NEC S.C. Rating

Hi,

I don't know exactly what you're getting at but I guess you're asking whether or not short-time short circuit rating of 25KA is OK at an electrical panel to which a load of 10KVA is connected(??).

Short circuit rating at a panel is not related to the connected load per se but to the fault current that will flow in the event of a short circuit occurring at the panel. This prospective short circuit current is dependent on the capacity of the electricity supply transformer, it's p.u. impedance and the impedance due to cables between the transformer and the point of fault. Having said this, however, rotating electrical machinery downstream of the point of fault do contribute to the prospective fault current.

Regards

I dont think that you can close out the document even if somebody tell you .

The 2014 NFPA 70®: NEC® is the standard/document you need.

Are you really qualified to close this document out, if you don't have a copy of the code?

I think not.

Thanks i will check it out the NFPA 70.

Non of the figures given make any sense what so ever. 25KA MPFC where the rated load is only 10KVA

Addendum:

25KA MPFC I would associate with somewhere around 1000KVA at 433V.

One hell of a miscalculation if this is to close a document of any standing.

sprabakar22,

More information will help! What type of document are you trying to close out? Do I assume this is for equipment subject to NEC rules (you are in India and I am unaware of the NEC being the law there)?

Also, as others have said, the S.C. amps is really dependent on the characteristics of the power source, and not the actual equipment load.

If this is a piece of equipment you are working on, for sale, then you can specify that it must be connected to a source circuit with a maximum S.C. amps. Doing this requires you to ensure that all the components will withstand a fault current of that magnitude (or a lower magnitude if downstream of a current-limiting device). After doing this, the customer and installer now assumes the responsibility for supplying the power with the proper S.C. rating.

--John M.

Dear Mr. John M,

Thanks for the Reply. Not for the India project its for project in Europe. Yes this equipment is subject to NEC Rules.

And to find out the upstream contractor and the person and the supporting docs will be very difficult. The Scope we are having here is very small only stuck with the NEC rules i already checked with the Max assumption of calculation with IEC 60909 its comptiable.

Only thing is need rules for supporting doc. like any previous project r Rules In NEC.

IF i have the supporting doc from NEC rules for short circuit its easy.

Thanks,

Prabakar

Mr. Prakabar,

The NEC (NFPA 70) is a document that lists the conditions or specifications that a completed electrical project must meet. It does not tell the electrician how to do the work. If includes phrases such as telling you that all equipment must be installed in accordance with the manufacturer's instructions and any instructions or requirements that are part of its listing.

That word "listing" has a special meaning--it is the certification by an independent agency that a component has been tested to perform according to a published standard. In the USA this is often by Underwriters Laboratories (UL) or Factory Mutual (FM). In Europe there are many, but two come to mind (TUV and DIN).

I'm wondering if the information you have been given has confused the need for a UL type of listing with the installation requirements that should comply with the NEC. I worked for a number of years for a UL panel-building shop. We did custom panels. Each one had to be built with materials and methods that complied with the UL requirements. The completed panel was then given a UL label. With this label on the panel, it could go to any customer's location and be installed without any further worry about an inspector's or customer's wondering if the panel was acceptable.

Also, look back to the last part of my earlier post. If the customer has already told you what the available S.C. fault current is, then you must use components that can safely handle or interrupt a short circuit of this magnitude. Many people can confuse the S.C. fault current with the much lower currents that may typically flow during an overload. A fuse or breaker may be rated to open with a downstream overload current of 30 amps, but it has to be built to withstand the stresses and heat produced when a downstream short circuit current of 1000, or 6000, or 12000, or possibly even 35000 amps is momentarily flowing through it.

If your customer has specified the available fault current, then you have to build to handle it. If all that your customer has given is the available S.C. fault current at some point upstream from your equipment, there are programs available via free download that will calculate the fault current at the line lugs on your equipment. To do that, however, you need a number of details (wire size, wire length, conduit type, number of wires in parallel, system voltage, incoming available fault current). If a transformer is there, you need the impedance. If you have equipment nearby with a large motor or a capacitor bank, these will increase the available fault current in the circuit. The process of doing this sort of calculation is not easy.

--John M.

I am also from Chennai, India. In this part of world, the panel building has become cottage industry. Any body and every body makes panels here without knowing technical importance. Many people here believe in providing 35 kA MCCB for incomer and 25 kA/16kA MCCB for outgoing irrespective of the location these panels in the Electrical distribution system.

May be the person who posted the query is end user or certifying agency for acceptance.

This is how things are happening here. No scientific and systematic approach for any issues. No designers are involved in Electrical systems design ,people simply copy drawings and specifications from other projects without really and deeply going into the details of requirement for the new Project.

This is one of the reasons for major failures like building collapse recently happened here .

Some thoughts came to my mind and I am recording here .

If this continues , even one day there will be major failure in Electrical Installation also.

For your kind information im from chennai but im not working over there.

Dont belive that other side of the person will not be technically sound, If he also from your place.

IF you found panel manufacturing company is not capable in chennai Pull them to court or compalint about them in Association or the Report to them who certified.

In my opinion, NEC does not indicate any way to calculate the short-circuit current the cable

will withstand. The cable cross section area is chosen only according to continuous load [and the overload protection setting. The grounding conductor cross section area is chosen according to ungrounded conductor[s].

However, in art. 240.4 Protection of Conductors it is an informational note:

Informational Note: See ICEA P-32-382-2007 for information

on allowable short-circuit currents for insulated copper and aluminum conductors.

In General Cable SPEC F125/ January, 2010 -see:

http://www.generalcable.com/NR/rdonlyres/D60F2C04-B580-4C2C-A4DC-07B638A1DA02/0/SpecF125.pdf

it is shown how to verify the short-circuit withstand current for copper and aluminum conductors

insulated with XLPE or EPR.