Is a series rated panel right for you?

It is not always necessary to have every branch circuit breaker rated for the full available fault current in an electrical panel or switchboard.

05/28/2015


Learning objectives: 

  • Understand the difference between a "fully rated" and "series rated" electrical panel.
  • Know how to properly choose between the two options.
  • Learn about cost effective alternatives to series rating panels. 

Introduction

Designers, owners, and contractors are motivated by budget constraints to save costs wherever possible. Circuit breakers found in electrical panels and switchboards become increasingly expensive as the rated current interrupting capacity (AIC) increases. In facilities where the available fault current is high, this will lead to increasingly expensive circuit breakers. All of the available options to reduce cost should be considered.

Series-rating a panel or switchboard and using lower AIC-rated circuit breakers is one option that could reduce the electrical installation costs of a project. 

Because of the great risk to the facility and personnel in the event of an electrical fault, properly rating a panel or switchboard's current interrupting capacity must be a priority. An underrated circuit breaker could fail to interrupt a fault event, and poses a serious fire and personal safety hazard to the facility. The resulting cost of an improperly rated circuit breaker in the event of a fault could be substantial.

Finally, if all the required conditions cannot be met to series-rate a panel, there are other options to reduce the necessary rated current interrupting capacity of a panel. Although a panel may be fully rated, if the calculated available fault current at a panel or switchboard is reduced, the cost to install and maintain that equipment may be reduced initially and well into the future.

Overcurrent protective devices and fault current

During an electrical fault, there is a sudden surge of current through the faulted conductors. It is the job of the upstream overcurrent protective device to clear that fault as quickly as possible. The upstream overcurrent protective device must be capable of clearing the maximum possible available fault current. 

Circuit breakers are rated by their interrupting rating or ampere interrupting capacity (AIC). This is often expressed in kilo-amperes  as the KAIC rating or simply "Interrupting Rating " as seen in Figure 1. This rating indicates the maximum current the circuit breaker is capable of clearing in the event of a fault, and is generated after testing by either UL or the IEC . Because the testing specifications differ between the two, often times the UL or IEC ratings will be provided together. 

In the event the fault current exceeds the circuit breaker's capacity to clear the fault, the results could be disastrous. The circuit breaker could fail to open during a fault and its contacts may fuse together causing catastrophic failure. This scenario poses significant risk to the facility and its personnel. 

For this reason, accurate maximum fault current calculations are imperative. While erring on the side of conservatism is safe and prudent, beware of the tendency to be overly conservative in these calculations. If the maximum available fault current is calculated to be higher than reality, it may force the affected equipment to be needlessly overrated. This can lead to substantially increased construction costs over the lifetime of the equipment. On the other hand, improper calculations can result in underestimating the available fault current. This could lead to overcurrent protective devices that are not prepared to handle the maximum available fault current with consequences listed above.

What is a series rated circuit breaker?

It is not always necessary to have every branch circuit breaker rated for the full available fault current in an electrical panel or switchboard. After substantial lab testing, many circuit breaker manufacturers have the ability to install circuit breakers rated for a lower fault current, but maintain the panel or switchboard's overall AIC rating. A panel or switchboard with this setup would then rely on the main circuit breaker to clear the fault. A panel with this arrangement is called a series-rated panel. 


The National Electrical Code (NEC) defines this in Section 240.83, Series Ratings, as when "a circuit breaker is used on a circuit having an available fault current higher than the marked interrupting rating by being connected to the load side of an acceptable overcurrent protective device having a higher rating..." Figure 1 shows an example of a typical series rating set up. Note that the fault current may exceed the 10KA rating of the branch circuit breaker, as long as the upstream circuit breaker is rated appropriately. 

Currently, there is no way to accurately calculate the outcome when two circuit breakers are paired together in a series rating. The only way to determine if the two circuit breakers will operate properly together is through extensive lab testing. Two specific circuit breakers are then paired together and must be from the same manufacturer. The pair can then be listed by UL as being safe for use in this application. 

A fully rated panel would not use this method of pairing tested circuit breakers together. Instead, all circuit breakers in the panel, including the branch circuit breakers, would be rated for the maximum available fault current as shown in Figure 2. As we will see, although this increases the cost substantially, there are cases where series-rated panels are not allowable by the NEC or the local authority having jurisdiction. 

A fully rated panel would not use this method of pairing tested circuit breakers together. Instead, all circuit breakers in the panel, including the branch circuit breakers, would be rated for the maximum available fault current as shown in Figure 2. As we will see, although this increases the cost substantially, there are cases where series rated panels are not allowable by the NEC or the local authority having jurisdiction. All circuit breakers in the panel, including the branch circuit breakers, would be rated for the maximum available fault current as shown in Figure 2. Courtesy: Coffman Engineers

Related electrical code requirements

When considering a series-rated panel, the first step is to verify that the installation will adhere to the requirements of the NEC. Section 240.86, Series Ratings, introduces the definition of a series rating and states that the circuit breaker combination must "meet the requirements specified in (A) or (B), and (C)." 

Subsection (A) and (B) of NEC Section 240.86 states that the series-rated combination must be either selected and approved by a licensed professional electrical engineer or be tested and marked on the end-use equipment. Many manufacturers of panelboards and switchboards offer solutions that satisfy part (B).

The final governing subsection, subsection (C), is often misunderstood or overlooked. Although the available fault current on the line terminals of the equipment may be cleared by the main overcurrent protective device, the panel's motor contributions to the fault may not be seen by the main overcurrent protective device depending on where the fault is. 

NEC Section 240.86(C) states that a series rating on a panel may not be used if the sum of motor loads on the panel exceeds 1% of the interrupting rating of the lower AIC-rated circuit breaker. Consider a scenario where a fault occurs on a panel's furthest downstream circuit breaker; that is, the circuit breaker furthest on the bus from the main overcurrent protective device. If the upstream branch circuits on the panel contain significant motor load, the motor contribution to the fault could damage the branch circuit breaker even if the main overcurrent protective device is properly series-rated.  This is a common mistake made by contractors and designers of these systems, and one of the most common code issues in an electrical gear submittal for a project. 

As seen in Figure 3, although the branch and main circuit breakers may be rated for a series rating, this does not account for the motor load on that panel. Courtesy: Coffman Engineers

For example, on a panel where the series-rated branch breakers are rated to 10kAIC, a common series rating, this limits the panel to only 100A  of motor load. As seen in Figure 3, although the branch and main circuit breakers may be rated for a series rating, this does not account for the motor load on that panel. The two 52A loads, 104A in  total, exceeds 1% of the 10,000A rating of the branch circuit breaker. This installation would not be code-compliant per NEC 240.86(C).

Another related section of the code requires additional labeling of all series-rated panels. Section 110.22 requires that the panelboard or switchboard be labeled clearly with the text: "CAUTION—SERIES COMBINATION SYSTEM RATED ___ AMPERES. IDENTIFIED REPLACEMENT COMPONENTS REQUIRED."

Figure 4: an example of series rated panel. Courtesy: Coffman EngineersAdditionally, for safe future modifications to installations, the calculated available fault current should be well documented. In 2011, the NEC added Section 110.24, which requires the available fault current at the service to be "legibly field-marked with the maximum available fault current." This should include the date the fault current calculations were performed. Future calculations could then be based on this original calculation.


Effects on future modifications to the panel

One drawback to consider if series-rating is performed by the manufacturer, is that the panel is now limited to that manufacturer when selecting replacement or additional circuit breakers in the future. Currently, no manufacturer goes through the process of having their circuit breakers UL-listed for use with a competitor's circuit breakers, and this will probably not happen anytime in the near future. 

It is possible that over the lifecycle of the equipment, a manufacturer may no longer produce replacement circuit breakers, making maintenance and future expansion difficult or impossible. This also poses a particular issue if a new panel installed in an existing facility is provided by a different manufacturer than the existing equipment
[VS1] . That facility now not only has to stock replacement circuit breakers specifically for the new panel, but also must remember to use only that manufacturer's circuit breakers on that panel. For this reason, proper labeling and marking is not just for the convenience of future maintenance, but also can prevent a code violation and potentially placing the facility and its personnel in danger. 

Safety when performing maintenance or upgrades on a series-rated panel is also a concern. If a spare circuit breaker is added to a series-rated panel from a dissimilar manufacturer, the installer can no longer be sure the main circuit breaker will clear the fault before the new branch circuit breaker has a catastrophic failure. For this reason, it is recommended that a new series-rated panel be filled with spares to prevent any future concerns. 

Furthermore, if significant motor load is added to the facility, any panel to which this load is added will need to be re-evaluated to prevent violation of NEC 240.86(C) as described above. In a facility with a large amount of motor load and relatively low available fault current, series-rating panels could limit the panels available that could take on additional motor load. If significant motor load is added to a series-rated panel without taking this into consideration, the additional motor load could exceed the 1% rule and become a code violation and safety hazard. 

Consider reducing the calculated available fault current
An alternative to series rating a panel is to reduce the facility's calculated available fault current. This would allow less expensive circuit breakers for the facility's electrical panels and switchboards. If the calculated available fault current of the facility is substantial, it could also significantly reduce the initial construction cost as well as the cost of maintenance well into the future. 
Reducing the calculated available fault current has the added benefit of reducing the potential arc flash hazard. This could reduce the danger of arc flash as well the severity of damage caused during an arc flash event. Additionally, the required Personal Protective Equipment (PPE) could be reduced when performing maintenance on live equipment. Although the cost savings to the project would be a benifit, the added safety to the facility's personnel is immeasurable.
A comprehensive list of options for reducing the calculated available fault current is beyond the scope of this article, however below we discuss some common and cost effective options. One of the most common options when reducing the calculated available fault current is to employ current limiting fuses. Current limiting fuses are designed to interrupt a fault condition before the fault can reach its peak current. They have been tested extensively by the manufacturer and are UL labeled for this application. When used in the design of a facility, this could ensure that the calculated available fault current downstream from the fuses will never reach the maximum calculated level. 
Another option is the careful selection of an upstream transformer. If the upstream transformer serving a facility, panel or switchboard is being selected as a part of the project, it might be more cost effective to deliberately use a transformer with an increased internal impedance. The losses that occur from the increased transformer impedance could be offset be the reduced cost of fault current bracing downstream. Since a higher impedance transformer produced more heat, this would not be an effective option if the transformer is in a conditioned space and the cooling load of the facility is critical, such as in a data center environment. 
A third option, and often times the most cost effective option, is to increase the linear length of the feeder serving the panel or switchboard. If the fault current calculations approach one of the standard AIC ratings, even a small amount of additional feeder length could reduce the available fault current to a level below the next interval. For example, a panel with a calculated available fault current of 43kAIC would require a rating of 65kAIC, the next highest standard AIC rating. If the feeder to that panel was increased an additional ten to twenty feet, it could increase the impedance of that feeder enough to decrease the available fault current and allow a rating of 42kAIC for the panel. This can be accomplished by exploring alternative conduit routing. If the panel is a major panel and required to be fully rated, a small adjustment to the upstream feeder length could result in substantial equipment cost savings. 
The final option for reducing the calculated available fault current is to use more accurate fault current calculations. It is common for the engineer on record to perform calculations that are not only excessively conservative, but are calculated before the conduit and feeders can be routed. As a result, the calculated available fault current often times represents a "worst case scenario" instead of actual conditions of the installation. If possible, the calculations should be done with the aid of accurate system modeling. Numerous modeling software packages exist but can be complicated and should be operated by someone with experience and training with that software package. Additionally, 3D modeling software may be used when routing conduit in the design phase of a project in order to accurately model the feeder lengths in the fault current calculations. Finally, accurate field measurements of the installed feeders can result in reduced calculated available fault currents. These measures can reduce the potentially costly situation where the facility is overrated for an excessively conservative and inaccurately calculated available fault current. 
In order to save money, series rating every panel in a new installation might appear to be an effective way to save money on a project. There are however, a number of code requirements to consider before a panel can be series rated. Furthermore, a series rated panel makes future modifications difficult and all related alternative should be explored. A series rated panel may not be right for every facility and the decision to series rate a panel should be carefully made. 


Aaron Hesse is a BSEE graduate of Eastern Washington University and a professional electrical engineer with Coffman Engineers in Spokane, WA. Edited by Joy Chang, digital project manager, CFE Media, jchang@cfemedia.com



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