One of the more significant and controversial changes to makes its way into the 2005 NEC can be found in 240.86 — Series Ratings. Throughout the 2005 Code cycle process, numerous proposals and comments were brought forth from all corners of the industry regarding the change, and some people, associations, and organizations debated it until the bitter end during the Code Panel Reports at the necforum last spring in Salt Lake City. In fact, it was a floor action at the meeting that ratified the change.
The broad change is intended to provide a new method for determining the combination series rating of overcurrent devices in existing installations for electrical overcurrent devices that haven't been evaluated or tested as a unit for a given application. The concern among those in favor of the change seemed to be the use of circuit breakers to interrupt current at levels above the circuit breaker's interrupting rating.
The revision of 240.86. Although it hasn't been completely rewritten, 240.86 underwent considerable updating. The new and improved section reads as follows:
“240.86 Series Ratings. Where a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected on the load side of an acceptable overcurrent protective device having a higher rating, the circuit breaker shall meet the requirements specified in (A) or (B), and in (C).
“(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer. This documentation shall be available to those authorized to design, install, inspect, maintain, and operate the system. This series combination rating, including identification of the upstream device, shall be field marked on the end use equipment.
“(B) Tested Combinations. The combination of line-side overcurrent device and load-side circuit breaker(s) is tested and marked on the end use equipment, such as switchboards and panelboards.
“(C) Motor Contribution. Series rating shall not be used where
“(1) Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and
“(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.”
Generally speaking, a circuit breaker shouldn't be applied where the available short-circuit current at its line side terminals exceeds the circuit breaker's interrupting rating. This is a requirement of 110.9, which requires overcurrent protective devices to have an interrupting rating sufficient for the nominal circuit voltage and the available (short-circuit) current at the line terminals of the device.
Interrupting rating is defined as the highest current at rated voltage that a device is intended to interrupt under standard test conditions. Where fuses or circuit breakers that don't have adequate interrupting ratings are misapplied, they can rupture violently and damage equipment. In addition, such an event can pose health hazards to personnel, such as damage to eyesight and hearing, as well as burns from flammable clothing.
Sec. 240.86 does have provisions for fuses or circuit breakers to protect downstream circuit breakers where the available short-circuit current exceeds the downstream circuit breaker interrupting rating. This is known as a series-rated combination, series rating, or series combination rating. The application of series ratings does have technical limitations, as well as additional NEC requirements that must be complied with for proper application. Series-rated combinations used as permitted in 240.86 should be used sparingly.
Lighting and appliance branch-circuit panelboards are common applications of series-rated combinations. They're also appropriate for existing installations in cases where purchasing new electrical equipment as fully rated systems isn't economically feasible.
Series-rated systems for existing facilities. Series-rated systems in existing facilities are covered by 240.86(A). Because building electrical systems may change — sometimes drastically — after the initial installation, it isn't uncommon for the increased available short-circuit current to exceed the interrupting rating of existing circuit breakers.
In many cases, existing electrical equipment isn't evaluated when a change is made to the service transformer or utility feed. The increased available short-circuit current may be a result of changes to the electrical system, including the addition of new transformers with a higher kVA rating, lowering the impedance of the new transformers (as compared to the transformers replaced), the use of higher efficiency transformers, and, in general, the addition of more loads. Therefore, it's important to do a thorough review of the short-circuit currents associated with the changes to the electrical system to avoid possible misapplication of the overcurrent devices.
Prior to the revision of 240.86 for the 2005 NEC, the only option available to the owner appeared to be replacement of the existing electrical equipment, including circuit breakers or fusible switches with adequate interrupting ratings for the increased available fault current. This is often cost prohibitive and disruptive. It can also place electrical workers in a very hazardous situation by making them work on or near energized, improperly rated equipment.
How is protection provided for existing circuit breakers that have inadequate interrupting ratings? Let's look at three possibilities.
If the existing electrical system of panelboards, switchboards, or load centers were initially listed for use with specific tested series rated fuse/circuit breaker combinations, the testing was completed with umbrella fuses. The licensed professional engineer engaged primarily in the design or maintenance of electrical installations may be able to use commercially available fuses to provide appropriate protection. For example, if the existing system used series ratings with Class R fuses, the licensed professional engineer could analyze where a specific fuse manufacturer's RK1 fuse or Class J fuse might provide the protection at a higher short-circuit current rating. The series ratings for panelboards (where marked in manufacturers' information) that are protected by Class R fuses have been tested with special Class RK5 umbrella fuses.
It's possible that the existing circuit breakers may have been series-rated with upstream devices, but the equipment may have never been marked as such. The equipment may have information that provides all the possible listed combinations of fuse/circuit breaker or circuit breaker/circuit breaker series ratings. If this is the case, a licensed professional engineer engaged primarily in the design or maintenance of electrical installations may be able to select a circuit breaker or fusible switch to be installed on the line side that complies with one of these listed series-rated combinations adequate for the available short-circuit current.
For the most part, an industry-wide accepted method doesn't exist for comparing the let-through characteristics of a current-limiting fuse or circuit breaker with the interrupting rating of modern molded-case circuit breakers that exhibit dynamic impedance during the first ½ cycle after the initiation of a fault. A licensed professional engineer engaged primarily in the design or maintenance of electrical installations could possibly oversee short-circuit testing with current-limiting overcurrent devices (fuses or circuit breakers) to ensure that the combination is acceptable.
Whichever method the licensed engineer decides to use, proper documentation is required. The documentation for the selection must be stamped by the engineer and be readily available to those involved in the design, installation, inspection, maintenance, and operation of the equipment.
Tested combinations. In new installations, a tested series-rated combination that has been evaluated for use in specific panelboards, switchboards, and load centers must be used. Several permutations of fuse/circuit breaker and circuit breaker/circuit breaker combinations have been tested and found acceptable for use in specific equipment. For new installations, where tested combinations are typically used, there's no need to enlist the services of a licensed professional engineer for the design or maintenance of electrical installations.
For circuit breaker/circuit breaker combinations, the specific circuit breakers are tested. For fuse/circuit breaker combinations, special umbrella fuses protect the circuit breakers during testing. These umbrella fuses ensure that every commercially available fuse will be able to protect the circuit breakers. This means that if a specific circuit breaker is series-rated with Class R fuses, a Class R fuse made by any manufacturer can be used in this device. This includes both Class RK5 and Class RK1 fuses. It's important to note that Class RK1 fuses are more current limiting than Class RK5 fuses. However, since they're the same physical size, all series-rated combinations are tested using Class RK5 umbrella fuses.
Series-rated combinations have additional NEC requirements that must be considered. Switchboards and panel boards that are part of a series-rated combination must be factory marked by the manufacturer with the series combination rating, and they must also be legibly marked in the field by the installer to indicate the equipment has been applied with a series combination rating per 110.22 and 240.86(B) of the NEC.
Series-rated combinations are also subject to certain limitations because series-rated systems aren't selectively coordinated; the line side overcurrent protective device must operate to protect the downstream circuit breaker. Therefore, series-rated combinations may not be applied on those systems that are required to be selectively coordinated by the NEC, such as emergency systems (700.27), legally required standby systems (701.18), essential systems in health-care facilities (517.26), and elevator circuits that have a single feeder for multiple driving machine disconnecting means (620.62). The first three included changes in the 2005 NEC.
Motor contribution. Another limiting factor in the application of series-rated combinations is the motor load that's supplied by the system. The sum of the motor full-load currents connected on the load side of the higher rated overcurrent protective device and the on-the-line side of the protected circuit breaker can't exceed 1% of the protected circuit breaker's standalone interrupting rating per 240.86(C). The concern is that running motors will momentarily contribute current to the fault. This added current can result in a short-circuit current in excess of what the protected circuit breaker can safely interrupt.
To protect buildings and structures from potential electrical fires, and in the interest of safety for workers in the workplace, the 2005 NEC includes revisions to 240.86 to allow series ratings for existing installations under the supervision of a licensed professional engineer engaged primarily in the design or maintenance of electrical installations.
Two methods are acceptable for meeting the interrupting rating requirements of 110.9: fully rated systems and series-rated systems. When applying series ratings in compliance with 240.86, remember to apply part (C) Motor Contribution. Don't forget to field mark the equipment enclosures to indicate the equipment has been applied with a series combination rating per 110.22 and 240.86(A).
For installations in supervised locations be sure to read and follow the requirements of 240.93, which is very similar to 240.86.
Owen is the owner and president of National Code Seminars in Pelham, Ala.