is there a code requirement for health care occupancies served by single engine systems to be provided with means for connecting a portable/roll up generator in event of failure?

Jensen: Now there is. New for NFPA 70-2017, Article 700.3(F) states that a temporary source of power for maintenance or repair of the alternate source of power is required and per NFPA 70 Article 517.26, the life-safety branch must meet the requirements of Article 700. A health care occupancy must also have the portable connection.

Question: NFPA 110 prohibits interconnection of sources. Does this include banning closed-transition transfer?

Jensen: No. NFPA 100-6.2.3 says that mechanical interlocking or an approved alternate method shall prevent the inadvertent interconnection of the primary power supply and the EPS, or any two separate sources of power. This requirement does not preclude the use of closed transition switches, but rather requires that if these type of switches are used, the proper protection such as reverse power relays are used.

Question: What type of software should be used for the design and calculation of a power distribution system?

Jensen: All of the major manufacturers have their own sizing program. It is best to use the manufacturer that is actually being chosen for the site as they have the best insight as to how their engine/alternator will react to the different type of load parameters.

Question: What are some good products to comply with NEC section 700.3(F)?

Jensen: All of the major engine and switchgear manufacturers now make an integrated cabinet that works for quick and easy connection of a temporary roll up generator, or they can be custom-built by the electrical contractor. The most important consideration is to plan for the appropriate space where it is accessible by a roll-up generator. And plan for the proper protection within the switchboard it connects to in order to avoid any revers power back on to the utility system or other power source.

Question: Have you any knowledge of retrofitting 2600 kW generator to Tier IV? What does it entail?

Kutsmeda: I have added SCR and particulate filters to an old two-stroke diesel engine to reduce emissions. It involved removing and reconfiguring the exhaust and muffler sections to be able to install the SCR and filters. SCR will have a urea system that can either be a tote system or permanent tank. The urea is temperature sensitive so may need to be heat traced if in an exposed generator room.

In this case, there was not enough room between the exhaust on the engine and where the exhaust left the building. Therefore, the exhaust had to be turner so that is went south and then turned 180 deg north and went through the SCR and out the building. Back pressure was a big issue in this case.

Question: When it comes to generator sizing tips, loading the system with the largest motors first, does this “first” mean before all other motors or all other smaller loads? For example, in a hospital project largest motors are typically part of the equipment branch and won’t be loaded until after life sand critical loads are connected.

Jensen: This is correct. In a hospital application, it may not be applicable to load the largest motor loads first. This would be more of an option in an industrial plant where you can load the largest motors first so that the rest of your load does not see the large voltage dip when the motors are added to the system.

Question: Can you explain wet stacking?

CSE editors: Read this article, “A close look at wet stacking” for a basic understanding.

Author Bio: Danna Jensen is one of the founding principals at Certus. She has extensive experience in the design of safe, reliable and efficient electrical systems for complex health care projects. Jensen is a member of the Consulting-Specifying Engineer editorial advisory board.