he new requirements for selective coordination of
circuit were to expand and take down an entire panelboard.
emergency and legally required standby systems in
However, selective coordination won’t prevent the smaller­
Sections 700.18 and 701.27 of the 2005 National
scale problem from occurring, nor will it ensure that
Electrical Code (NEC) are challenging electrical
additional lives will be saved if an escalated problem was
engineers to intensively consider how systems respond to
prevented. Even with selectivity, it’s still possible for a large­
short circuits.
scale problem to occur, such as a feeder fault, and create the
Meeting this challenge raises questions regarding the
code requirements, how other aspects of system design are
same type of situation that would be mitigated by selective
coordination.
compromised and what constitutes compliance. While the
For a selectively coordinated emergency system to benefi t probability of owners benefi ting from selectively coordinated the occupants of a building other than a hospital, a connected
electrical systems is small, the likelihood of incurring
chain of events must occur:
signifi cant costs to ensure selective coordination is a certainty with the 2005 NEC.
As electrical engineers consider the impact of selective
coordination, there is a growing consensus that this is the
1. A building must be occupied.
2. There must be an event that mandates exiting the
building.
wrong direction for the NEC, and that this area of design
3. The event must disrupt a portion of the emergency
should be returned to engineering judgment. In fact, there
electrical system, in which no other life safety
were more than a dozen proposed changes to the 2008 NEC
support exists.
to delete these new requirements or better defi ne them. Unfortunately, the just-released NEC Report on Proposals
4. The disruption must be of a nature that it would
interrupt more of the electrical system than
showed that there were not enough Code Panel votes to return
would occur if the system were not selectively
this design area to engineering judgment. However, there is
coordinated. This type of event is usually an
an opportunity to address and respond to this issue, as NFPA
infrequent, high-level fault.
Code Panels will vote again in December and are accepting
public comments on these proposals until Oct. 20.
5. The extended disruption of the electrical system
must result in injury or death of an occupant.
The probability of all of the above occurring is miniscule.
Selective coordination might prevent a small-scale
If the fi rst four occur, it may still be possible to avoid injury problem from becoming larger — e.g., if a fault on a single or death through the use of alternative systems — e.g., using the normal lighting system to exit, using a redundant exit
or how to determine if overcurrent devices will selectively
path or exiting in the dark.
coordinate. The methodology for coordination studies is
An emergency system is most likely to fail under everyday
described in several publications such as ANSI/IEEE Std
operation. In this scenario, the building’s normal power 242, IEEE Recommended Practice for Protection, and IEEE
system is available to support alternatives, and an emergency
Coordination of Industrial and Commercial Power Systems.
system failure can be treated like the failure of any other
However, these publications are not acknowledged by the
circuit.
NEC.
Extending this logic, it becomes obvious that if selective
Determination of selective coordination is dependent on
coordination were needed, it would be required as much or
information provided by manufacturers, who describe the
more for normal systems than for emergency systems! If
operating characteristics of fuses and circuit breakers using
faults on non-selectively coordinated systems frequently
graphical curves that plot overcurrent performance data
created unwanted large outages, we would see it in our
vs. time curves (TCs). The de facto standard for TCs has
everyday lives; building blackouts would halt workplace the time axis beginning at 0.01 seconds. Most overcurrent
production and create unlimited safety hazards.
protective devices start to operate in time periods of less than
More than 99.9999% of electrical systems in existence
0.01 seconds, and all current-limiting fuses operate in less
today were designed using engineering judgment and do not
than 0.004 seconds. To address time periods of less than 0.01
meet the 2005 NEC’s strict defi nition of being selectively seconds, manufacturers have developed overcurrent device
coordinated; yet once energized, they operate with high coordination tables. There are no industry standards for the
reliability. While these systems are not selectively coordinated
test procedures that are used in developing these tables, and
for every fault condition, the vast majority will selectively
no guarantee that various manufacturers are developing these
coordinate for the most common types of faults, which are
selective coordination tables under similar circumstances.
low-level arcing faults.
The tables are often subject to disclaimers such as “reasonable expectation of selective coordination”. A selective coordination standard would need to be
Demonstrating compliance with the code places engineers
similar to the series rating portion of UL 489. A series-rated
and inspectors in somewhat of an untenable position. The NEC
circuit breaker can be applied at fault current levels that are
gives exact requirements for determining loads, sizing wires
above its listed rating if used with a tested upstream breaker.
and overcurrent devices, and many other aspects of design,
When applied in the fi eld, series­rated circuit breakers are but it does not describe how to perform a coordination study
specifi cally labeled and referenced to an upstream protective device.
Two Design Methods
Eventually, the NEC
will have to include similar
requirements
for
selectively
coordinated devices and require
testing per standard criteria. Until
appropriate standards are in place,
engineers cannot ascertain if a
system is selectively coordinated.
Figure 1. Method A will not selectively coordinate for high-level faults that rarely occur.
Method B will selectively coordinate, subject to manufacturer verifi cation.
If testing standards for selective
coordination
are
developed,
the public would benefi t from Page 2
the assurance of independent verifi cation of selectively upstream equipment to pass through more energy
coordinated systems.
while waiting for downstream equipment to operate.
Manufacturers could submit their
devices to a testing laboratory, which would subsequently
verify selective coordination in accordance with the
standard.
Unfortunately, the quantity of devices combined with
performance variations at different fault levels leads to a
huge number of permutations and associated testing expense.
Manufacturers would not likely submit their devices for
verifi cation of selective coordination with the devices of other manufacturers. Accordingly, it may be necessary for
all overcurrent protective devices in a selectively coordinated
system to be made by a single manufacturer.
Facility owners are often locked into proprietary systems
that can only be modifi ed by using the products of the original manufacturer. Fire alarm systems are a good example. The
listing or communication protocol requirements for fi re alarm systems force the entire system in a building to be made by a
single manufacturer.
For the most part, once you get outside of a single
panelboard or switchboard, electrical power systems are
• Larger equipment will increase the maintenance
requirements.
• Multiple panels will increasingly be strung together
to achieve selectivity ratios.
• If circuit breakers are used, arc­fl ash hazards will increase, and automatic transfer switches may be
forced to withstand fault currents longer than the
three-cycle duration required by UL Standard 1008.
• Introducing additional impedance into a system helps
circuit breakers to selectively coordinate. Some
undesirable methods of achieving this include higher
impedance transformers or current-limiting reactors
that increase energy losses; extending circuit runs longer than is necessary; and forgoing spare capacity to minimize wire size.
• If fuses are used where circuit breakers may have
been the original choice, the advantages of circuit
breakers are lost, including rapid resetting after
a known overload trip; compact size; prevention of single phasing; and no risk of incorrect fuse or unavailable fuse replacements.
being an exception. However, this may change due to further
• Selective coordination increases the cost of electrical
systems.
developments in selective coordination.
For expansions,
Selective coordination of systems with ground fault
renovations and similar changes to an existing system
protection is very problematic. Ground fault protection is set
where selective coordination is mandated, it’s quite possible below the rating of its associated overcurrent device, which
that a manufacturer can now — or in the near future — set often makes it impossible to maintain trip ratios that permit
pricing with limited or no competition. Competition would
selective coordination with downstream devices.
immune from proprietary restrictions, with series ratings
only exist if a complete change-out was an alternative. If a
proprietary bid was set for the emergency system, it would
also be possible for a manufacturer to extend additional costs
Selectively coordinated systems will benefi t building onto the normal power system as part of a packaged system
occupants only under a narrow set of circumstances for which
quote.
there is no record of any signifi cant occurrences. While the NEC requirements for selective coordination are clear, the
methods of compliance are vague, and the groundwork has
When one aspect of a design becomes an absolute
been set to further limit competition in the industry. Prior
requirement, compromises are often made to other aspects
to 2005, the benefi ts of selective coordination could be of the design. Ideally, engineers could simply improve a
compared against other safety and performance aspects of an
system. However, it’s likely that the following compromises electrical system. With the 2005 NEC, selective coordination
will arise with selectively coordinated systems:
• Electrical equipment will increase in size, either
to meet a manufacturer’s minimum ratios between upstream and downstream equipment or to allow
takes precedence and overrides engineering judgment.
To respond to the NFPA proposals on selective
Page 3
coordination of legally-required
Sample Space Requirements
standby systems. At this time
the proposal is being rejected
by Code Panel 13, and selective
coordination of legally required
standby systems will be retained
in the 2008 NEC.
Whether you’re for or against selective coordination,
you
can
provide
useful
feedback to the appropriate
Code Panels by commenting
on the above proposals. Even
if the Code Panel is supporting
Figure 2. All four layouts show a 75-kVA transformer and a 225-amp branch circuit panelboard. The two layouts on
the left represent what most designs would look like prior to meeting the selective coordination criteria. The two
layouts on the right represent selectively coordinated designs.
your viewpoint at this time,
they would still like to hear
your input. Of particular value
coordination for the 2008 NEC, visit the www.NFPA.org.
to the Code Panels is verifi able information that identifi es The deadline for comments is Oct. 20, 2006.
specifi c instances where selective coordination has or has A summary of key proposals is as follows:
Proposal 13-77. This proposal adds selective coordination
not been a problem. Inadequate ground fault settings are the
to fi re pump feeders as part of a proposed alignment with a most common problem and should be distinguished from
draft of NFPA 20. At this time it is being rejected by NEC
other problems.
Code Panel 13.
Proposal 13-135 (also see Proposal 13-137). This proposal
was to delete Section 700.27 and the requirements for
selective coordination from the 2008 NEC. At this time the
Jim Degnan, P.E., LEED®AP is
proposal is being rejected by NEC Code Panel 13. Unless
principal at Sparling, an electrical
the panel receives comments that substantiate a reversal,
engineering and technology
selectivity will be retained for the 2008 NEC. Code Panel
consulting fi rm with offi ces in Seattle 13 refers to their statement in Proposal 13-135 for all other
and Portland. He can be reached at
proposals regarding article 700.27.
jdegnan@sparling.com.
Proposal 13-139 (also see Proposals 13-146). This proposal
would keep selective coordination in the NEC but only require
it for specifi c types of faults, most notably high­resistance faults, which are the most common type.
If accepted,
the concerns listed in this article regarding verifi cation, competition and design consequences would be signifi cantly relieved. At this time the proposal is being rejected by Code
Panel 13.
Proposal 13-159. Similar to proposal 13-135, this proposal
calls for removal of Section 701.27, which calls for selective
Page 4