[RE-wrenches] Utility inverter rating

[Brian Teitelbaum]

Dave's answer is correct, but can also be fleshed out a bit.

The SI6048 has an internal transfer switch that is rated for only 56A at 120VAC. All power from the production sources need to go through that relay, at least if the two inverters are set up normally where both are connected in the critical loads panel. In order to protect that transfer relay from overcurrent (load draw in the critical loads panel or high production from the PV array), it needs to be protected with a 50A single-pole breaker in the Main panel (assuming a breaker with 100% duty rating and that you can't find a 56A breaker). That's only 6000W at 120VAC, which would be the max AC output into the Main panel. Make sure that you are not overloading the bus in the Main panel.

This is off-subject but still relevant:

Since the Island is a 120VAC inverter, and the SB7000 has 240VAC output, there will have to be a transformer somewhere in the system: either a step-down between the SB and the critical loads panel, or a step-up between the critical loads panel and the Sunny Island. Either way, the connection to the Main is at 120VAC, unless you add a second transformer to step the Island's 120 up to 240, or use two Sunny Islands.

If you have a PV array that could max-out the SB7000, you may have a nuisance tripping issue with that 50A single-pole breaker. In the installation manual for the new SMA Smartformer for the Sunny Island, it only recommends using the SB7000 or SB8000 in off-grid use for this very reason. I suggest that the system design be revisited to either use a smaller PV inverter, or two Sunny Islands. 

I understand that SMA is working on a 240VAC version of the Island inverters, and hopefully they will include a transfer relay with a higher current rating.

Brian Teitelbaum
AEE Solar

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Click
Sent: Thursday, October 18, 2012 11:56 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Utility inverter rating

Erika, I think the takeaway here is that the Island only has that surge power capacity on the protected load side- you wouldn't see this surge on the utility side. SMA should be able to provide this sort of documentation for you.

On 2012/10/18 9:38, Erika Weliczko wrote:
> Greetings all.
>
> I recently received this back regarding an interconnection application 
> proposing a Sunny Island 6048 and a SB7000:
>
> "we have to rate the system at the max output of the inverters, and 
> since that battery backup has an inverter that can export 11kW, 
> (albeit for 3 seconds) I have to review the system for a total of 18kW."
>
> This is a new one on me. I might argue that the SB7000 is upstream of 
> the SI6048 and really the SI6048 is the point of interconnection, so 
> the
> SB7000 is irrelevant. And 3 seconds is not really continuous operation.
>
> It seems that batteryless inverters show continuous output ratings, 
> though I know there are some that specify a larger than expected OCP 
> for various reasons, but that is not what the utility cares about.
>
> Any insights here?
>
> Thanks,
>
> Erika
>
> REpower SOLUTIONS
>
> www.repowersolutions.com
>
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>
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