RIch;
I think you're exactly right on your interpretation. Mike Holt's
"Guide to 2011 NEC requirements for PV..." has an 'author's comment' on
page 305, that the 120% buss bar rule of article 705 only applies if
there are loads in the panel. If all slots are filled with PV breakers,
you're off the hook, at least according to Mike Holt, and my self for
the little that matters ;-)
Now you have to convince the AHJ, so you might want to show them Mike's
book on the subject.
Ray Walters
On 1/22/2012 2:13 PM, Rich Nicol wrote:
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-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@lists.re-wrenches.org>
[mailto:re-wrenches-boun...@lists.re-wrenches.org]
<mailto:[mailto:re-wrenches-boun...@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-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@lists.re-wrenches.org>
[mailto:re-wrenches-boun...@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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