[RE-wrenches] Paralleling Multiple Inverter

[Mark Frye]

William,
 
I think we have it all out on the table. Below you cite 2008 690.64 (B) (2).
But the requirements of (B) (2) do not come into play if it is excluded by
the provisions of 690.64 (B). And as you already pointed out, this all
revolves around the interpretation of the Code vis-a-vis the word "capable".
 
What is the intent of the Code? What reflects reality? In reality, you
cannot find any point in the entire feeder circuit including the load center
itself where the response to a ground fault or short circuit failure would
not overwhelming be controlled by the feeder breaker. On one side of the
feeder breaker you have some inverters, whose ability to produce excess
fault current is limited short term discharge of it's capacitors. On the
other you have an almost unlimited source of excess current and uncontrolled
rise in the rate of energy flowing into a fault. I suggest that in this
case, it will always be the feeder breaker that will open in response to a
fault and to that extent it is perfectly reasonable to size the load center
and feeder to the feeder breaker sufficient to carry the maximum continous
ortput of the inverters.
 
So what was the intent of the Code makers? Why did they include the language
"capable of supplying multiple branch " if their intent was other than to
allow the use of dedicated load centers?

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.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: Sunday, January 22, 2012 11:28 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Paralleling Multiple Inverter


Mark:

I would respectfully disagree, based on my reading.  If you have code
citations that inform me otherwise, I would be very grateful to expand my
knowledge.

Pending hearing otherwise from you, here is what I know, based on 2008 code,
and assuming load side connection  (2008 citations in italics):

1. 690.64(B)(1) Dedicated Overcurrent and Disconnect. Each source
interconnection shall be made at a dedicated circuit breaker
or fusible disconnecting means.

Each inverter will require a 40 amp circuit breaker.  The value of that
breaker is calculated by adding a 25% continuous duty rating to the maximum
AC output, which is 25 amps: 25 * 1.25 = 31.25.  The next breaker size up is
40 amps, so you need 40 amp breakers.

2. 690.64(B)(2) Bus or Conductor Rating. The sum of the ampere
ratings of overcurrent devices in circuits supplying power
to a busbar or conductor shall not exceed 120 percent of the
rating of the busbar or conductor.

The designer must add the values of the circuit breakers to determine the
back-feed value.  We just calculated the circuit breaker size in step 1,
above.  Four inverters means four 40 amp circuit breakers, therefore: 4 * 40
= 160.  

3. In systems with panelboards
connected in series, the rating of the first overcurrent
device directly connected to the output of a utility interactive
inverter(s) shall be used in the calculations for
all busbars and conductors.

Therefore the designer must use 160 amps as the total back-feed value for
all panels and feeders in series all the way back to the service.

Mark, I would really appreciate it if you could reply today with any
information I am missing.  I could use my new-found knowledge to modify the
permit application I am submitting tomorrow morning.

Sincerely,

William Miller

PS:  I used to think, erroneously, that I need only consider the actual
maximum AC amperage from a given inverter.  Some time ago I bid on and
started a job based on that fallacy.  Mid-way into the job the AHJ informed
me that my calculations were incorrect, that I needed to use the breaker
value.  This job used SB6000 inverters and the value required was 40 amps.
I researched this thoroughly and discovered they were right.  In order to
comply with 690.640(B), I had to downgrade the main breaker at my own
expense.  The breaker was not inexpensive, so this is a lesson I learned the
hard way.

This is why I am very interested in any knowledge that might prove otherwise
in this scenario.

Wm 
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