[RE-wrenches] down-sizing main breaker

Sat Aug 1 18:19:18 PDT 2009

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?

Allan Sindelar

Allan at positiveenergysolar.com

NABCEP Certified Photovoltaic Installer

EE98J Journeyman Electrician

Positive Energy, Inc.

3201 Calle Marie

Santa Fe, New Mexico 87507

505 424-1112

www.positiveenergysolar.com

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of William
Miller
Sent: Saturday, August 01, 2009 6:11 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] down-sizing main breaker

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

At 03:47 PM 8/1/2009, you wrote:

William,

I don't know if I've solved Brian's dilemma yet either. But, with regard to
your
example below, if the 100 amp meter main is accepting 20 amps, or even all
of
the 50 amps of PV, that would mean that the house, at that moment, presented
no
load at all, and that all the PV would be backfeeding the grid, turning the
meter backwards. That, of course, would mean the main 100 amp breaker is
only
handling 50 amps, well below its rating. Likewise the feeder from the house
would only be carrying the 50 amps from the PV, instead of anything near
it's100
amp rating. The code can be perverse, but it's my understanding that the
point
of 690.64(B) is to ensure none of the distribution components are exposed to
overcurrent, not to arbitrarily throw numbers around.

I was just kidding about offering you those 200 amp Milbank breakers. I've
got
to figure out how to list them on Ebay.

Dick

--- You wrote:
Dick:

I don't believe you have solved Brian's dilemma yet.  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.  In the scenario you suggest, that will not be the 
case.  The 100 amp meter main on Brian's project will still  accept only 20 
amps of PV AC feed, still not satisfying his requirements.
--- end of quote ---
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