This multi-part article is designed to help engineers, contractors, and other specifiers understand the stringent requirements for hospitals—as people’s lives are on the line when the power goes out. In Part 1: Hospital Emergency Power Design, Generac addressed Circuit Requirements and Selective Coordination. In Part 2, Generac will highlight Separation of Circuits and Disconnect at Point of Entry.
Separation of Circuits
Separation of circuits is the concept in the National Electric Code that prevents a critical failure in one circuit from impacting others. This is implemented through separation of wiring and separation of components. The wiring from the load side of an emergency transfer switch must be kept completely isolated from other load circuits. This is covered in NEC 700.10(B) which requires the wiring from the emergency source or emergency source distribution overcurrent protection to emergency loads to be kept entirely independent of all other wiring and equipment. Exceptions are provided for wiring inside a transfer switch, exit lights, emergency luminaires, and unit equipment.
What has been somewhat confusing over the various code cycles is the concept of the separation of equipment. It has always been well understood that the transfer switches for emergency and non-emergency circuits needed to be isolated into separate transfer equipment. What was less clear was the separation of the circuit breakers feeding these transfer switches from the emergency side. The NEC clarified this with NEC 701(B)(5) that required these breakers to be in “separate vertical sections”. This is illustrated in the diagram below.
NEC 700.10(B)(5) clarified that the common bus feeding this vertical section could be fed from a common generator feeder. This is a fairly typical configuration for a single generator application. The generator feeder may be a main lug configuration without a generator breaker or it could be fed from a generator breaker, provided the breaker is selectively coordinated. NEC 700.10(B)(5)(d) also clarified that the emergency distribution equipment can be fed with multiple feeders, which is the case when parallel generation is utilized.
What is not clearly identified within the NEC is the requirements for separation of circuits within the generator connection box. NEC 700.10 makes it very clear that external to the generator each circuit type must be isolated. But what about applications that utilize multiple breakers within the generator? This is very common for smaller applications that have only 2 or 3 transfer switches. This ultimately is an Authorities Having Jurisdiction (AHJ) interpretation, but Generac believes good generator design should extend separation of circuits within the scope of the generator connection box. Generac utilizes separate connection boxes for each breaker to ensure the most stringent interpretation of separation of circuits is met.
Disconnect at Point of Entry
The power conductors from the generator are classified as a feeder within NEC 100. Often times the generator is located outside; thus, creating an outdoor feeder entering the building. NEC 225.31 requires these feeders to include a disconnect and be located at the point of building entry (NEC 225.32). NEC 700.12(B)(6) allows this disconnect to be located at the generator provided the disconnect is readily assessable and within sight of the building. Given that the generator breaker(s) are generally within the generator enclosure and not visible, most applications should require an additional disconnect at point of entry. One will see the implementation of this to be extremely varied within the market. Many markets accept the close proximity of the generator and its easy accessibility as sufficient to meet the intent of the code for feeder disconnect. In essence, the AHJ may consider the internal generator disconnect(s) much more accessible than a feeder breaker secured inside another building. This easy accessibility meets the local markets interpretation of the codes intent. Please coordinate with your local market AHJ to ensure requirements for disconnect are being met, whether it is a breaker integral to the generator assembly or a disconnect at point of building entry.
NEC 700.12(B)(6) does specifically allow the generator disconnect to be not visible when specific conditions are met: “For installations under single management, where conditions of maintenance and supervision ensure that only qualified persons will monitor and service the installation and where documented safe switching procedures are established and maintained for its connection, the generator set disconnecting means shall not be required to be located within sight of the building or structure served”.
NEC 225.36 traditionally required the disconnect at point of entry to be service entrance rated which requires overcurrent protection. This section was revised for the 2014 Code. The disconnecting means is now only required to be suitable for use as service equipment when the bonded neutral is also used as the return path for ground-fault current. This is the case for a non-separately derived (3-pole transfer switch) configurations. Configuring the system as a separately derived system (4-pole transfer switches), would allow the disconnect at point of entry to not include overcurrent protection. This is potentially helpful in providing additional design choices in supporting selective coordination design challenges.
The implementation of generators to hospitals applications involves many design considerations that will continue to be discussed as this multipart series continues.
If you would like more information about emergency power design considerations, contact your local Generac Industrial Power Distributor/Dealer. You may also call 1-844-ASK-GNRC or email ASKGNRC@generac.com.