Multiple Points of Interconnection

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mcnut

Member
Location
Florida
First off I am in Florida and still working off the 2008 NEC untill July.

I have a micro inverter system that will have 3 - 20 amp breakers at two different points of connection in the existing electrical system of a single family dwelling.

The house has a standard 200 amp extended service, consisting of a 200 amp meter main combo (with breaker spaces) feeding a 200 amp MLO panel (via feed through lugs in the meter main) wtih 4/0 aluminum conductors. The bus in the meter main is rated 200 amps and the bus in the MLO panel is rated 200 amps.

The first point of interconnection to the PV system is 2 - 20 amp back fed breakers in the MLO panel inside the house. The second point is a 20 amp backfed breaker on the bus bar within the meter main combo enclosure.

Is this code compliant? My fear is this does not meet the 120% rule for the 4/0 feeders because they have 260 amps in overcurrent devices supplying power to them. Am I correct in this and should be be aware of any other possible issues?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
First off I am in Florida and still working off the 2008 NEC untill July.

I have a micro inverter system that will have 3 - 20 amp breakers at two different points of connection in the existing electrical system of a single family dwelling.

The house has a standard 200 amp extended service, consisting of a 200 amp meter main combo (with breaker spaces) feeding a 200 amp MLO panel (via feed through lugs in the meter main) wtih 4/0 aluminum conductors. The bus in the meter main is rated 200 amps and the bus in the MLO panel is rated 200 amps.

The first point of interconnection to the PV system is 2 - 20 amp back fed breakers in the MLO panel inside the house. The second point is a 20 amp backfed breaker on the bus bar within the meter main combo enclosure.

Is this code compliant? My fear is this does not meet the 120% rule for the 4/0 feeders because they have 260 amps in overcurrent devices supplying power to them. Am I correct in this and should be be aware of any other possible issues?
No matter how you split up the backfed breakers you have a problem. If the main panel has 200A busbars and a 200A main breaker, you can only put 40A of backfed breakers into it irrespective of whether they are in the main panel, through a subpanel, or any combination thereof. You will have to go to a supply side connection.
 

DanS26

Member
Location
IN
What about downsizing the main breaker if load analysis allows? Replacing the main breaker with a 150 amp breaker should be sufficient.
 

Carultch

Senior Member
Location
Massachusetts
What about downsizing the main breaker if load analysis allows? Replacing the main breaker with a 150 amp breaker should be sufficient.

Or just mark it as "existing 150A main breaker" on your permit set. And make sure it will be "existing" by the time the inspector gets to your site.

KIDDING!

But seriously, most often main service breakers are oversized, in excess of any instantaneous load that they will realistically experience. If I calculate that I need a 150A main breaker, I'd likely put in a 200A main breaker in the event that someone in the future adds more circuits. Easier to do that now, than to upgrade a service later.

I wonder just how many main breakers in non-PV situations are sized lower than the busbar of the main panel.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I've downsized main breakers a few times, with and (admittedly) without the knowledge of the AHJ. In the former cases, I've never had an AHJ ask for a calculated load analysis, although I would be careful going with a main breaker less than 100A because that's the minimum requirement for a single family home. (I should say I've only done this on single family homes. With commercial you should really do a load analysis, IMO.)

It is correct that you'll have to downsize the main if you can't do a supply side tap. (That's based on the assumption that the 20A solar breaker at the meter/main is loadside, which I believe is what you've described).

As far as the ampacity and protection of the 4/0 AL feeder...

In the 2008 wording, downsizing the main to get under 120% of the ampacity (from both ends) would be enough.
In the 2014 wording, supposing your meter/main busbar rating weren't a problem, you would still have to either downsize the main or install a new overcurrent device on the feeder at the meter/main end. The latter could be done, for example, by taking the conductors from those feed through lugs and diverting them to a fused disco before they head to the existing subpanel.

Solving these problems on 200A services like this can be difficult due to the lack of 200A branch breakers.
 

Zee

Senior Member
Location
CA
I agree with other comments on downsizing most likely being fine.

2p175A would be the very next step down, I have done it many times, and a 175A main breaker should suffice to feed the house loads.

60 amps of PV can be added to that very nicely to get 235A .....which is just under the allowable 240A.:)

(just ordered a Siemens 4pole 175 for my next job...it will be existing by the time i add pv:roll:
It'll allow me to add an 11.4 kw inverter)
 
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