Selective Coordination Compliance –
Methods for Evaluation and Mitigation
Dave Bradley, PE, LEED AP
August 8, 2008
New requirements for Selective Coordination became a part of the National Electrical Code® in 2005.
Since then the topic has been often misunderstood, avoided, discussed and debated. Selective
Coordination is an enforceable code requirement in many areas, and as such is also a design
requirement. This document will attempt to give the engineer and designer basic background,
methodology and mitigation methods to assist in designing to these new requirements.
BACKGROUND
What is selective coordination?
Selective coordination is a design requirement where only the overcurrent device immediately upstream
from the fault will operate, minimizing the outage caused by the fault. 2005 NEC® Article 100 defines
“Coordination (Selective)” as “Localization of an overcurrent condition to restrict outages to the circuit or
equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or
settings.” In the diagram below, only device C would open under the fault. For selective coordination,
the system would be designed so that for a fault of any magnitude (up to the available fault current at
device C) device C is guaranteed to clear the fault with neither B nor A opening.
What do the new code sections say?
In addition to the definition above, new requirements were added in the 2005 and 2008 National Electrical
Code to Article 700 - Emergency Systems (see 700.27), Article 701 - Legally Required Standby Systems
(see 701.18), and by the 2008 NEC to Article 708 - Critical Operations Power Systems (COPS) (see
708.54). These Articles also apply to Health Care Facilities as noted in 517.26. The new requirements
state that the overcurrent devices of those systems “shall be selectively coordinated with all supply side
overcurrent protective devices”. There is also a clarification on the scope of the required coordination for
700.27 in the 2008 National Electrical Code® Handbook – “This coordination must be carried through
each level of distribution that supplies power to the emergency system.”
What’s different from the way we’ve designed these systems for years?
Prior to 2005, selective coordination was only required for some elevator applications by Article 620. Until
the 2005 NEC, a properly coordinated system was generally regarded as ensuring the timely operation of
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overcurrent devices to assure safety and protection of equipment (using damage curves) while minimizing
nuisance trips. We’ve tried to minimize nuisance trips as much as possible by ensuring that there was no
overlap in coverage in the thermal region of the time-current curves, and by adjusting the instantaneous
range for as little overlap as possible. However, some overlap almost always existed and was accepted.
Most of us have heard of a nuisance trip or two in our careers, but probably not too many. To get two
breakers to open simultaneously (or have only the upstream breaker open), a very low impedance fault
would have to occur to supply a very high fault current. Low impedance faults almost always occur due to
bus or cable wiring mistakes at the time of construction or during a repair – these faults are quickly
identified and remedied. The vast majority of real world faults are higher impedance and generally do not
cause multiple breakers to operate. It is a safe bet that the building you are in right now has breakers
with overlap in the instantaneous range of their time-current curves, and that virtually all the buildings you
have designed in your career do as well.
However, now in order to comply with the NEC for Sections 700, 701 and 708, the engineer needs to be
able to prove, using curves or other manufacturer’s data, that there is NO overlap in curves at the
available fault current at that point in the system.
Fuse vs. circuit breakers?
Systems can, and are, being designed using either (or both) circuit breakers or fuses that selectively
coordinate. In some cases fuses may more easily selectively coordinate than circuit breakers, however
fuses still have inherent drawbacks that continue to make designing with circuit breakers the preferred
method. These drawbacks include increased training of owners to safely replace fuses, requirement for
spares and storage, single phasing, increased arc flash hazard and increased PPE requirements.
Because circuit breaker design is by far more common, this paper will address system design using circuit
breakers.
Is it being enforced?
There are several areas of the country that are currently enforcing selective coordination, and many more
areas that are not – these areas can begin active enforcement at any time, and it could start with your
project tomorrow. If you are in an area that has not yet begun enforcement but has adopted the
requirement, remember that the Engineer and his firm may still be liable if someone is injured due to a
selective coordination accident for the life of the building, even if the code inspector did not enforce the
requirement.
Applicable codes vary by location – be aware of the local requirements. Only areas that have adopted
the 2005 or 2008 versions of the NEC will be affected by the changes discussed in this paper. There are
also several municipalities, counties and states that have amended, or are in the process of amending,
code requirements to modify or eliminate the NEC selective coordination requirements. Examples of
proposed modifications include:
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requiring selective coordination only where determined “practicable” by the Engineer
not requiring selective coordination of time current curves below 0.1 seconds
eliminating the ability of the inspector to deny a Certificate of Occupancy based on noncompliance at the time of construction
This is a period of change and adjustment in the industry, so you should always consult the code
enforcement officer or authority having jurisdiction in the project location to make sure you are following
the correct standards.
What if I already call for coordination studies?
Coordination studies are performed after construction so that the actual equipment, fault current levels,
conductor lengths and trip settings can be used to evaluate the system coordination. In the past these
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studies have allowed for proper adjustment of the breakers to minimize nuisance trips and establish the
base system settings. However if the study indicates that there are areas that do not selectively
coordinate (which is almost assured if not designed properly), it is too late to fix easily. All the study can
do is to confirm that the system was designed in a non-compliant fashion. In many, if not most, cases
redesign and replacement of many system components (switchboards, panelboards, ATS, etc.) will be
required to meet NEC requirements and allow you to get a Certificate of Occupancy.
Can I require the manufacturer to provide selectively coordinated breakers in the project
specifications?
No, and for several important reasons.
There’s no such thing as a selectively coordinated breaker, only two breakers that selectively coordinate
at a given available fault current. In order to even evaluate if two breakers selectively coordinate, one
must know the available fault current at each applicable point in the system. If the Engineer has provided
the calculated available fault currents (not to be confused with AIC ratings) at each of the panels, it is
possible to evaluate the sizes he has scheduled. Without these values, no evaluation can be made.
If a breaker must increase in size to selectively coordinate (a very common and likely condition), a series
of additional changes must occur that are outside the scope and ability of the contractor or manufacturer
during the bidding period.
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The new breaker may not fit in the specified panel type and the new panel may not fit in the
space anticipated by the engineer. In some cases rooms may need to be enlarged.
The wiring and conduit size must change due to the increased breaker size – the increased
breaker size and the upsized conductor are design changes that must be approved by the
Engineer. Other components that are not supplied by the manufacturer may also be required to
change in size (automatic transfer switches, generators, etc.) – these changes and their possible
impact on space requirements can not be accounted for during the bidding period.
The increased conductor size requires recalculation of the available fault current, which in turn
may require another increase in breaker size – only the Engineer can do these design
calculations.
In the majority of projects the bidding package drawings and specifications are never seen by the
manufacturer. In most cases the contractor does the take-off and asks a distributor or
manufacturer for a price on the bill of materials the contractor created.
The Engineer can not delegate design responsibility to a contractor, distributor or manufacturer,
especially during the bid period. In most states, sealed drawings require that the Licensed
Engineer be in direct supervision of project designers and engineers that produced the contract
documents.
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NUTS & BOLTS
How do I evaluate a system for compliance with NEC?
Let’s take a look at a very simple system and evaluate it for compliance with 700.27.
The riser above shows a 480V system with an automatic transfer switch (ATS) and a 35kW natural gas
fueled generator. The main switchboard has a 2000A insulated case main circuit breaker, and has a 60A
molded case circuit breaker feeding the 60A ATS. The ATS feeds a 100A main lug only panelboard with
20A branch circuit breakers that feed egress lights, exit signs and the fire alarm panel. The building has
an occupancy that requires these emergency loads to meet the building code requirements, and the
community has adopted the 2005 National Electrical Code. The Engineer has calculated the available
fault current at the main switchboard to be 9,000A and the available fault current on the normal source at
the emergency branch panelboard to be 900A. This is a typical design that many of us have done in the
past (and may have out to bid right now). As shown, does this meet code? No. Let’s evaluate it and see
what has to change.
We have the essential first piece of information – the available fault currents computed by the Engineer
(remember, the Engineer of record is the only person who can issue these values for design purposes).
The first step is to evaluate the largest branch breaker in the emergency branch panel and the next
supply side breaker (the 60A feeder in the main switchboard). You need to evaluate the largest breaker
in the branch panel because that’s the worst case – it would be harder for the supply side breaker to
coordinate with a 90A breaker than a 20A. Since a 20A branch is the largest in our example, we’ll
compare the curves for that and the 60A feeder.
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We always evaluate breakers in pairs for selective coordination, and only look at the curve to the left of
the available fault current. Imagine taking scissors and cutting the right side of the paper off at the current
available at the downstream breaker – if the curves on the left side of the available fault current do not
overlap, the breakers selectively coordinate. The 20A (red) and 60A (orange) breaker curves shown
above clear each other in the entire thermal range (curved part), however the instantaneous ranges begin
to overlap at just under 600A. This makes sense, since a typical 60A molded case breaker will have an
instantaneous pickup about 10 to 12 times its frame rating minus a +/- manufacturing tolerance (the width
of the curve). Since we have 900A of available fault current in our example, we will need an upstream
breaker that does not trip on instantaneous at that value – the first breaker with an instantaneous pickup
(left edge of the vertical section) above 1000A is a 125A (see green curve below). The 125A breaker will
now selectively coordinate with the 20A branch at an available fault current value of 900A.
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A - Our first change is to replace the 60A feeder breaker with a 125A breaker (the red A refers to the
designation on Page 4). In this case, a 125A breaker will physically fit without problem in our
switchboard, but had this been a regular branch panelboard with a maximum breaker size of 100A, a
larger panel would have had to be installed.
The next breaker pair that must selectively coordinate is the new 125A breaker and the next supply
side overcurrent device, the 2000A main breaker. Comparing the curves below (125A – green,
2000A – blue), we see that these two breakers do selectively coordinate at 9,000A of available fault
current (no overlap to the left of the available fault current at the downstream device), so no
additional changes are needed. Note that these two curves actually do not overlap at any current,
so they would selectively coordinate in any situation where the available fault current didn’t exceed
their AIC ratings.
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B - There are now several things that must change due to increasing the size of that feeder breaker.
The branch panel, a 100A, must now be increased in size to at least a 125A. Make sure there is
space to mount this larger panelboard.
C - The 60A ATS must also be increased in size so its rating is at least 125A. Remember to check for
additional mounting space for the larger transfer switch.
D - The conductor and conduit size between the feeder breaker and the ATS, and between the ATS and
the branch panel must also change because of the increased size of the breaker. Now comes a fun
part – you must recalculate the available fault current at the branch panel because the conductor got
larger, then recheck the curves to make sure the 20A branch breaker still selectively coordinates
with the 125A feeder at the new, larger available fault current. Sometimes they will, sometimes they
won’t and you must upsize again. In our example they did coordinate so an additional increase is
not needed.
E - We have now modified the design to comply with code on the normal supply side. Now we must
evaluate the emergency source side. On emergency source, the next supply side overcurrent
device from the 20A branch is the 60A output circuit breaker at the generator. In most cases the
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manufacturer of this breaker will be unknown, but for this exercise we’ll assume it is the same as the
60A curves above. Will this 60A breaker selectively coordinate with the 20A? It all depends on the
available fault current, in this case the fault current delivered by the generator.
Since we need to evaluate these breakers in the instantaneous range of operation, we need to know
the fault current the generator can produce in the 1-6 cycle range. To find this we must know the
sub-transient reactance (not transient or steady state) of the alternator on the generator. This per
unit value, X’’d, is available from the generator manufacturers and varies a good bit between
alternators and generators. Once you obtain the value of X’’d, you divide the rated output current of
the generator by this value and you will have the sub-transient available fault current at the generator
output – from this you can determine the available fault current at the branch panel. In our example
(based on real values from an actual project) the generator provided 700A of fault current at the
panel. The 60A output breaker curve overlaps the 20A at this value, so we must increase the size of
the output breaker. The only problem is that the output breaker protects the alternator windings, so
the only apparent alternative is…
F - increase the size of the generator so that the output breaker can get larger. A larger generator has
a larger fault current and a different X’’d, and in the real world project a 50kW generator with its
larger output breaker finally did selectively coordinate with the 20A branches. Anything else that can
go wrong in the process of meeting code for this example? Remember to check to see if there was
enough natural gas pressure at the jobsite to increase from 35kW to 50kW! The example has now
been modified to meet National Electrical Code requirements.
This example was of a small project with a relatively small available fault current from the utility. If the
design were of a more complex system with a distribution level on the load side of the ATS, and 50,000A
of available fault current at the service entrance, and had the same emergency requirements (60A),
another selective coordination issue may appear. A large normal source available fault current can cause
the breakers to grow very quickly. If we had 5,000A at our example branch panel instead of 1,000A, the
feeder breaker would have grown from 125A (won’t trip on 1,000A instantaneous) to a 600A (won’t trip on
5,000A). There are methods given of reducing these “growing” breakers later in this paper, but many
breakers will still have to increase in size in systems with high available currents. If the output breaker of
the generator is less than twice the size of the largest downstream breaker (a reasonable ratio in many
cases to keep the curves from overlapping) it will probably not selectively coordinate. Even though the
output breaker only has to coordinate in the instantaneous range based on its own available fault current
producing ability (X’’d), it still has to selectively coordinate with the downstream breakers in the thermal
curve area. If we have a 125A breaker downstream of the 60A output breaker and we get a 70A fault, the
output breaker will trip before the downstream breaker and we lose selective coordination.
As pointed out by this example, the evaluation and correction of this design to meet code is something
that can only be reasonably done by the Engineer during the design phase. If this project were to bid with
the uncorrected drawings, and the specifications were written to require the manufacturer (or study
provider) to provide “selectively coordinated breakers”, one can see that it is just about impossible to
comply. If the project were to be built as shown and the code enforcement officer asked for proof of
compliance before a Certificate of Occupancy would be given, one can easily add up the extras and
liquidated damages resulting from making the changes A - F above in the field vs. on the drawing board.
If someone gives you assurances that their breakers selectively coordinate without having evaluated the
entire design with you, for the sake of your firm’s name, insurance, and your license, make sure that you
go through all the steps yourself before issuing drawings for bid.
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MITIGATION
What are some ways to design the system to meet code?
The best way to design for selective coordination may be to change how we approach the design from
the start. New ideas and methods will continue to appear as long as the requirements exist. There are
many complex applications, such as hospitals, that will truly challenge the engineer, but there are also
many smaller applications where mitigation is more easily achieved. Here are a few methods that may
help mitigate some of the problems associated with selective coordination:
Is selective coordination really needed? - Let’s look again at our example. Selective
coordination was required because of the presence of components covered under Section 700. If
I called for the installation of unit equipment to provide the minimum allowable egress illumination
and exit signage, and my fire alarm panel had an integral emergency power supply (as required
for listing), I could have bid the project as drawn with no requirement for selective coordination.
Here’s why – NEC 700.12 states in part “Unit equipment in accordance with 700.12(F) shall
satisfy the applicable requirements of this article.” For a simple installation such as our example,
installing unit equipment satisfies the code requirements – I can then install the generator and
transfer switch to feed any loads I wish, even additional egress lighting in excess of the bare
minimum provided by the unit equipment. This approach will not be reasonable for large projects,
but may be perfectly applicable to many smaller ones.
Minimize the emergency system. - Carefully evaluate the loads on the emergency ATS. Have
only the loads required by 700 and 701 (and 708 if applicable) on the emergency transfer switch.
Optional loads may then be supplied by a second ATS. Smaller loads with smaller overcurrent
devices are easier to selectively coordinate than larger ones.
Use less levels of distribution. - Fewer levels are far simpler to coordinate. The fewest is
achievable by feeding the ATS from a second main disconnect (one of the six allowable). That
provides the smallest load with the smallest conductor running the longest distance which
minimizes the available fault current to the emergency side and further facilitates selective
coordination.
Use “turned down” solid state circuit breakers. - Molded case circuit breakers are required
to have instantaneous devices, and in the majority of devices this pick-up level is 10-12 times the
breaker’s frame size. If you have a circumstance where you need a larger breaker for its higher
instantaneous pick-up but really want a smaller breaker, consider adjustable solid state breakers.
For example, 175A breaker will have an instantaneous curve where the leading edge (left side)
picks up at about 900A. As an alternative you can take a 600A solid state breaker with a 0.3x
long time setting (180A) and turn the instantaneous setting to maximum – the left edge is now at
5,000A.
Use insulated case circuit breakers for high fault current applications. - You may find that
fault currents are so high that coordinating the instantaneous band with molded case breakers is
impractical. Using an insulated case breaker (available in both switchboard and switchgear
versions) in many cases allows you to turn off the instantaneous range simplifying selective
coordination. These are larger, higher end breakers and should be evaluated by the Engineer for
practicality.
Use “special” breakers. - There are some circuit breakers whose curves have shorter
instantaneous “feet”, such as the Siemens NGB breaker. These make coordination easier than
standard breakers, but they must be specifically called for by the Engineer.
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Use transformation to reduce high available fault currents. - If you have a very high normal
source available fault current it can make selective coordination very difficult. Inserting an
isolation transformer or step down transformer will immediately limit the downstream available
fault current to a more manageable level. There is, however, a catch to using transformers in any
selective coordination application. The code required secondary overcurrent device for three phase
transformers is almost always too small to selectively coordinate with any other breakers in that
panel. For example, a 75kVA 480V transformer has a rated output of about 90A, an available
fault current at 4.5% of about 2,000A, and a maximum secondary overcurrent device of 125A.
Even if you use the actual primary available fault current to calculate the secondary fault current,
you still can’t selectively coordinate with a typical 20A branch breaker. This is a perfect
application for an adjustable solid state breaker for the secondary protection.
What information and aids are available from manufacturers?
There are aids available from all the manufacturers to assist the Engineer.
Curves - Most manufacturers provide time-current curves for all their breakers. Siemens
provides their curves in electronic form in its EasyTCC software available for free download on its
website – http://www2.sea.siemens.com/support/consulting-engineers (don’t forget the www2).
This software allows the user to overlay curves and adjust all settings for all Siemens circuit
breakers. The example curves in this paper were created using EasyTCC.
Charts - Charts are available which list, in varying styles, breakers that selectively coordinate
with other breakers at specific available fault current levels. An advantage of the charts is that
they are often more accurate than curve comparisons as they are based on more specific test
data derived for selective coordination. If there is disagreement between overlaid curves and
charts, the chart data should be used. These charts do not always list all breakers from the
manufacturer. Siemens breaker coordination charts are available for download on its website
and are included on the Siemens Design Assistant Disc.
Tools - Siemens has developed a Selective Coordination Tool (part of the Available Fault
Current Tool) to assist in designing code compliant systems. This tool provides very conservative
(and more likely to be competitively biddable) sizing for design. It is available for free download
on Siemens’ website and is included on the Siemens Design Assistant Disc.
What else should I be aware of as the Engineer?
The rules changed slightly from the 2005 to the 2008 NEC regarding overcurrent devices in series. It was
observed, and changed in 2008, that devices in series of the same ampere rating and the primary and
secondary overcurrent devices for transformer protection may not themselves selectively coordinate, but
do not affect the selective coordination of the system (the scope of the outage is not increased by the
upstream device or both devices operating). However, if your project is still governed by 2005 code make
sure that the AHJ will accept the variance if you use the 2008 rules.
Arc flash hazard is also a much discussed topic recently, and it should be. There are people injured each
day from arc flash incidents, including contractors, building occupants, and even consulting engineers.
Some important variables that determine arc flash energy are the voltage (can’t change that), the current
(can’t do much about that) and the duration of the flash (this we have some control over). In reducing arc
flash energy the object is to drive the overcurrent devices feeding the arc fault into their instantaneous
operation as soon as possible to more quickly extinguish the arc. High available fault currents are
actually better for this application because overcurrent devices can be driven into their instantaneous
ranges and clear more quickly. Even though it sounds backwards, low available fault currents will
actually increase the arc flash energies in many circumstances because overcurrent devices may stay in
the thermal range of operation allowing the arc to stay “lit” longer. Ironically, the same attributes that
make thermal magnetic breakers harder to apply in selective coordination applications can make them
more suited for arc flash hazard reduction. Accepted methods of reducing arc flash energy include
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lowering the instantaneous pick-up of adjustable breakers (moving the instantaneous band to the left) and
having overlap in the instantaneous ranges of multiple breakers. Contrast these goals to the goals of
selective coordination design – it mandates that there be no overlap in operation, and to do that the
engineer will deliberately keep the instantaneous settings as high as possible and limit the available fault
current in the system to facilitate coordination. Once again the Engineer must decide between the two
extremes of design – better coordination (higher pickup values, longer clearing times, less nuisance trips)
or greater safety (lower pickup values, shorter clearing times, more nuisance trips). In the case of
selective coordination, however, there is now a code requirement to bias the design in favor of
coordination.
What’s the conclusion?
Selective coordination is a requirement in most areas of the country for emergency systems, and barring
legislation that modifies or eliminates it, will be a requirement in the future. The only person who can plan
and design a system to comply with this requirement is the Engineer – responsibility for selecting
components can not be passed on to the contractor or supplier. There is technical help available to the
Engineer and designer. Use these early days of enforcement and compliance to change your design
approach and meet the required codes – do not let your firm be the first to be exposed to liability when a
code that has been in effect for the past several years begins to be enforced in your area tomorrow
morning.
National Electrical Code® and NEC® are registered trademarks of the National Fire Protection
Association.
© 2008 by Siemens Energy & Automation
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