system expansion [RE-wrenches]

[Sky Sims]

Replace the main breaker in the main panel with a 150 amp breaker. Then
you will be well within the 120% of the bus rating rule by any ones
interpretation of the code. 

This raises an issue that has been on my mind for quite some time
though. 

A while back I was speaking to an engineer at square d and he informed
me that the bus rating is not actually a bus rating it is a tab rating. 
The tab is the point that the breaker attaches to on the bus. 
The bus is actually rated for far more than the tab. 

This means that the NEC is being misinterpreted by nearly everyone in
the country. It is impossible for the individual tabs to be overloaded
unless the breaker on that tab has a higher rating than the tab.

If a panel has a 200amp rating then that means each tab is rated for 200
amps and the bus itself is actually rated for far more (10x?? or more)
than the actual tab rating. Additionally the breakers are thermally
activated, so if the bus or the tab actually begins to approach capacity
(and heat up) the breakers will derate and trip off.

I look forward to more discussion on this topic. It is possible that I
am missing something here but I don't think so. If what I said above is
correct then there is no issue (aside from NEC misinterpretation by the
local inspector) with putting a 200amp solar back feed on a panel with a
200amp tab rating and a 200amp main breaker feeding from the utility
company.

Sky Sims
Ecological Systems
www.ecologicalsystems.biz
220 County Road 522
Manalapan, NJ 07726
ph)732-462-3858  fax)732-462-3962
 

-----Original Message-----
From: Larry Brown, Sunmountain [mailto:sunmountain at netstep.net] 
Sent: Saturday, August 18, 2007 12:00 AM
To: RE-wrenches at topica.com
Subject: system expansion [RE-wrenches]


Wrenches,

I just finished a system for a client with 36 Sanyo 200"s.  4 strings  
of 6 modules each feed a SunnyBoy 6000.  2 strings of 6 modules each  
feed a SunnyBoy 3000.  We did a tap on the main service entrance  
conductors between the utility meter and the main ( new 200 amp )  
service panel.
Now that wen are done the customer wants to add batteries to the  
system.  So we are looking at a Sunny Island and a step up  
transformer or possibly two Sunny Islands depending on critical loads  
to be run.
The questions are how to reconfigure the wiring from the Sunny Boys  
since they will now go to the critical loads panel rather than to the  
tap on the service entrance wiring. The Sunny Island will go to the  
main service entrance panel.
We have 60 amps ( 40 amps for the Sunny Boy 6000 and 20 amps for the  
Sunny Boy 3000 ) coming from the two inverters combined into one 60  
amp outside disconnect to satisfy the local utility requirement of  
only one disconnect switch for the whole system.
The Sunny Island requires a 70 amp breaker.

Any suggestions on how to set up the critical loads sub panel so to  
not violate an code requirements?
Does the Sunny Island fall under the 120% rule since it is a 70 amp  
breaker and a 200 amp main service panel.
Anyone done this set up before?


Thanks

Larry Brown
Sun Mountain
Olivebridge, New York


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