NFPA 110: Standard for Emergency and Standby Power Systems extended Q&A

Recent Consulting-Specifying Engineer webcast presenters Tom Divine, PE, Project Manager, Smith Seckman Reid Inc., and Kenneth Kutsmeda, PE, LEED AP, Jacobs Engineering, answer reader questions about what new code requirements will mean for consulting engineers.


Tom Divine, PE, Project Manager, Smith Seckman Reid Inc., and Kenneth Kutsmeda, PE, LEED AP, Jacobs EngineeringQ: When is it required to have a 3 pole and 4 pole ATS?

  • Tom Divine: When the generator is a separately-derived system, a 4-pole ATS is required for circuits with a neutral.  If the circuit has no neutral, a 3-pole ATS will suffice.  The purpose of the requirement is to avoid multiple connections between neutral and ground, which could provide alternate paths for neutral current, and cause improper operation of ground fault protection for equipment.

Q: With respect to breaker coordination; what are the preferred electronic trips: LI, LS/I, LSI, or LSIG?

  • Kenneth Kutsmeda: LSI or LSIG are preferred because they provide the most flexibility when trying to coordinate.  If you are trying to serve Article 700- or Article 708-type loads, pay careful attention to the selective coordination requirement when selecting trip units.

Q: Do you recommend dual hot-standby control PLCs when installing parallel gensets?

  • Kutsmeda: Yes, if the project can afford the additional cost, I would recommend dual control PLCs.  This prevents the PLC from becoming a single point of failure.

Q: Are schools considered to be Level 1 or Level 2 facilities? Typically, Level 1 is needed for health care or mission critical facilities only.

  • Divine: “Level 1” describes a system whose failure could result in death or serious injury to a person.  That might apply to a mission critical facility, or might not.  It definitely applies to any facility that provides critical care.  Intuitively, I’d expect a school to be a Level 2 facility, because it doesn’t provide services whose sudden failure would leave someone exposed to injury.  I’d recommend getting the opinion of all the AHJ’s before proceeding with a design, though.  It’s not impossible to make a case that the failure of an emergency system during an evacuation event could be catastrophic, and an AHJ could take the view that it’s Level 1.

Q: What is the intent/purpose of physically separating the critical branch and life safety branches? Can one have a detrimental impact on the other?

  • Kutsmeda: The separation is mostly to protect against damage and/or a cable fire. You do not want a cable fire on a general circuit taking out a critical circuit.

Q: Is mineral-insulated (MI) cable needed for optional standby generator feeder circuits?

  • Divine: MI cable isn’t required by the NFPA codes.  It’s one of the methods of complying with requirements for fire rating of emergency feeders in NEC 700.9.  It’s my understanding that some jurisdictions require it for certain applications, but that requirement doesn’t arise from NFPA 99 or NFPA 110, or from the NEC.

Q: Does the 10-second power restoration start time apply to the main service or to the end use device?

  • Kutsmeda: The NEC states the current supply to emergency lighting and power shall be available with time required for the application but not to exceed 10 seconds.  In my experience the AHJ has interpreted that to mean power/current available at the end use device.

Q: Does the emergency system include conductors and other equipment upstream from an ATS on the normal side of the ATS?

  • Kutsmeda: No, the emergency system does not include equipment or feeders upstream from the ATS on the normal side.

Q: Should the emergency service switchboard that feeds the ATSs be in a separate room from the room that contains the ATSs?

  • Divine: It doesn’t have to be.  NFPA 110 requires that the EPS, which would include generators and paralleling gear, to be installed in a separate room for Level-1 systems.  But it specifically permits EPSS equipment and transfer switches, to be installed in that room.

Q: If the only important issue is egress lighting, can that be addressed with battery lights, and if so, can my system be standby only?

  • Kutsmeda: Yes, the system can be standby rated if the lighting is addressed via individual batteries or a central inverter system.  This is quite common with paralleled systems that can’t meet the 10 second requirement.

Q: When referring to the NEC as related to the EPSS, do you need to also consider article 695 elevators and 517 hospitals for additional requirements?

  • Kutsmeda: Yes, there are specific requirements in both of those sections for EPSS-type systems.

Q: How long does the crank test need to be done on the generator?

  • Divine: That requirement appears in in both the 2010 and 2013 editions of NFPA 110.  They reference the crank and rest cycles described in, which calls for 15 seconds of cranking, followed by 15 seconds of rest, repeated three times.

Q: Is the use of closed transition transfer switches recommended for Level 1 facilities? Any issues with using closed transition switches?

  • Kutsmeda: Closed transition is not a requirement.  It is recommended for those systems that do not want to take another outage transferring back to utility.  For example, power systems that serve life support or surgery type equipment may want to consider closed transition.  Many mission critical type facilities use closed transition to prevent the mechanical systems from shutting down during the transfer back to utility (power backup by UPS).  Closed transition switches need to have some type of synch check to prevent closing two sources out of phase and protection to prevent back-feeding the utility.

Q: What are the requirements for the life safety and critical branch systems when the building—specifically a skilled nursing facility—has a full building backup generator? Do you still require separate ATSs, even though we have separate panels and wiring within the building? Contractors feel this is not needed.

  • Divine: Articles 517.40 through 517.44 describe requirements for “nursing homes and limited care facilities.”  Article 517.41(B) requires a separate transfer switch for each branch, unless the total load is below 150 kVA.  There’s no exception for a full building backup, and there are requirements for load to be staged onto the system.

Q: If I have a system per Article 701 and Article 702 application, what branches do the battery charger, fuel pump, and dampers get connected to?

  • Kutsmeda: Article 701.  The equipment required to operate the generator shall be connected to the highest level for which that generator serves.

Q: If a hospital is to be 100% backed up by Generators, is it a violation of NEC 517.30(B)(4)-Transfer Switches to eliminate all transfer switches by providing Medium Voltage Generators and tie them thru a paralleling gear to a Medium Voltage Distribution Switchgear at the Central Utility Plant? Transfer of power will be at the Medium Voltage switchgear.

  • Divine: Presuming a load of more than 150 kVA, this scheme doesn’t meet the black-letter requirements of 517.30(B)(4), which calls for separate transfer switches for the emergency branches and the equipment system.  I don’t see that anything in the system can arrange for delayed connection of the equipment system, as required in 517.34.  Finally, this scheme will run afoul of NFPA 110 6.1.6, 2013 edition, which requires transfer switches to be listed assemblies where available, and permits medium-voltage transfer via interlocked circuit breakers for only mechanical and central plant loads. 

Q: Could the 517.34 system be considered as a 701 system?

  • Kutsmeda: Yes, the loads identified in Article 517.34 Section (A) could be considered as a 701 system.  Loads identified in 517.34 Section (B) would be considered as a 702 system because they are not required to be automatic.

Q: To test under on-site or portable load bank, do facilities have a TESTING transfer switch to connect the load bank to the system and provide that auto removal of load bank if utility power fails during load bank testing?

  • Divine: I’ve never seen that implementation.  The primary function of a transfer switch is to keep load energized.  There’s nothing special about a transfer switch as a means of de-energizing something like a load bank.  Typically, I’ve seen a dedicated breaker in the generator distribution switchgear for that purpose, or sometimes an outdoor disconnect switch connected to such a breaker.  The requirement is that equivalent loads—the load bank—be replaced with emergency loads—the facility’s load—if the primary source fails.  That might be done with a control relay that trips the load bank if normal power fails, or it might be done by connecting the emergency load as part of the test, and supplementing with a load bank.

Q: Can you combine Article 700-type loads and Article 701-type loads within the same switchgear and on the same OCPD of a generator?

  • Kutsmeda: You can combine 700 and 701 type loads on the same switchgear with a common OCPD on the output of the generator.  The 700 and 701 loads must be on separate feeders with separate OCPD on each feeder and the OCPD for the 700 type loads must be located in separate vertical sections of the switchgear.

Q: While performing the 2 hour load test per, I assume the generator OEM factory can perform only a 2 hour reactive load test at rated power factor to comply with, correct?

  • Divine: NFPA says that the acceptance test can be performed at unity power factor using a strictly resistive load bank\ if the manufacturer has tested the generators at rated load and rated power factor.  

- Edited by Jessica DuBois-Maahs, associate content manager, CFE Media, jdmaahs(a)

ROMAN , Non-US/Not Applicable, Poland, 06/05/14 10:17 AM:

Very informative.
Anonymous , 10/20/14 09:18 AM:

I have a followup question regarding ATSes in a separate room. Are Level 1 ATSes required to be in a separate room from the normal power switchboard? I understand that per NFPA 110, EPS shall be in a separate room and EPSS is permitted to be installed in the EPS room... but is EPSS required to be separate from the normal side?
LEONILO , MD, United States, 09/09/15 03:46 PM:

I have a followup question regarding ATSes in a separate room. Are Level 1 ATSes required to be in a separate room from the normal power switchboard? I understand that per NFPA 110, EPS shall be in a separate room and EPSS is permitted to be installed in the EPS room... but is EPSS required to be separate from the normal side?
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