[RE-wrenches] Combining Multiple Inverters

Jason Szumlanski jason at fafcosolar.com
Fri Jun 27 09:41:52 PDT 2014


Dave,

I looked at the BR24L70RP a while back. It's really a 2-space, 4 cuircuit
panel. I think my conclusion was you actually need a single quadplex
breaker with common trip center and outer breakers to get (2) 2-pole
breaker functionality. The quadplex breakers come in a variety of mixed
rating (i.e. 15A center and 20A outer breakers). They are not readily
stocked and not cheap. We stuck with Sq D QO load centers for that reason.

Am I missing something?


Jason Szumlanski

​Fafco Solar​





On Fri, Jun 27, 2014 at 11:32 AM, Dave Click <daveclick at fsec.ucf.edu> wrote:

> 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 <daveclick at fsec.ucf.edu> 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, <re-wrenches-request at lists.re-
>>>>> wrenches.org>
>>>>>
>>>> wrote:
>>
>>>
>>>>> Message: 5
>>>>> Date: Wed, 25 Jun 2014 19:17:57 -0700
>>>>> From: William Miller <william at millersolar.com>
>>>>> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
>>>>> Subject: Re: [RE-wrenches] Combining Multiple Inverters
>>>>> Message-ID: <04F4B8FD-E280-4BF2-B5A4-C2FCA8D1C7A6 at millersolar.com>
>>>>> 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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