This is an interesting conversation, but I'm not sure it's going to
really impact anything.
When an inverter is listed, it is tested with some "maximum output
overcurrent protection" as part of its rating per UL 1741. Typically
inverter manufacturers set this to be just above the inverter's max
current + 25%. As you know, many inverters can work at more than one
voltage, so for the SB5s their max current ratings at 277/240/208V are
18/20.8/24A, respectively. The max overcurrent protection is 50A. This
figure often appears in the manual but not in the data sheet. For SMA,
it's in the manuals. For some manufacturers, it's only available from
tech support.
For the Sunny Boy line, the only way you could have more than one
inverter on a shared OCPD is if you have two SB5s operating at 277V
(18.1A rated current each, so 2x18.1x1.25 < 50A). The SB6, SB7, and SB8
all have currents too high to make this work, as they share the 50A max
OCPD. The SB3/3.8/4 have a max OCPD of 30A, which is too low to
double-up since each SB3 requires an OCPD of at least 20A. The TLs don't
work either, as the max for any unit is too low. I don't see it in the
SMA manual, but I know that in other manuals I've seen a manufacturer
requirement for a dedicated OCPD. The only inverters I know of that have
a max OCPD rating far beyond their rated current are micro-inverters.
As for the shared disconnect, since at least the 2005, 690.15 has noted,
"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." That being said, the NEC requires ac
modules and micro-inverters to have ac disconnects (connectors,
typically) per 690.6 and 690.15(A). 690.15 notes that an inverter be
able to be disconnected from all sources, but if doubling up these SB5s
you could argue that this shared disconnect accomplishes that via the
anti-islanding protection... so I agree now that this is a grey area.
Anyway, it comes down to whether it's worth worrying about this for what
is an extremely small subset of compatible inverter configurations, when
you can just do something like install a $50 Eaton BR24L70RP (70A bus,
240V, 4 spaces) with two 30A breakers and be on your way.
DKC
On 2014/6/27, 6:53, Richard L Ratico wrote:
I may be playing devil's advocate here, but I want to add a few thoughts. Jay, I
just looked at Fronius and SMA inverter data sheets. I found no spec. for an
output OCPD, only a spec for max. output current.
William, most inverters now come with integrated DC/AC discos. Dave, 2014 NEC
705.12(D)(1), IMHO, specifies ONE OCPD for the entire interconnected power
system, not individual OCPD for each inverter.
Generally speaking OCPD is provided for the circuit conductors in a system, not
the individual pieces of equipment. If the equipment manu. specifies OCPD, that
is a different story.
I understand the 2014 code to require appropiate OCPD for all the conductors
from the panelboard to the inverter. If that can be accomlished with one device
at the panel, it meets code. That said, code is a MINIMUM standard.
Bottom line, I agree with Corey that there is no code requirement for individual
OCPD for each inverter. If there is, unfortunately, it is insufficiently
explicit such that we are having this conversation.
Dick Ratico
Solarwind Electric
--- You wrote:
I have never seen an inverter that does not specify a OCPD size.
Jay
Peltz power.
Sent from my iPhone
On Jun 26, 2014, at 3:20 PM, Dave Click <[email protected]> wrote:
Corey,
The line of reasoning is faulty. It's 705.12(D)(1). Micro-inverters are the
exception because they are specially listed to share a breaker. As for the other
inverters, doubling them up on a single breaker / disconnect probably goes
against their installation instructions [110.3(B)] and it's unlikely that you
could put multiple units on a single breaker anyway because when you take (2
inverters) x (rated current) x (1.25) you will probably come up with a minimum
breaker size that is larger than the maximum allowed under the NRTL listing to
UL 1741.
Dave
On 2014/6/26, 16:18, Corey Shalanski wrote:
We considered the necessity to shut down individual inverters and determined
that the added costs of an inverter output combiner panel were not merited. I
agree that in theory it seems beneficial to be able to switch each inverter
individually, but how often does this occur in practice? For the relatively
infrequent cases where we need to return to a jobsite and shut down an inverter
- for troubleshooting/removal/etc. - we do not mind shutting down the other
inverters (up to a limit) for what is hopefully a short period of time.
Again, this whole line of reasoning may be shown to be faulty if someone can
directly point to the Code section that requires OCPD on each individual
inverter.
--
Corey
$E1eB
On Thu, Jun 26, 2014 at 9:20 AM, <[email protected]>
wrote:
Message: 5
Date: Wed, 25 Jun 2014 19:17:57 -0700
From: William Miller <[email protected]>
To: RE-wrenches <[email protected]>
Subject: Re: [RE-wrenches] Combining Multiple Inverters
Message-ID: <[email protected]>
Content-Type: text/plain; charset="utf-8"
Not allowed. You need a dedicated OCPD. Plus seems like a really bad idea.
How do you shut down just one inverter?
Miller Solar
--- end of quote ---
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