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

[Mark Frye]

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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