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Figure 01
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For EC&M Magazine
By Mike Holt, NEC® Consultant
Do you know how the NEC requirements differ for the three kinds of standby power systems?
In the hierarchy of electrical systems, emergency systems (Article 700) receive first priority. Legally required standby systems (Article 701) take the number two spot, followed by optional standby systems (Article 702).
Article 700 applies to systems or equipment required to protect people who are in an emergency and trying to get out.
Article 701 applies to systems or equipment needed to aid the people responding to the emergency. For example, Article 700 lighting provides an exit path. But Article 701 might control the elevator used by fire fighters to reach the applicable floor.
Legally required standby systems must supply standby power in 60 seconds or less after a power loss [701.12], instead of the 10 seconds or less required for emergency power systems [700.12]. The time limit for an optional standby system to kick in is typically determined by a business risk assessment.
Emergency power systems are generally installed to provide illumination for safe exiting and for panic control in buildings subject to occupancy by large numbers of persons, such as hotels, theaters, sports arenas, and health care facilities. These systems may also provide power to maintain life, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where interruption would produce serious life safety or health hazards, and similar functions.
Legally required standby systems provide electric power to aid in firefighting, rescue operations, control of health hazards, and similar operations. These systems typically supply loads such as heating and refrigeration, communications systems, ventilation and smoke removal systems, sewage disposal, lighting systems, and industrial processes that, when stopped, could create hazards, or hamper rescue or firefighting operations.
Optional standby systems protect public or private facilities or property where life safety doesn't depend on the system. These systems aren't required for rescue operations. You'll see these systems in facilities where loss of power can cause economic loss or business interruptions. A data center, for example, can lose millions of dollars from a single minute of lost power.
Tests and maintenance
These systems must be tested and maintained.
- Emergency system. The authority having jurisdiction (AHJ) must conduct or witness a system acceptance test [700.3(A)]. Thereafter, the system must be periodically tested [700.3(B)]. It must be maintained per manufacturer instructions and industry standards. And a written record must be kept of all required tests and maintenance [700.3(D)].
- Legally required system. The emergency system requirements also apply here [701.3].
- Optional standby system. The NEC doesn't provide maintenance requirements for standby systems. But failure to properly maintain such a system will probably mean a severe financial loss when the system fails to support the load.
Capacity
Emergency system. Must have adequate capacity to carry all emergency loads expected to operate simultaneously [700.4(A)]. If an alternate power supply has adequate capacity, it may supply emergency loads [700], legally required standby loads [701], and optional standby system loads [702].
If the alternate power supply lacks adequate capacity to carry the entire load, it must have automatic selective load pickup and load shedding to ensure adequate power in this order of priority: Emergency, Legally required, Optional standby [700.4(B)].
Legally required system. Must have adequate capacity to carry all loads expected to operate simultaneously [701.4]. Its alternate power supply can supply legally required standby and optional standby system loads if:
(1) The alternate power supply has capacity to handle all connected loads.
(2) There's automatic selective load pickup and load shedding to ensure adequate power to the legally required standby circuits.
Optional standby system. Must be suitable for the maximum available short circuit current at its terminals [702.4(A)]. Its power source must have capacity to supply the full load transferred. The calculated load must be per Article 220 or by another method approved by the AHJ [702.4(B)]. When a manual transfer switch is used, the system user selects the loads to be connected to the system and that determines the system kVA/kW rating.
Transfer equipment
With all three types of systems:
- The short circuit current rating of the transfer equipment must be field marked on the exterior of the transfer equipment (new with 2017 NEC).
- Transfer equipment must be automatic and be installed to prevent the inadvertent interconnection of normal and alternate sources of supply [700.5, 701.5, 702.5].
Transfer equipment for emergency systems must supply only emergency loads. Figure 01
Surge protection
A listed SPD must be installed for all emergency system panelboards and switchboards [700.8]. The NEC doesn't require this for the other two types of systems, but it almost always makes financial sense to do it.
Circuit wiring
The requirements are in Part II of the respective article. For legally required and optional standby systems, the system wiring can occupy the same raceways, cables, boxes, and cabinets with other general wiring [701.10, 702.10].
A further requirement for optional standby systems pertains to generators [702.12]. If you have a portable generator rated:
- Greater than 15 kW outdoors and it's equipped with a readily accessible disconnect within sight of the building, you don't need an additional disconnect for the generator feeder conductors that serve or pass through the building. Same for permanently installed.
- 15 KW or less and it's installed using a flanged inlet or other cord-and-plug type connection, you don't need an additional disconnect for the building.‚
For emergency systems, the requirements are much greater. Three highlights [700.10]:
1. Emergency circuits must be permanently marked as components of an emergency circuit or system so they'll be readily identified.
2. All wiring to emergency loads must be kept independent of other wiring (unless permitted in 700.10(B)(1) through (5), including wiring in transfer equipment and luminaires supplied from two sources)..
3. Emergency wiring circuits must be designed and located to minimize the hazards that might cause failure due to adverse conditions (e.g., flooding, fire, icing, etc.).
This is where the requirements for optional standby systems end. But emergency and legally required systems have more.
Power sources
Part III of each Article then lists which power sources are permitted and what the requirements are for each source. One difference is Article 701 lists connection ahead of service [701(E)] while Article 700 does not. There aren't many other differences.
Selective coordination
Overcurrent protection devices for emergency power systems must be selectively coordinated with all supply-side overcurrent protection devices [700.32]. The design must be made by an engineer or similarly qualified person. It must be documented and made available to those authorized to design, install, inspect, maintain, and operate the system [700.32]. Same thing applies to legally required standby systems [701.27].
At this point, the requirements for legally required standby systems end. But there are more requirements for emergency standby systems.
Circuits for lighting and power
Emergency circuits must supply only emergency loads [700.15].
Emergency lighting must include egress lighting, illuminated exit signs, and all other luminaires specified to provide the required illumination [700.16].
Emergency lighting systems must be designed and installed so the failure of any individual lighting element (e.g., a lamp) won't leave dark any space that requires emergency illumination. Consequently, a minimum of two lighting heads is required. This is why an individual emergency lighting unit (sometimes called a lunchbox) always has two lighting heads.
When an emergency system is installed, emergency illumination must illuminate the service or building disconnect if the disconnect is indoors.
Multiwire branch circuits aren't allowed for emergency systems [700.19]. Emergency lighting supplied by branch circuits rated not greater than 20A can use a listed branch circuit emergency lighting transfer switch [700.25].
Different functions, different priorities
Among these three systems, many of the requirements are the same. Yet legally required systems have more requirements than optional systems, and emergency systems have more requirements than legally required systems.
Because emergency standby systems protect life, the NEC gives priority to these systems over the other two. And the legally required systems take priority over the optional ones regardless of other considerations. Understanding this hierarchy will help you prevent mistakes in applying this trio of articles.
None of this is to say optional systems are unimportant. Or actually optional, from a business standpoint. Data centers make theirs extremely robust. At the same time, data centers meet their NEC obligations for the other two types of systems.
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