Solar Inverter Line Side Connection Questions

arcturusk1

Hey everyone; first time poster here. I was wondering if I could get some advice and help on a possible line side connection for an install that we're going to do in the near future.

We'll be installing a 55kW multipole-mounted system for a municipal building with a Solectria PVI 60kW central inverter as the heart of it and I'm looking at our options for connecting this thing. Their feed from the utility is 208V 3-phase and their main panel is rated for 1200A. The Solectria's Installation & Operations Manual states that the PVI 60 requires a 250A breaker. There is an existing 400A breaker that isn't being used.

Here has been my train of thought on this one:

>The output of the PVI 60 @ 208V is 166A per phase. Multiply by 1.25 to get 207.5A. The next possible breaker size is 225A, I believe, but this is difficult to get. Solectria has also stated that a breaker of this size may cause nuance tripping (might be messing up the wording there). So, they bump it up to 250A.

>The panel is rated at 1200A. If you summed up all the capacities of the breakers on the panel, it would be far greater than 1200A, probably closer to 2200-2400A. This is fine because there will literally never be a time during which every load in the building will be on. Is this mentality correct? Well, I suppose it has to be fine because that's currently how it's set up.

>For the sake of my calculations, I need to assume that 1200A of the panel is being used as I should always assume the worst-case scenario and design for that.

>NEC code says that panels allow you 20% of room above their rated amperage. For this 1200A panel, that means we have 240A of overhead, or wiggle room.

>The requirements need a 250A breaker, not a 400A breaker, so we'll have to swap the 400A breaker out and put the 250A breaker in. Why is this again? If you need 250 and you have 400, why is the extra room violation of code? Is it because the breaker will never trip given how much higher it is vs what the inverter will ever put out (opposite the issue of nuance tripping)? I understand this may be a stupid question, but I just can't wrap my head around it.

>Since we only have 240A of overhead, the requirement of 250A is 10A over.
This seems to be the crux of my problem.

>I considered other inverters but I think I ruled out Satcon's Powergate 50kW for some reason and Fronius' CL line is not out yet.

>Solectria suggested one option is to ask the inspector if the 10A discrepancy is ok

>Solectria suggested the other option is to do a line side connection
This seems to be the safer and more code-compliant solution

I get that a line side connection is between the meter and main panel and completely gets around the limitations imposed by the main panel at the same time as being totally code compliant (690.64). What I don't get is how this connection is made. Is a panel put in series between the main and the meter? Is it a J-box instead? I haven't had much luck searching the net as all I'm getting are links to definitions about line and load and two links to archived posts on this forum that don't actually explain how to do it. I need to know how this may work so I can pitch it to my boss as an option. I also need to know because I need to come up with a Scope of Work for the municipality. They're going to be handling the electrical contractor work post-AC-disconnect while we handle everything up to the AC disconnect.

I apologize for the length of the email but I try to be very detailed with my questions and explanations. Thank you for your time!

drees

Re: Solar Inverter Line Side Connection Questions

Non-residential installs do not allow you to exceed the busbar rating of the enclosure - so if you have a 1200A panel and want to feed 250A of PV through it, you need to downsize the main breaker to 950A at most.

Having no experience with commercial installs, I would imagine that doing a line-side install would make sense in a lot of cases because of this limitation.

arcturusk1

Re: Solar Inverter Line Side Connection Questions

Thanks for the reply, drees. A few questions, though:

Where in the code is the restriction on non-residential installs? I'm reading 690.64(B)(2) and it states that "The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor..." The quote finishes by talking about sub-panelboards.

I do not have experience with downsizing breakers. What exactly do you mean by that? I do not know if this is an option with this install given the panelboard/switchgear in place.

As you have given a bit of a nod to the line-side connection, could you direct me to any really good resources that describe how to do this? If we need to do this, I'm still unsure as to how this takes place. Is it like the following picture? (On reflection, the right side is basically just the left side minus a box over top of it)

dwh

Re: Solar Inverter Line Side Connection Questions

Hrmm. The rule changed...

http://solar2solar.com/resources/CODE%20CORNER/INTER%20CONNECTION%20NOW%20AND%20THEM.pdf

"In the 2008 NEC, 690.64 was rewritten and the
120% allowance was applied to commercial installations
if an additional requirement was met. That requirement
[690.64(B)(7)] says that the PV backfed breakers must
be mounted at the opposite end of the bus from the
main breaker or feeder. This location prevents overloading
the busbar. If this requirement cannot be met, then
the sum of the breakers will be limited to no more than
the busbar rating on commercial installations."

dwh

Re: Solar Inverter Line Side Connection Questions

arcturusk1 wrote: »

Is it like the following picture?

Yup, pretty much.

http://www.nmsu.edu/~tdi/pdf-resources/IAEI-1to2-06.pdf

drees

Re: Solar Inverter Line Side Connection Questions

dwh wrote: »

Hrmm. The rule changed...

Good to know - tough to keep all the revisions in check! I wonder what improvements the 2011 NEC will bring...

arcturusk1

Re: Solar Inverter Line Side Connection Questions

dwh wrote: »

Yup, pretty much.

http://www.nmsu.edu/~tdi/pdf-resources/IAEI-1to2-06.pdf

Awesome. Thanks for the link! That was very helpful; it was what I figured it would be, although I was assuming there would be a lot more complexity. It really isn't so bad. I'll probably be contacting the local inspector to run this by him, too.

drees wrote: »

Good to know - tough to keep all the revisions in check! I wonder what improvements the 2011 NEC will bring...

Yeah, with some of the really nice advances in solar tech, I wonder how the Code will change to adapt to these (or restrict their use).


With what seems like few (or easy) restrictions, it seems like supply-side/line-side connections are much simpler for larger kW installs. Why bother with busbar ratings and frying a main panel when you can just tap into the supply side?

Thanks for the help, folks.