[RE-wrenches] Combining Multiple Inverters

[boB at midnitesolar.com]

When this subject came up, the first thing that I thought of was our 
micro-inverter combiner  that
takes 3 strings of many inverters, (each inverter W/OUT OCPD), but does 
have an OCPD for each string.

That would have to be code compliant of course.  (MNPV6 Disco combiner 
Micro for example)

Enphase for instance can have something like 16 micro-inverters on one 
string going
into the combiner using their cable.

boB




On 6/27/2014 8:32 AM, Dave Click 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 ---