Series v Fully Rated

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buck

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I find some very knowledgable individuals on this site. That being said, can anyone help me sort through the confusion I have with series and fully rated systems?

It seems I find more and more industrial/
commercial systems being designed as series rated.
I have done some research but none that really explains it in a VERY clear manner.

Also, on a fully rated system, does that tell me that all overcurrent devices installed have to be rated with the same stand alone rating (A.I.C.)as that supplied from the utility? Let's say, the utility supplies 42k aic, do all devices including the main have to be rated for 42k or designed as series rated (where the downstream devices can be rated less according to the listing)?

One other question,can a fully rated system be combined with a series rated system?

Appreciate any meaningful responses from the experts here.Thanks in advance.
 
Re: Series v Fully Rated

Bussman has very good information on this subject, I would download their Electrcial Protection Handbook. Review it and then come back with any questions, it will give you the background to understand series and fully rated systems
 
Re: Series v Fully Rated

I trust thast this will give you some insight on what a series rated system consists of:
You first must establish the KAIC available at the service entrance which is commonly provide by the power company. The disconnects, i.e., the Main breaker, must be rated with a minimum kaic of the available fault current. However, in that main panel the feeder and/or branch devices may have ratings that are considerably less than the main device, that is these devices are series rated with the main breaker. What the manufacturer has done is to test the main breaker with the downstream devices to assure that the main breakers current limiting capability (not necessarily a "current limiting" circuit breaker) will be current limiting enough to protect those down stream breakers from a fault current that could damage them. However, the manufacturer simply cannot make this determination but the combination must pass UL tests and be listed as such. Thus, this panel is considered to be series rated. Likewise, down stream panels and breakers can be series rated with those that are upstream provided that they, too, have been tested and listed by UL to be applied as such. One thing that must be noted is that you cannot mix manufacturers products unless they too have been tested and listed with one another as quite often fuses may be series rated with breakers by some manufacturers. To "Fine tune" a distribution beyond the use of series rated equipment the available kaic within a distribution system can be taken even farther by doing a fault current study within the system which impart takes the impedance of the cable into consideration.

One of the goals of using series rated devices is to reduce the cost of the installation. I'm sure you're well aware of the premium that must be paid for higher kiac devices.
 
Re: Series v Fully Rated

Fully rated means that each device is rated individually to handle the fault current available at its location. Series rating is when a manufacturer tests devices together and give a ratings that they can be applied, which doesn't correspond to the actual rating on the device itself.
For example, a 65kA main and a 22kA branch breaker, may have been tested together (series tested) by the manufacturer to be applied at a point in the system that has less than 65kA fault current available. Whereas, if that system was to be specified as fully rated, it could not be installed unless that point in the system was calculated at less than 22kA.
 
Re: Series v Fully Rated

Hmmmm, thanks for your responses guys, they help but it appears to me that it is very common for electricians to install lower aic rated breakers downstream on FULLY rated systems, which would be a dangerous installation. If utility supplies say 42kaic, what would prevent a 10kaic installed downstream? The installer and/or inspector would have to know the supplied utility aic(on a new or existing system).

Any suggestions on handling this?
 
Re: Series v Fully Rated

Buck,
You are correct. You should never apply a new device without knowing the available fault current, or an estimate of such.
It amazes me that many folks blindly design/install equipment of an equal (or lower) AIC value as compared to equipment that is existing. The big assumption is that the existing equipment was designed/installed correctly.
 
Re: Series v Fully Rated

buck,
If by "downstream" you mean in a subpanel, then the breaker only has to be rated for the fault current available at the subpanel. The impedance of the subpanel feeder may make a major reduction in the available fault current at the subpanel. For example a 20' run of three #2 copper conductors in steel conduit will reduce the available fault current from 42,000 to 17,000. The breakers at the subpanel would only be required to be rated for 17,000 and not the 42,000 that is available at the service. If the feeder run is 45', then the available fault current drops to 9,750. These numbers assume a 120/240 volt single phase system.
Don
 
Re: Series v Fully Rated

You should have fault current calculations with the point of fault at the line side of all equipment as per 110-9 & 110-10 of the 99 NEC. If series rated systems are to be installed, they must be listed and labeled by a NRTL and comply with 110-22 & 240-86 of the 99 NEC. To try to put this in very simple terms, fully rated is whenever the AIC rating of a device whether passive or active is rated equal to or greater than the available AIC at the line side of that equipment. Any time the AIC rating of a piece of equipment is less than the available AIC at the line side terminals of that equipment, it must be series rated or it is missapplied and would be in violation of several sections of the NEC 90-1, 110-3(b), 240-86.
 
Re: Series v Fully Rated

Great info guys, appreciate the responses.
Don, how did you come up with your reductions based on length of feeders? Are there any tables or maybe rules of thumb to use when faced with field situations? Do any of you know of seminars or classes about series and fully rated systems,always looking to learn more about questionable applications.

Had a situation recently where engineered called out for series rating on a commercial building and low and behold the electrical contractor installed different remote panelboards(mfg) than the MSB(42kaic) which according to the listings wouldn't work as series. The engineer then required contractor to make the system fully rated which he said required upgrading(at a substantial cost) all downstream breakers to the 42kaic of the utility supplied 42kaic. Don, does this sound like reasonable or did the engineer do his homework per se? Have the feeling installers, inspectors and maybe even some engineers are confused or should I say misinformed on this subject, which IMHO could be as dangerous of situation as say installing undersized conductors with larger breakers.
 
Re: Series v Fully Rated

buck,
I got the formulas from a Bussmann publication called Electrical Plan Review. I have an old paper copy, but the link is to the current copy that has the same calculations. The publication has infromation on short circuit calculations, series systems and more.
Don

[ March 30, 2004, 01:16 PM: Message edited by: don_resqcapt19 ]
 
Re: Series v Fully Rated

Aren't All breakers fully rated? If it is marked 10kaic then that is the full rating. Series rating can't always be used even if the breakers have been tested together. You have to make sure your installation meets the code requirements that allow series rating. Going through a commercial dry type transformer will also knock the fault current down. But motors can add to the fault current which I believe is why you can't use series rating if there are lots of motors.
 
Re: Series v Fully Rated

Don't kill yourself performing the calculations, here is an excellent section of Mike Holt's website with excel spreadsheets that will do the calc's for you, simply plug in the values.
Also, you have to figure your point-to-point for your values. formulas
 
Re: Series v Fully Rated

buck,

the reason the engineer required the contractor to install a fully rated system is that manuf lists which combination of their circuit breakers are listed as series rated. The also have a list of their circuit breakers and fuses which are series rated. The series rated test is a UL approved test. However, there is no such list for a series reated system between two different manuf circuit breakers.
 
Re: Series v Fully Rated

bwyllie,
Thanks for the responses(along with everyone else) but I realize the basics for series rating. I did have a concern about the lack of knowledge in general of those installing,inspecting and designing these systems. The example I gave appeared to me that the engineer just made a blanket requirement to the electrical contractor to fully rate all breakers at 42kaic(no matter the distance as Don pointed out from the MSB)which resulted in a substantial cost to the contractor. Some remote panelboards were as far away as 100 to 125 ft from MSB.
 
Re: Series v Fully Rated

Buck,
I'm sure the electrician could have paid the additional consulting cost to have the engineer recalculate and respecify the panels.
Sounds like the electrician should have installed the series rated combination to begin with.
 
Re: Series v Fully Rated

without knowing the specifs it is hard to comment, but if the Fault current at the service was 42K and thats what the engineer wanted everything rated at then I would say the engineer got lazy and did not want to perform the calcs on downstream devices. However, that still does not mean the contractor should have deviated fromt he plans without the consent of the design engineer. The construction documents(contract documents) called for a fully rated system and thats what the contractor should have priced and installed.
 
Re: Series v Fully Rated

Reading this thread at 4 am (insomnia) has just made me realize a problem that many folks may have been over looking. It is a common practice when designing additions / renovations to commercial buildings to add a second main panelboard / switchboard to a building to serve the new addition. This often requires the POCO to change out their xfmr with a larger one. In doing so, the available fault current my go way up and the interrupt rating of the old main switchboard may be inadequate.

[ April 03, 2004, 04:36 AM: Message edited by: sceepe ]
 
Re: Series v Fully Rated

You can have a fully rated system based upon the service entrance available fault current which is the simplest but costliest method,
coordinated system is where an engineering study is done to the distribution system by a qualified person to determine the available fault current at a given point within the distribution system to assure that a given component is rated correctly,
series rated system where the OCPDs are tested in series with one another and listed by UL.
The objective of a coordination study is either to assure that all devices a applied correctly for their ratings as well as the ability to use OCPDs that are less costly. Series rated devices when applied correctly also reduce the cost of the installation.
 
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