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#212271 - 12/27/13 04:50 PM Short Circuit Current Rating  
shortcircuit  Offline
Joined: Jun 2004
Posts: 608
SCCR of equipment must be coordinated so as to not cause damage during a fault.

How many of you are checking your installations for this?

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#212274 - 12/27/13 08:32 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
All complex jobs are engineered -- and the EE performs the math.

The small stuff doesn't need it: virtually no motor loads -- and the worst faults possible are way under the capacity of the one-panel SERVICE.

It would be a big deal if one is working to a Design-Build scheme. Back EMF from hefty motors is the typical headache.

I've never seen any print that sophisticated that was not drafted by an EE -- an in-house EE for D-B shops.


#212282 - 12/28/13 09:49 AM Re: Short Circuit Current Rating [Re: shortcircuit]  
shortcircuit  Offline
Joined: Jun 2004
Posts: 608
Small jobs with a single panelboard can be subject to concern for SCCR of equipment. For example, a local coffee shop has a 150 KVA transformer located close to the building to provide 3phase 120/208 service to the building. The available fault current at the secondary of the transformer will be over 40K short circuit amps. Not all these jobs are engineered and the contractor must check to be sure his service equipment can handle the fault current.

Here is a link to some general information on SCCR...


#212284 - 12/28/13 04:36 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
I've never seen such a grossly oversized pad mounted Poco transformer near such a trivial load in my career.

I don't know how, when, or why such a beast would pass muster.

In my area, such secondaries would be routed underground. EUSERC standards would apply. Cheesy schemes are not allowed.

The number one concern, as ever, earthquakes.


Franchisees build to engineered prints -- as far as I know.

Certainly, that's all that I've witnessed.


The only time that SCCR comes up is when Design-Build -- in house -- is demanded.

It's so rarely done, by electricians, that I would not trust the average j-man to get it right. I'd expect him to miss somehting.


Out my way, coffee shops, and the like, are but a part of strip shopping centers -- just one out of four or more demised spaces -- powered by a NEMA3R SERVICE featuring a house panel and tenant panels. The MAIN OCPD is robust enough to stop shorts in the secondaries -- and was engineered by the developer/ landlord.

I can't recall a SINGLE commercial job that wasn't either engineered, professionally, or was not a trivial sub-set of an engineered scheme.


There was a time I spent a fair amount of effort to learn how to wire up motor controls.

And I have done so -- on my personal heavy equipment; as for on the job: never. Just on the economics, everything is shipped from the factory -- pre-wired. All that is necessary is to land the field wiring. Even that is challenge enough. (I've got a few war-stories about even getting it right when landing to a terminal strip... like 500 man-hours lost... dealing with a mere 12 terminal strips... not a typo! -- When you're lost, everything snowballs out of sight. In this case, the crew had to send for a factory tech to un-screw-up what they'd screwed up.)

So, I just let Ugly's be my guide in the very, very, few instances when I need to revisit the matter.


#212287 - 12/28/13 11:30 PM Re: Short Circuit Current Rating [Re: Tesla]  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Over yonder here, this is not the job of an engineer, but the installing electrician, it is your job to ensure the circuit protection equipment you install, fits in with not only what you have installed after the main isolator, but that the largest protection device you have will co-ordinate with anything the PoCo has installed for their system.

We are also required to perform various "Earth-Loop current" tests to make sure that all wiring and connections are up to scratch.
If there are problems in this respect, you are then required to bring the installation up to the latest level of safety, under the existing standards.

This can be a bit of a mine-field at times, but it does ensure that fuses or CB's operate when they should. smile

#212294 - 12/29/13 10:34 AM Re: Short Circuit Current Rating [Re: shortcircuit]  
ghost307  Offline
Joined: May 2005
Posts: 921
Chicago Illinois USA
Trumpy's right on this side of the big pond as well.
If something's specified that's obviously wrong and the EC blindly installs it, they have a lot to be blamed for.

At the very least the EC should, on seeing something wrong, send in a question to the designing Engineer for confirmation.
If the answer comes back "just put it in" and it later turns out to be wrong the EC is off the hook.


#212305 - 12/29/13 03:22 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
sparkyinak  Offline
Joined: Jul 2007
Posts: 1,328
I usually run the numbers during the review process using the attached above. Doesn't hurt to double check. It's my understanding that this version is a "get you in the ballpark" method. If it came to splitting hairs with it, is be talking with a PE about it. From what I found least with well known equipment, the AIC. Ratings have been fine for small to medium systems. For major power users, it maybe a bigger issue at the MDP

"Live Awesome!" - Kevin Carosa

#212309 - 12/29/13 03:39 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
During Plan Review, AIC is checked, I use the Bussman system

Any questionable issues are RFI back to the EE or Arch for correction or clarifications. All equipment is verified in field inspections.

Occasionally, what is not what's there.


#212324 - 12/30/13 01:11 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
JBD  Offline
Joined: Jul 2001
Posts: 599
The NEC very clearly requires all, not just service entrance, protective devices (110.9) and equipment (110.10) to be installed with the appropriate AIC or SCCR.

These requirements have been in the NEC since at least 1975, yet I still run into electricians that act like they have never heard of them.

#212327 - 12/30/13 03:48 PM Re: Short Circuit Current Rating [Re: shortcircuit]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA

That's because -- for pure practicality -- every EC I've ever known has addressed the matter at a higher pay grade.

The biggest SCCR situations 'always' involve colapsing magnetic fields: transformer and motor windings.

(Mathematically -- the laws of physics -- are identical for three-phase transformers and three-phase motors. Tesla was so brilliant that he could 'see' this in his mind -- even before the first induction machine was ever built.)

Outside of industrial settings, motor loads are too trivial to impact SCCR calculations; a couple of fractional horsepower motors aren't a worry.

Residential distribution systems -- at least out my way -- are already tuned by the Pocos to the AIC of plain vannila SFH Services. A residentially oriented electrician would never encounter the matter outside of a textbook or a forum on the Internet.

This is also true for all engineered commercial work. Out my way, the EEs have to perform the calculations -- which are tucked into a detail in the prints... right along with Title 24 calculations -- and the certification that these prints meet said standards as of XX-XX-XX.

The ONLY time that electricians have to jump to it: Design-Build. In such instances, the EC becomes the EE for his own installation.

EUSERC rules the rules for Service gear out my way. They are so picky that I don't know quite how you're going to be able to screw things up. EUSERC Pocos only permit a few standardized schemes:

480Y277 3-phase
208Y120 3-phase
240/120 1-phase: center tap grounded, 2 legs

All other Services are either grandfathered ( 240/120 3-phase, center tapped, A to B phases) or by special permission and engineering. (all ungrounded schemes)

(EUSERC Pocos grandfathered their A to B center tapped Services -- leaving the C hot as the wild leg/ bastard leg/ stinger. This is permitted by the NEC as an exception.)


Given the above, it's extremely easy to see why countless electricians can go through their entire career without facing SCCR/ AIC issues.

As for legal liabilities: they'd fall all over the EE/EC and his license. The impact is so drastic that no-one wants to trust his j-men to perform said calculations.

The NEC language merely gives the EC no excuses whatsoever. That's why it's in there.

Craft skills taught in school/ apprenticeship are lost when never used. Most of the j-men I know stumble when confronted with even "AIC." They don't even know what exactly that means... for the very same reason.

Their lives are filled with issues of execution -- not design.

The reverse is true. I've run into budding EEs fresh out of college honestly astounded to see their paper plans reduced to practice. (by me, of course) Out comes the camera. It's just too pretty. (if you're an EE or EC)


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