[RE-wrenches] Paralleling Multiple Inverter

[William Miller]

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