1. You need to know the actual rating of any sub-section buss that you
will be backfeeding with PV. Do not backfeed this buss with more than
100% of it's ampacity.
2. Evaluate 120% of the 4000A buss against the 3500A main and all PV
backfeed all of the subsections.
Mark Frye
On 2/17/2015 7:37 AM, Allen Frishman wrote:
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 <tel:%28917%29%20699-6641> - cell/
/(888) 460-2867 <tel:%28888%29%20460-2867>/
/www.aeonsolar.com <http://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 <tel:%28917%29%20699-6641> - cell/
/(888) 460-2867 <tel:%28888%29%20460-2867>/
/www.aeonsolar.com <http://www.aeonsolar.com/>/
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