NYC Con Edison Service Switchgear KAIC (SCCR) Rating

[shespuzzling]

Looking for some enlightenment regarding short circuit current in NYC. The installation in question is very typical; it's a ConEd Secondary Service with (6) 265/460V transformers, network protectors, and (6) 4000A Service Switchgear Lineups. There are no main devices on any of the services (also very typical for NYC).

Con Edison gives you a maximum available fault current from their network of 200K. It may actually be much lower than this but in my experience ConEd won't budge on this value. The problem is, the maximum SCCR rating of Service Switchgear is 200K, you can't get a UL listed line-up with an SCCR higher than this. Our fuses are rated for 300KAIC, but the bus bracing is only 200K. When you take into account motor contribution, your available fault current at the Service will always be more than 200K!!

Wondering if anyone knows how to get around this from a code or technical standpoint. Adding a main with a current limiting fuse is not an option. Help!

 

[Ingenieur]

Any conductor between xfmrs and sg?
xfmr size?

 

[Ingenieur]

What voltage?

con edison service terms
they will calc a fault current if requested in writing
all installs > 800 A must be submitted for approval

Their transformer spec for > 1 mva 5.32 to 6.18%
if not in this range rejected

 

[infinity]

We recently installed 3-4000 amp 480Y/277 services (no mains) and all of the switchgear was rated for 200,000 KAIC. Between the Con-Ed stabs from the network protection system we had 10 sets of 600 kcmil all about 12' long. For us this is a pretty common setup, give or take a few feet on the SEC's, so is more than 200,000 KAIC really necessary?

 

[augie47]

We recently installed 3-4000 amp 480Y/277 services (no mains) and all of the switchgear was rated for 200,000 KAIC. Between the Con-Ed stabs from the network protection system we had 10 sets of 600 kcmil all about 12' long. For us this is a pretty common setup, give or take a few feet on the SEC's, so is more than 200,000 KAIC really necessary?

May I ask purely for my own education,.... taking your numbers to one of the AIC programs I have, the available only drops to 193,000. If one had a decent motor contribution would the 200k not pose a problem ?
I don't recall encountering a fault current above 200k. When one does, what is the answer ?
Is a bolt-lok fusible switch braced beyond 200k ?

 

[infinity]

May I ask purely for my own education,.... taking your numbers to one of the AIC programs I have, the available only drops to 193,000. If one had a decent motor contribution would the 200k not pose a problem ?
I don't recall encountering a fault current above 200k. When one does, what is the answer ?
Is a bolt-lok fusible switch braced beyond 200k ?

Honestly I have no idea but would add that the distances mentioned in my first post are from the network protection stabs not the utility transformer. Between the transformer and the network protector there is probably 20' of cabling on the POCO side not to mention the collector bus and the feeding the stabs. If the Con-Ed numbers are from the network stabs then I would guess that one could ignore everything upstream of the stabs.

 

[Ingenieur]

The 200k is not realistic imo
not knowing xfmr rating if we use the 4000 A as i rated
Z pu = 4/200 or 2%, not likely for a xfmr this large
their own spec calls for >5% for >1 mva
the actual i fault is likely <100 kA

the 200 kA is a fault MVA of 167 !!!!

you need to request i fault in writing
we are speculating on the 200k

 

[augie47]

In my small town Southern City world it would be unlikely but we have seen available currents above 100k when the POCO underground grid of inter-conencted transformers is encountered.
I assumed with ConEd 200 might be plausible.

 

[iwire]

In my small town Southern City world it would be unlikely but we have seen available currents above 100k when the POCO underground grid of inter-conencted transformers is encountered.
I assumed with ConEd 200 might be plausible.

That is when we run into those high fault currents as well. When the utility supply comes from a network of interconnected transformers.

Often the power company requires cable limiters in those applications.

http://www.cooperindustries.com/con...l_library/BUS_Ele_Tech_Lib_Cable_Limiters.pdf.

 

[NewtonLaw]

LTN and high AIC

LTN and high AIC

The 200k is not realistic imo
not knowing xfmr rating if we use the 4000 A as i rated
Z pu = 4/200 or 2%, not likely for a xfmr this large
their own spec calls for >5% for >1 mva
the actual i fault is likely <100 kA

the 200 kA is a fault MVA of 167 !!!!

you need to request i fault in writing
we are speculating on the 200k

I have for many years designed and supplied services for Customers supplied by our Low Tension Networks. These systems are not one transformer, so asking for transformer data only is valid for Spot Networks, i.e., networks made up of two or more transformers supplying one bus system. LTNs typically are made up of many transformers, our smallest was 23 transformers and four primary supply cables, our largest was about 68 transformers supplied by 10 primary cables. All secondary from these transformers supply a bus which connects to all the secondary's of the network. Some of my manholes had ring bus cables that exceeded 200,000 amps of fault current. All transformers on the grid helped to supply the fault current. All this to say, Con-Ed's system which is the largest LTN in the US as far as I know, is more than reasonable to suggest you use a 200K SCCR. Until you have seen the energy developed by a fault in the LTN, you really have little idea of what kind of energy and over pressures can be developed.

Considering that Con-Ed may have to add additional transformer vaults and cables in a developing area, you may start off at as little as 30k Amps and in 5 years find the fault currents have reached the 200k mark. Now consider what the 2017 NEC codes and the requirements that you must change out equipment for any updates where the new AIC exceeds what you have in service. Con-Ed's requirements are not unreasonable.


For SHESPUZZULING - I have specified Pringle Bolted Pressure Switches for cases like this in the past using current limiting fuses, but I am not sure if this would apply in your case.

Hope this helps.

 

[Ingenieur]

I can see the 200k if the xfmr sec are bused (not clear in OP)
that is effectively a 24000 a service

one of my profs worked for con ed, Gregory Reed
might try to sit with him and discuss to have a better understanding

 

[augie47]

Still one nagging question in my mind. Even the Pringle switches I look at have a rating of 200K. If, after calculations, you exceed 200k, what type of main can be used ??

 

[GoldDigger]

Still one nagging question in my mind. Even the Pringle switches I look at have a rating of 200K. If, after calculations, you exceed 200k, what type of main can be used ??

My guess would be that current limiting fuses would be mandatory, whatever type of switching was used.

 

[Bugman1400]

Why can't simple line reactors be used?

 

[infinity]

I'll leave the fancy calculations to you engineers but in my experience in working with Con-Ed the 200,000 K seems to be the standard for 480Y/277 volt services fed from a network. The last three major jobs that weve done with them have had 13.8 feeding three or four 13.8-480Y/277 transformers which in turn feed a network made up of network protectors and collector bus. The collector bus feeds the services through 4000 or 5000 amp fuses.

The first photo is the fuses from the collector bus to the service stabs inside the network protection vault, the second are the stabs feeding the service.

 

[ron]

The struggle is real.

200kA is the Con Ed standard when fed from network transformers. It is acceptable to do your best regarding equipment ratings.

Most bolted pressure switches that are used for con Ed service switches have a short circuit rating of 200,000 rms symmetrical amperes at rated voltage when protected by Class L fuses.

200kA is the best you can do for now, so even though your motor loads may drive you higher, it is acceptable by the AHJ that the equipment be rated at 200kA.

 

[mbrooke]

IS it actually 200,000? If so I agree, what about motor contributions? Yes I know old thread- but I to would like to know.

 

[electrofelon]

IS it actually 200,000? If so I agree, what about motor contributions? Yes I know old thread- but I to would like to know.

Fyi, three year old thread ...

 

[jrohe]

Infinity: Any chance you could post the photos you posted in this thread a couple years back? I'd be interested in seeing those.

 

[mbrooke]

Fyi, three year old thread ...

I know But relevant to me as 3 seconds ago.