NEC 240.86 Series Ratings?

[chuck@apsinc.biz]

NEC 240.2 gives a definition for a curren-limiting overcurrent protective device and the commentary in the handbook eludes that its purpose is to cut off a fault current in less than one-half cycle, thus preventing fault currents from building up to their full avaiable values.

NEC 240.86 requires series rating for circuit breakers used on a circuit having an available fault current higher than it's marked rating.

I interperated this to mean that if the let-thru value after the system's main disconnect is more than the down stream distribution's marked rating then the system must be fully or series rated for the aviable fault-current before the main. However, if the the let-thru value after the system's main disconnect is less than the down stream distribution's marked rating then series rating is not required.

The city of Denver has interperted 240.86 to mean that electrical systems will be fully or series rated reguardless of let-thru fault current values.

If this is the case then what is the purpose of Current-Limiting Overcurrent Protective Devices?

Am I missing something or is the City off on it's enforcement?

 

[gwz2]

Re: NEC 240.86 Series Ratings?

I imagine you have the Bussman SPD book ( and other Bussmann bulletins) which helps explain and determine the Available Short Circuit Current (ASCC) at points along a circuit.

I am not sure of your evaluation of Series Rated OCPD's.

The first OCPD interrupting rating in the circuit must be equal to or greater than the ASCC at that point in the circuit. On Series rated OCPD's the ASCC at the point of the lower rated device , say a 10Kair CB, maybe actually 20000 Amps, but if series rated with the upstream OCPD, both the upstream OCPD and the 10Kair OCPD are to open at the same time so that the 10Kair does not "see the 20000 amperes", is not interrupting the 20000 amperes.

Problem, almost always a black-out condition with Series Rated components under fault conditions.

 

[don_resqcapt19]

Re: NEC 240.86 Series Ratings?

The "series rated" system must be physically tested. You can't make your own "series rated" system using a current limiting OCPD upstream. You can do your calculations based on the limiting caused by the circuit impedance.
Don

 

[hurk27]

Re: NEC 240.86 Series Ratings?

Glen I dont want to get off the subject but if I instaled a main breaker that has a 20,000 air rating and then had a branch breaker with a 10,000 air would this prevent the blackout problem when the branch circuit were to fault? It sounds like this would not be allowed. with 240.86?

 

[ron]

Re: NEC 240.86 Series Ratings?

You are not permitted to determine the let through of a current limmiting device, and decide that is all that is left to work with down stream.
As mentioned, you must have a series rating on a combination of devices, or a fully rated device to pass muster.
The only way to reduce short circuit current to a device is to run additional conductor footage (increase the net impedance) before you get to the device, add a reactor upstream, convert any transformer (utility et al) to a higher impedance and/or pray a lot.

 

[hurk27]

Re: NEC 240.86 Series Ratings?

This was a hard one to explain to a home owner when he faulted a 20 amp breaker and triped the 100amp main also. this was why I was asking

[ April 22, 2003, 10:28 PM: Message edited by: hurk27 ]

 

[templdl]

Re: NEC 240.86 Series Ratings?

Wayne,
The kaic rating has nothing to do with the magnetic (instantaneous)trip pickup of a breaker. As a rule of thumb the breaker instantaneous pick up value is approximately 10x the rating of the breaker for illustration purposes. As such a 100a breaker would be 10ka and a 20a would be 200a. (it is not unusual for a 15-20a breaker to be significantly higher though).
If you had a nasty fault in a branch circuit that generated 10ka of fault current both the branch and main would see it at the same time. Since instantaneous is just that, no time delay, it would become a race as to which one trips first. Quite often both trip but it is not uncommon for a main to trip and a branch will not.
As far as the kaic rating of a breaker is concerned it is the breaker's ability to interrupt that fault without destroying itself, that is possibly dislocating itself from the panel as it tries to interrupt a fault that is greater that it is capable of. A 100a breaker with a 10kaic rating will have the same magnetic pickup calibration range a one that has a 22kaic rating. Both will trip a the same point, that is the contacts will open at the same point, but the 22kaic is capable of interrupting up to 22ka if the spike of current continues up to that value.
Both interrupting rating as well as a breakers ability to be current limiting is all in the contact design, their ability to clear an arc be splitting it up and cooling it and the speed of which it is done determines its current limiting capability. There's all sorts of magic that is done with the arc chutes and moving and stationary contact design.

 

[chuck@apsinc.biz]

Re: NEC 240.86 Series Ratings?

Don, I understand the limiting caused by the circuit impedance. But doesn't the upstream "current limiting" OCPD cuts off a fault current before it builds up to it's full available value,thus limiting the let-through energy to within the rating of the downstream components,in spite of it's available ground fault current potential?

If not then what is the purpose of a "current limiting" OCPD?

What happens if your remodeling, and backfeeding existing equipment and the only way to get the Ground fault potential to an acceptable level at the old equipment is through the use of "current limiting" OCPDs since a "series rating" of the old and new devices does not exist? Am I NEC required to replace the existing system, since the "current limiting" devices don't apperently limit the ground fault current into the system?

 

[ron]

Re: NEC 240.86 Series Ratings?

What type of remodel is causing you to deal with higher fault current availability? Has the utility transformer been replaced with larger? More motor load?
If for what ever reason equipment in the system, existing or new, does not have the correct AIC rating, it must be replaced worked around.

 

[gwz2]

Re: NEC 240.86 Series Ratings?

Wayne,

For a SERIES RATED System, the circuit breakers must be tested together and the CB's are on the panelboard label.

In a two tier Series Rated systems, almost always both CB's trip at the same time. If the lower Kair rated one trips first and if the fault current is higher than it's interrupting rating, then the system is not functioning as designed and the lower unit may explode.

I have not been around any three tier system when there was a fault. I would assume the system must be designed to at least trip the highest rated (Kair) under a fault condition, other-wise one or both of the lower Kair units may " see " more current than it's interrupting rating depending where the fault is.

 

[hurk27]

Re: NEC 240.86 Series Ratings?

Thanks glen It is still a hard subject to try to explain to a homeowner that doesn't understand it.
One of these days I would like to go to one of your IEAI meetings and meet you.