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#128032 - 05/19/02 08:24 PM Systems / Methods to address Short Circuits.  
Gwz  Offline
Joined: Apr 2002
Posts: 197
What methods, other than a Fully Rated system and Series Rated system, are used to address NEC 1999 110-9 and 110-10 [ NEC 2002 110.9 and 110.10 ]?

Can a System in which the Available Short Circuit Current ( ASCC ) at the Service Equipment is greater than 50,000 amperes with GFP as per 230-95 [ 230.95 ] using 3500 Ampere Fused Main OCP and Current-Limiting fuses such as Class RK as feeder OCPD in the service equipment line-up and then 14Kair CB's in subpanels where the ASCC at the subpanel location is greater than 30,000 amperes be considered as a Fully Rated System, or is the system a Series Rated System, or is there another system (name?) that would not require the system to be considered a Series Rated System and not need the labeling required by 110-22 ( 2nd paragraph ) and 240-86(a), [ 110.22 ( 2nd paragrph ) and 240.86(A) ] ?


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#128033 - 05/19/02 09:21 PM Re: Systems / Methods to address Short Circuits.  
Ron  Offline
Joined: Mar 2002
Posts: 582
White Plains, NY
The 14KAIC CB's are not fully rated, because they are not rated higher than the available fault current of 30KA by themselves. If the CB's rated @ 14KAIC are not series rated with the upstream fuses (from your example) then you have a code violation. No labeling in the world will help if there is no fully rated device or series rated combination at the panel (with 30KA available). The only agency that determines a series rated combination is UL and it would be in the associated literature with the device(s).

[This message has been edited by Ron (edited 05-19-2002).]


#128034 - 05/20/02 10:23 AM Re: Systems / Methods to address Short Circuits.  
Dan Neeser  Offline
Joined: Mar 2002
Posts: 29
St. Louis, MO, USA

To properly apply overcurrent protective devices with an adequate interrupting rating, there are only two ways to do things.

The first and preferred way to comply is to select a fully rated system as required by NEC 110.9.

The second way to comply is to provide a fully rated system with limited applications of series rating combinations provided the additional requirements and considerations are met. There really is no such thing as a "series rated system", I would prefer to think of it as stated above. This is kind of a play on words, and I know 110.22 refers to a series rated system because the first rule of thumb should be to fully rate all devices with an adequate interrupting rating in compliance with 110.9, then look for special situations where series rated combinations may be applied.

Series rated combinations may be used, provided the requirements of 240.86 and 110.22 are met and other considerations are reviewed. Because of the requirements of the NEC, additional considerations, and the available tested, listed and marked series rated combinations, the application of series ratings is often limited to lighting and appliance branch-circuit devices and the upstream feeder.

The specific application you have suggested may be permitted. To see if it is permitted, please see the attached article on the Bussmann Website and the series rated charts (showing fuse/circuit breaker series rated combinations for GE, Sq. D, C-H, and Siemens).

The link for the article is:
(note this article will be updated evently to reflect the discussion above).

The link for the charts and additional information is:

#128035 - 05/20/02 12:06 PM Re: Systems / Methods to address Short Circuits.  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
I stand corrected as needed, but with some[?] molded-case circuit breaker series-rated systems, i.e., 65kA feeder serving a group of 14kA branch breakers, you fun the likelihood of a branch fault tripping the both breakers.

#128036 - 05/21/02 09:02 AM Re: Systems / Methods to address Short Circuits.  
JBD  Offline
Joined: Jul 2001
Posts: 599
Series rating does not always sacrifice coordination, likewise full rating does not always provide coordination.

For example: Square D's standard and high interrupting industrial breakers (I-Line) share the same ampere trip curves. This means the coordination with upstream devices is identical regardless of AIR rating.

#128037 - 05/24/02 04:52 PM Re: Systems / Methods to address Short Circuits.  
Dan Neeser  Offline
Joined: Mar 2002
Posts: 29
St. Louis, MO, USA
With regards to the likelihood of a fault on a branch breaker tripping an upstream breaker. The important thing to consider is the amperage size of the devices and the instantaneous setting of the upstream device. Any fault that occurs that is equal to or greater than the instantaneous setting of the upstream device could trip both breakers. Since a small percentage of overcurrent conditions are short-circuits, the chance is relatively small, however, most people have seen this situation happen.

You bring up a good point. Applying series rated combinations does not always cause a lack of coordination. However, the possibility of a lack of coordination always exists with a series rated combination, especially if a short-circuit occurs. That is how the upstream device can protect the underated downstream device, it opens.

Also, yes not all fully rated systems are coordinated. In fact, the majority of systems that use fully rated molded case circuit breakers are NOT selectively coordinated, because of the instantaneous trip feature with molded case circuit breakers.

However, fully rated systems can be selectively coordinated if you use either fuses and apply proper selective coordination ratios or if you use the more expensive (and bigger) low voltage power circuit breakers with short-time delay.

The important thing to remember, when applying series rated combinations, selective coordination is NOT the only thing to consider. You must consider other things such as marking, selecting proper combinations and application of these combinations, as well as other requirements and considerations. See the documents we have on the Bussmann Website for more information.

#128038 - 05/25/02 02:43 AM Re: Systems / Methods to address Short Circuits.  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Some of the points to ponder - as in regards to SCA figures are:

  • When dealing with a Multi-Customer used Transformer, and the KVA size needs to be increased a significant level (such as upgrading a 300 KVA 3.0%Z Transformer with a 500 KVA 1.2% Z Transformer), this might skyrocket passed the limits of a Series Rated system's capability (or even a system without Series Rating in place, but with MCCB's which just made the original minimum KAIC),
  • Increasingly higher Induction Motor loads at any point[s] in the system - either from other Tenants, or within the Client / Tenant of concern. Such Motors being placed in the "End Load Sides" of Series rated systems, being of 10 HP or more with highly Dynamic loads connected to their shafts, contributing a high level of SCA into the system,
  • An inexperienced or "Cut-Throat" installer whom installs equipment which is incompatable to the Series Rated system, or falls below the minimum KAIC Rating at a given point. These installs may be done by persons both unaware of the consequences and unconcerned with the results, or be installed by a person with good intentions whom is willing to try and save the Client money (not a "Cut-Throat" person!).

These are a few things to get threads started and possibly burning [Linked Image]

Like to see some feedback.

Personally, I try to use these instances in mind when designing a Client's Power System. Also am a stickler to design selectively!
Can't even count how many times I have heard questions from persons in the field asking why the Subfeeder main, or as far as the Main breaker or Main Fuse[s] tripped / blew when someone Ground Faulted a Branch Circuit. This is another thing to consider when planning a Power System's setup and equipment coordination.

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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