Author(s): Jeff Sargent. Published on March 1, 2013.

Electrical Safety, by Jeffrey Sargent

The Source, Part II
The heart of an emergency power supply system

NFPA Journal®, March/April 2013 

In my last column, I pointed out that the heart of an emergency power supply system is the emergency power source (EPS). Requirements covering the installation and operation of the EPS are found in the National Electrical Code® (NEC®) and, depending on the type of EPS, in either NFPA 110, Emergency and Standby Power Systems, or NFPA 111, Stored Electrical Energy Emergency and Standby Power Systems.

Electrical Safety

FROM THE ARCHIVES

January - February 2013
A feature of any emergency power system is a standby power source

November - December 2012
A proposal for 2014 NEC fine-tunes the requirements for clear work space 

September - October 2012
The 2014 NEC considers electrical design for people with disabilities

July - August 2012
Part II of a review of proposed articles for the 2014 edition of the NEC®

May - June 2012
A closer look at a new proposal for the 2014 National Electrical Code

March - April 2012
New certifications for electrical workers and supervisors

Section 700.12 of the 2011 edition of the NEC provides emergency power supply options, including storage batteries, generator sets, uninterruptible power supplies (UPSs), separate utility services, fuel cells, and unit equipment. A small building or occupancy may opt for a simple emergency system composed of self-contained unit equipment, where emergency illumination of the means of egress and exits are the only electrical loads that require a normal and alternate source of electric power. Larger buildings such as high-rises or theaters have additional emergency power needs and require a higher capacity source. 

NEC Section 700.4(B) permits a single generator, or a set of generators connected in parallel, to be used for multiple purposes, provided the loads that have been designated as emergency by the applicable code or authority having jurisdiction take precedence over those connected for business continuity or comfort. This requirement can be accomplished by either sizing the generator to carry all the emergency, legally required standby and optional standby loads that will be connected and operating concurrently, or by installing equipment that selects and sheds loads based on priority. 

NFPA 110 provides sizing and performance requirements for the emergency system, including the EPS itself. Section 4.1 states that the system “shall provide a source of electrical power of required capacity, reliability, and quality to loads for a length of time as specified in Table 4.1(a) and within a specified time following loss or failure of the normal power supply as specified in Table 4.1(b)”.  

These two tables cover the classification and type of emergency system. NFPA 110 does not assign the classification and type for specific occupancies; that is a function of occupancy or equipment-based codes and standards such as NFPA 20, NFPA 99, NFPA 101®, and NFPA 5000®.

For example, paragraph 7.9.2.2 of NFPA 101 requires newly installed emergency systems that supply emergency lighting to be not less than a “Type 10, Class 1.5, Level 1 system.” Table 4.1(a) provides the emergency system classification. The emergency power supply system (EPSS) classification is simply the minimum amount of time the system is to be in operation to supply the emergency loads.
 System classification directly affects the required capacity for the fuel supply. Table 4.1(b) assigns the maximum time delay between loss of normal power and the restoration of power to the load terminals of the transfer equipment. The system level relates to NFPA 110 requirements covering the installation, performance, and maintenance of the emergency system.

NFPA 110 covers two levels of those systems, with Level 1 systems providing building safety functions critical to the safety of life and thus requiring a higher standard of installation, performance, and maintenance requirements. Paragraph 7.9.2.2 of NFPA 101 calls for a Level 1 system, with a minimum operational time of 90 minutes and a maximum time delay of 10 seconds for restoration of power.

Because these EPSS designations are used in other NFPA codes and standards in addition to NFPA 101, familiarity with Chapter 4 of NFPA 110 is necessary to provide the required level, class, and type of emergency system.


Jeffrey Sargent is a regional electrical code specialist for NFPA.