[RE-wrenches] Supply Side Tap Overcurrent Protection

R Ray Walters ray at solarray.com
Sun Mar 28 21:45:13 PDT 2010


This brings up a related question, that I can't seem to find an NEC answer to: How do we calculate the available fault current, for picking our AIC rating?
I used to always throw 20,000 amps AIC rating fuses and breakers for large battery banks, but on grid, it seems we would have to know things like the rating of the utility's upstream transformer, etc.
to really know the proper AIC to use.
The code mentions how important AIC is in several sections, but never explains how to calculate it. Did I just fall a sleep that day in Code class?
Seems that even a fused disconnect could be suspect, if they put in the wrong fuses. 

R. Walters
ray at solarray.com
Solar Engineer




On Mar 28, 2010, at 1:08 PM, Bob-O Schultze wrote:

> Dave,
> OK, here's my shot at this. Your mileage may vary. I think 240.86(A) applies here. Unless the CB in the Xantrex XW has a higher interrupt ratting (AIC) than the available fault current from that supply side tap and the CB is so marked (240.83(C), a disconnect using appropriate current limiting OCD devices is required. I've never done a supply side tap without the disco being fused with current limiting fuses, nor have I ever heard of one that wasn't current limit protected being approved.
> Good Luck, Bob-O
> ps you travel without your Code book? :)
> 
> 
> On Mar 28, 2010, at 10:52 AM, Dave Palumbo wrote:
> 
> Wrenches,
>  
> I am tasked with reviewing an installation by a licensed electrician in Vermont and would like help with the following:
>  
> Grid-tied PV system with battery back-up. AC point of connection is a supply side tap (done on customer side of utility meter ) then traveling 5 feet of #2 THHWN in 1” EMT to the 60 Amp 240vac Lockable Utility Disconnect (unfused) and continuing 15 feet #2 THHWN 1” EMT to Xantrex XW 60 Amp breaker.
>  
> I had said that the 60amp 240vac lockable disconnect needed to be fused, and received the following reply from the installing electrician.
>  
> << “The Xantrex XW Power Distribution Panel contains the required over current protection (60 amp
> double pole circuit breaker) negating the need for a redundant fuse at the Utility Disconnect switch.” >>
>  
> I am on a short trip in Florida and away from my code books. Can someone provide guidance and code reference please? I am concerned about the 20 foot distance of the un-protected circuit from the supply side tap and also the interrupt rating of the 60 Amp breaker in the Xantrex distribution panel.
>  
> David Palumbo
> Independent Power LLC
> Offices in Lamoille and the Champlain Valley
> 802.888.7194
> www.independentpowerllc.com
> 
> _______________________________________________
> List sponsored by Home Power magazine
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Options & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out participant bios:
> www.members.re-wrenches.org
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20100328/48cacce1/attachment-0003.html>


More information about the RE-wrenches mailing list