[RE-wrenches] Paralleling Multiple Inverter

Sun Jan 22 13:13:53 PST 2012

Wrenches

Thanks for the detailed responses.

My understanding is the same as William's that my total back feed value is
based on the over current devices for each inverter which is 40 amps X 4 =
160 Amps.  

Where I'm having a problem is choosing a panel based in respect to
considering both the inverter feed and the utility side or just the inverter
feeder side.  Considering just the feed of the inverters is certainly
preferable as Mark suggests since that would allow for use of a 200amp
panel.  

My take on it in that respect is a 200 amp panel, with no additional slots
other than those for the 4 inverters with clear signage that no load
circuits are permitted in this panel. A second small panel would then be
employed if any load circuits were required for lighting or monitoring
devices.

Thanks

Rich

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 2:28 PM
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

At 10:36 AM 1/22/2012, you wrote:

William,

The SMA6000 @ 240V has an AC output rating of 25A. 4 times 25 times 1.25
equals 125 A. If the solar accumulation panel is a dedicated load center,
then the load center, feeder and feeder breaker need only be rated to 125 A.
If the dedicated load center is located in the same building as the service,
then the dedicated load center can be main lug without main breaker. If the
dedicated load center is located on a buidling remote from the the service,
then it will need a main breaker rated at 125A.

Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City,  CA 95959
(530) 401-8024
www.berkeleysolar.com <http://www.berkeleysolar.com/>  

-----Original Message-----
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 10:20 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Paralleling Multiple Inverter

Rich:

Here is the first consideration you might want to make:  You state a value
of 100 Amps for this system, which I assume is the maximum AC output at 240
VAC (25 Amps) times four inverters.  The value of 25 amps per inverter is
irrelevant to your design, however.  You must take the 25 amps times 1.25
(for continuous duty) which means your required breaker is 40 amps (unless
you can find 35 amps breakers).  40 Amps is the relevant value.  40 times 4
equals 160 amps.  Therefore, for purposes of 690.64(B) calculations, you
have a 160 amp system.

Others on this list will no doubt provide advise on how to deal with that
160 amps.

William Miller

_______________________________________________
List sponsored by Home Power magazine

List Address: RE-wrenches at lists.re-wrenches.org

Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org

List-Archive:
http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org

List rules & etiquette:
www.re-wrenches.org/etiquette.htm

Check out participant bios:
www.members.re-wrenches.org <http://www.members.re-wrenches.org/> 

No virus found in this incoming message.
Checked by AVG - www.avg.com <http://www.avg.com/>  
Version: 8.5.454 / Virus Database: 271.1.1/4158 - Release Date: 01/21/12
19:34:00

Miller Solar
Voice :805-438-5600
email: william at millersolar.com
http://millersolar.com <http://millersolar.com/> 
License No. C-10-773985

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20120122/378105eb/attachment-0003.html>