[RE-wrenches] Supply Side Tap Overcurrent Protection

[Darryl Thayer]

I have always thought it was the same calculation as the Fault current as calculated in NFPA 70E.  but I will ask some one whom I respect.
Darryl

--- On Sun, 3/28/10, R Ray Walters <ray at solarray.com> wrote:

From: R Ray Walters <ray at solarray.com>
Subject: Re: [RE-wrenches] Supply Side Tap Overcurrent Protection
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Sunday, March 28, 2010, 11:45 PM

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. Waltersray at solarray.comSolar 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-Ops 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 ampdouble 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 PalumboIndependent Power LLCOffices in Lamoille and the Champlain Valley802.888.7194www.independentpowerllc.com
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