[RE-wrenches] Main Service Fuse rated higher than Bus Rating of attached sections?

[Allen Frishman]

Mark,
Thank you, your response is very helpful!     How does the 120% rule get applied to the 1000A sections in my example?     When grid connecting downstream on the load side of the switches on the 1000A sections do we ignore the rating of the 1000A bus bar being protected by a 3500 Amp Fuse?    If yes what section in the code allows this?


Al Frishman
AeonSolar

(917) 699-6641 - cell
(888) 460-2867
www.aeonsolar.com


On Feb 17, 2015, at 10:23 AM, Mark Frye wrote:

> NEC 2008, 240.21 (B)(1)(1) a.
> 
> Each of the smaller buss sections are feeders which are sized according to the tap rules. I believe the answer to your question is that the larger fuse can't protect the smaller buss. But, the likelihood of a short developing in that section of the circuit is very small and if one did, damage to the equipment would be limited that area. This is the essence of allowing taps.
> 
> Mark Frye
> 
> On 2/17/2015 6:17 AM, Allen Frishman wrote:Wrenchers,
>> I run into lots of Service Equipment that has a Large Service Switch which feeds multiple Sections with the Bus bar on each section rated significantly less than the main bus.    A specific example that I am currently looking at has a 4000Amp Switch with a 3500 Amp Fuse which feeds 3 (1000 Amp) switchgear sections.     How is it possible that the 3500 Amp Fuse protects the (3) 1000 Amp Sections?    In this particular case each of the 1000 Amp Sections has between 3 and 6 fused switches on them.     The sum of those switch/fuse ratings exceeds 1000 Amps in each case.    For reference - This service gear is over 40 years old. 
>> 
>> All thoughts are appreciated.
>> Al Frishman
>> AeonSolar
>> 
>> (917) 699-6641 - cell
>> (888) 460-2867
>> www.aeonsolar.com
>> 
>> 
>> 
>> 
>> _______________________________________________
>> List sponsored by Redwood Alliance
>> 
>> List Address: RE-wrenches at lists.re-wrenches.org
>> 
>> Change listserver email address & settings:
>> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
>> 
>> List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html
>> 
>> List rules & etiquette:
>> www.re-wrenches.org/etiquette.htm
>> 
>> Check out or update participant bios:
>> www.members.re-wrenches.org
>> 
> 
> _______________________________________________
> List sponsored by Redwood Alliance
> 
> List Address: RE-wrenches at lists.re-wrenches.org
> 
> Change listserver email address & settings:
> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
> 
> List-Archive: http://www.mail-archive.com/re-wrenches@lists.re-wrenches.org/maillist.html
> 
> List rules & etiquette:
> www.re-wrenches.org/etiquette.htm
> 
> Check out or update 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/20150217/7da5fc31/attachment-0002.html>