Brian, William & Allan,

Perhaps I've misunderstand the specific distribution equipment involved. I have
assumed the main SERVICE consisted of a meter and single, 100 amp main breaker,
as a combo unit or as separate enclosures, but with no busbar provision for
additional breakers there at the pedestal.  

The service disconnect, in other words, consists of a single switch, in an
enclosure that allows no other breakers. The feeder conductors to the house are
terminated on the load TERMINALS of that switch / breaker. The meter load side
is connected directly to the switch / breaker line side. There is no busbar in
the sense that additional breakers could be installed.

Dick Ratico
Solarwind Electric


>Brian wrote:
Hi Folks,

Can anyone provide code reference or advice on this question?
Our customer has 100 amp service at the meter pedestal, 100 amp main  
breaker feeding 200 foot underground run to a 100 amp panel at the  
house. We need to land a 40 amp breaker, so are proposing to replace  
main panel at house with 200 amp panel. The AHJ says we need a 200  
amp main breaker in this panel, which will work, but says this  
requires upsizing the feeder, main breaker at meter, and therefore  
utility service to 200 amps.

I've read on this list and elsewhere about downsizing the main  
breaker in a panel to create capacity to make the 120% rule. This AHJ  
says you can't put in a breaker smaller than the panel size, and  
furthermore, everything upstream would have to be upsized. Is this  
from the code?

Bruce Erickson
Mendocino Solar Service
PO Box 1252
Mendocino, CA 95460
707-937-1701


>Allan wrote:
William,

I have been following this back-and-forth thread and need you to please
explain one thing:

 Your 140A point makes perfect sense (from the point of 690.64 in requiring
that the busbar to which both utility and PV feeds are connected. No
question there. But you write: "It is my understanding that if one is going
to install a load side tape, the back feed calculations need to work for
every link of the distribution system upstream to the meter." It seems to me
that there's still no way to exceed main disconnect current rating. 

1.	100A utility (using 100A subpanel main) + 40A PV tied into subpanel
busbar = 140A, which is OK if the subpanel busbar is rated at least 125A
(residential) or 150A (commercial).
2.	100A + 40A tapped into subpanel ahead of 100A main breaker:

a.	maximum current passing through 100A main breaker = 100A. Adding 40A
of PV only reduces amount through conductors from main AC panel to 60A.
b.	no AC load in daytime = 40A of current back to main AC load center
c.	some subpanel loads in daytime reduces 2.b. above

3.	At the main panel, maximum current flow through breaker feeding
subpanel is 100A, as any PV reduces load at breaker. However, main panel
must itself meet 690.64, as it's still possible to feed 40A of PV (daytime
with no subpanel load) into main panel. So same busbar requirements apply in
the main panel as in the subpanel. But that's all. The subpanel feeders
don't need to be upsized.

 What am I missing here, please?

>William wrote:

Dick:

Your logic is correct but incomplete.  Consider it like this:  The buss 
bars in the service panel can handle 100 amps (they are rated for that 
amount).  The main breaker feeds 100 amps into that buss, maximizing it's 
capabilities.  If you feed an additional 40 amps into that buss bar, you 
have the potential of exceeding the ampacity of that buss assembly.  If no 
other loads are fed from that buss bar, there is no over current 
scenarios.  Say, however, someone puts more breakers on that buss bar and 
draws 140 amps from them.  100 amps comes in from the utility, 40 amps 
comes in from the PV and the buss is overloaded.

This is the scenario for which 690.64(B) was written.  This will occur only 
if the overload amperage is not drawn from a space on the buss bar between 
the utility feeder and the PV feeder.  If, however, the PV feeder breaker 
and the utility feeder are both on one end of the buss and the load is on 
the other end, then the loads are additive on the buss assembly.

Some want the code passage re-written to say that if the feeder is on one 
end of the bus assembly and the PV is on the other, there is no possibility 
of an overdraw on the buss bar and the installation is legal.  This has not 
happened yet.

William
--- end of quote ---

