[RE-wrenches] Combining Multiple Inverters

Fri Jun 27 08:32:02 PDT 2014

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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