[RE-wrenches] down-sizing main breaker

Sun Aug 2 07:22:36 PDT 2009

William,

Wow - you're a master at whipping up drawings!

OK, I get the concept, and it's just as I described. The scenarios in
drawings one and two are OK as long as both busbars are rated 150A
(commercial) or 125A (residential. 

You then show that the reason for the upsized feeders is to handle a short.
But wait! Even if the short could produce 140A, as you have indicated, the
conductors will still not carry more than the 100A that they're rated to
carry. One would carry 100A from one panel and 40A from the other, Nowhere
would that conductor carry 140A, and the mate to it would carry at most 60A.
Increasing conductor size wouldn't change anything in that (or any)
scenario. So why would you have to increase conductor size?

Thanks, Allan 

  _____  

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 11:13 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] down-sizing main breaker

Allan:

I whipped up three drawings that indicate three overload scenarios possible
with incorrectly sized distribution equipment.  They are on our web site at:
http://millersolar.com/case_studies/case_studies.html  Click on Point of
connection.  Click on any drawing to see a larger version.

I hope this helps with understanding the concept.

William Miller

At 06:19 PM 8/1/2009, you 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 tap, 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: 

1.	maximum current passing through 100A main breaker = 100A. Adding 40A
of PV only reduces amount through conductors from main AC panel to 60A. 
2.	no AC load in daytime = 40A of current back to main AC load center 
3.	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

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