[RE-wrenches] Solar accumulation panel (revisited)

[Andrew Truitt]

First - Thank you all for the useful information!  Bill - I'm sorry to hear
that this didn't make it into the 2011 NEC but I'm glad its on the radar!


In the process of getting a solar accumulation panel approved in a local
county we were challenged with something that I have not thought about
before.  The first part of NEC2008 690.64(B)(2) states:

"The sum of the ampere ratings of overcurrent devices in circuits supplying
power to a busbar or *conductor *(emphasis added) shall not exceed 120
percent of the rating of the busbar or conductor."

This means that the conductors between our solar accumulation panel (with
[3] 15A 2-pole breakers = 45A/pole) and the 40A interconnection breaker in
the main panel must actually be sized to 80% of 85A = 68A.  O.K., fine.
NBD.

But what does this mean for connecting to a sub-panel in an outbuilding?  If
you have feeders coming from the house that are sized to a 60A feeder
breaker in the main panel and you want to connect 30A of solar to the
subpanel in the outbuilding, you are out of luck since those feeders are not
rated for 72A (60A + 30A * 0.8).


Am I interpreting this right?



- Drew








On Wed, Jan 13, 2010 at 2:09 PM, Hans Frederickson <hans at fredelectric.com>wrote:

> 690.15, second paragraph supports Drake's method:
>
> "A single disconnecting means in accordance with 690.17 shall be permitted
> for the combined ac output of one or more inverters or ac modules in an
> interactive system."
>
> 690.17 says the disconnecting means can be a switch or circuit breaker.
> This
> seems workable, with a couple caveats... The inverter manufacturer
> directions might specify an individual branch circuit, and you would need
> to
> use the larger size wire between the AC combiner box to the fused AC
> disconnect at each inverter, so you would need to choose disconnects with
> big enough terminals.
>
> Anyone see any other issues with this idea?
>
> -Hans
>
> ________________________________
>
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
> Chamberlin
> Sent: Wednesday, January 13, 2010 9:59 AM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] Solar accumulation panel
>
>
> P.S.
>
> In many cases it would be necessary to use a fused AC disconnect to stay
> within the inverter manufacturer's specifications.
>
> At 12:53 PM 1/13/2010, you wrote:
>
>
>        Here is a wild idea.  If you  look at the Enphase inverter systems,
> several inverters connect together in parallel before going to an
> overcurrent device.
>
>        In all cases, the point of connection to a utility system must be at
> a dedicated breaker.  If there were multiple string inverters that would
> connect to one breaker, why not just use a junction box, instead of a
> combiner panel?  You could put in an AC disconnect for each inverter, if it
> didn't come with one.
>
>        The wiring would need to be sized for the combined output of the
> multiple inverters, which in many cases wouldn't be an issue.
>
>        Drake
>
>
> Drake Chamberlin
> Athens Electric
> OH License 44810
> CO License 3773
> NABCEP TM  Certified PV Installer
> Office - 740-448-7328
> Mobile - 740-856-9648
>
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-- 
"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough
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