Max Solar Backfeed on a 200A Residential Service

[Hv&Lv]

Not sure if you're being facetious or just not following the thread, but...

First, the POCO has no say about NEC governed service conductors, which was the subject of the post you quoted, if you read back.

Second, what kind of say the POCO has depends on state utility regulations. In California I can build anything I please up to 30kW, and as long as it's on a transformer shared by more than one customer, all the POCO can do is tell me to wait a year for them to upgrade the transformer. (Well, that's a big annoyance, but better than being told to pay something...) Of course, your state will vary, and 200A single phase is well over 30kW.

He said line side tap. To me, that is before the meter and main breaker. line side tap is POCO IMO

 

[jaggedben]

He said line side tap. To me, that is before the meter and main breaker. line side tap is POCO IMO

Line side tap is between the meter and service disconnecting means. It's NEC 705.12(A) (or 705.11 in the 2020). Whether it's POCO jurisdiction depends on all the points already mentioned.

 

[electrofelon]

One interconnect requirement is testing to make absolutely sure the solar generator doesn’t backfeed onto the lines and potentially get someone killed.

I would hate to know I connected an installation illegally and without the POCO knowledge, and without any regard for the lives of the guys working on the lines.

one can bet lawsuits will be many for the homeowner, electrician, and or solar company...

I doubt anyone here was talking about doing a renegade PV intertie without the POCO knowing.....That would be dumb IMO for several reasons, but perhaps most significantly because, unless its an older mechanical meter, the meter will probably count up regardless of the energy flow direction - at least that is what I have found everytime I have tried it.

In my experience, Line side taps (The NEC doesnt use that term, they say "supply side connection" referring to the service disconnect as which "side" you are on) are almost never made on the POCO side of the service point. I would find that very odd.

 

[electrofelon]

One interconnect requirement is testing to make absolutely sure the solar generator doesn’t backfeed onto the lines and potentially get someone killed.

Do you do a witness test on a UL 1741 inverter based system just to double check? Here POCO wants doc saying its UL 1741, then requires installer to fill and sign a test form, but its honor system as they dont come out and do any testing.

 

[GoldDigger]

...
....
In my experience, Line side taps (The NEC doesnt use that term, they say "supply side connection" referring to the service disconnect as which "side" you are on) are almost never made on the POCO side of the service point. I would find that very odd.

Unless there is a separate production meter certified by POCO, the interconnect is never going to be upstream of the meter.

 

[texie]

Technical issues aside, given the meager amount a POCO pays under most tariffs these days under net metering why would anyone have 200 amps of solar unless they had a massively large home/load? Am I missing something?

 

[electrofelon]

Technical issues aside, given the meager amount a POCO pays under most tariffs these days under net metering why would anyone have 200 amps of solar unless they had a massively large home/load? Am I missing something?

Not sure if this was just an academic question or not. But really its not that big of a system. IF you go with the 144 amps that is about 35 Kw, which might be around 40kw DC. Around here it works out for rough numbers you can take the DC watts and multiply by 1.25 to get the annual energy in KWH. So 40 Kw is maybe a $600 mo electric bill. I did a 25 Kw system last summer but that is the largest resi one I have done.

 

[Hv&Lv]

Do you do a witness test on a UL 1741 inverter based system just to double check? Here POCO wants doc saying its UL 1741, then requires installer to fill and sign a test form, but its honor system as they dont come out and do any testing.

We do.

 

[Hv&Lv]

but perhaps most significantly because, unless its an older mechanical meter, the meter will probably count up regardless of the energy flow direction - at least that is what I have found everytime I have tried it.

I get a report weekly showing every meter with reverse registration. We have every meter locked on secure forward, so you are correct in that regardless of energy flow ours (and many others) count up.
what this does is actually count up for generation so the customers bill actually goes up, rather than down with a solar install we didn’t know about.
The last report had 11 meters with reverse registration. 9 are known about, 1 is a single phase CT wiring error on a pole change out, and the last one is a new solar that was connected without contacting us first...

 

[Hv&Lv]

Line side tap is between the meter and service disconnecting means. It's NEC 705.12(A) (or 705.11 in the 2020). Whether it's POCO jurisdiction depends on all the points already mentioned.

I agree with that.

 

[Zee]

...... replace the meter socket with a class 320......

What is a 320 meter socket?

 

[Zee]

We have every meter locked on secure forward, so you are correct in that regardless of energy flow ours (and many others) count up.

Interesting.
.... ~15 years ago, we inadvertently added $1000 to a customer's first mo. bill due to above. Was a little unpleasant.

 

[electrofelon]

What is a 320 meter socket?

Its the larger self contained socket rated 320 A continuous/ 400 non continuous, has two sets of load lugs (or provisions for).

 

[Zee]

Its the larger self contained socket rated 320 A continuous/ 400 non continuous, has two sets of load lugs (or provisions for).

In your area do you typically see separate meter sockets and main service panels?
Here we have the combo versions AKA meter/main panels 99% of the time.

 

[electrofelon]

In your area do you typically see separate meter sockets and main service panels?
Here we have the combo versions AKA meter/main panels 99% of the time.

Yes typically separate. I rarely see meter mains here. I do put them in sometimes for pedestal services serving a house and outbuilding, or for easier temp power at new construction.

 

[Carultch]

Is there any ambiguity here though? I would have to double check the language as I don't remember the exact wording offhand, but doesn't it say "rating of the service"? Is that the ampacity of the service conductors, and what about the 83% sizing?

The rationale of the 83% rule (and the table in NEC2011 that preceded it) for dwelling service conductors is load diversity. I.e. the unlikelihood that a 200A service will operate all loads at once, or ever draw close to 200A. I feel the NEC's rules "double dip" the load diversity explanation, because load diversity is already considered in the algorithm that sizes the service in the first place, but that's a story for another time.

In any case, load diversity doesn't apply to a PV system, because it has the possibility of supplying the full nameplate amperes of the inverters. Even when you undersource the DC feeding an inverter, its output is still modeled as its full load amperes continuously. Therefore, it is best practice to limit the amount you can supply-side interconnect, to the ordinary ampacity of the service conductors. If building loads are temporarily zero, and the system produces full power, you will indeed put the full system current onto the service conductors. You'll ultimately transfer it onto the possibly-smaller utility conductors as well, but they aren't governed by the NEC.

 

[electrofelon]

The rationale of the 83% rule (and the table in NEC2011 that preceded it) for dwelling service conductors is load diversity. I.e. the unlikelihood that a 200A service will operate all loads at once, or ever draw close to 200A. I feel the NEC's rules "double dip" the load diversity explanation, because load diversity is already considered in the algorithm that sizes the service in the first place, but that's a story for another time.

In any case, load diversity doesn't apply to a PV system, because it has the possibility of supplying the full nameplate amperes of the inverters. Even when you undersource the DC feeding an inverter, its output is still modeled as its full load amperes continuously. Therefore, it is best practice to limit the amount you can supply-side interconnect, to the ordinary ampacity of the service conductors. If building loads are temporarily zero, and the system produces full power, you will indeed put the full system current onto the service conductors. You'll ultimately transfer it onto the possibly-smaller utility conductors as well, but they aren't governed by the NEC.

Right I agree that logically the "normal" ampacity should be used, so 144 amps for 4/0 AL but the code should clarify this.

 

[tortuga]

Technical issues aside, given the meager amount a POCO pays under most tariffs these days under net metering why would anyone have 200 amps of solar unless they had a massively large home/load? Am I missing something?

Rural areas where the POCO has large fees for service size upgrades for one, and the loads coincide with sunshine.
If its gonna cost you 10k to put in a larger service why not put that towards PV panels?
There is a farm I recently worked on that has I forget how many pole mount PV systems on a 200A service, they just added three more poles this spring.
They opted a long time ago to put in a 400A panel bus with a 200A utility main and slowly built their system.

The solar poles feed a combiner panel (out in a field) that feeds a 150A fused disconnect that feeds the lugs at the bottom of this service panel:

 

[wyreman]

Coming summer 2014!!


anybody running into the capacity limitations of
NEC705. 12(B)(2)(3)(b) s
and working with the new combos from siemens?
seems like they are expensive
and with Al bus... but

with the MC0816S1200SCT
The solar attaches directly
to the load side of the meter with
the auxiliary disconnect built into the panel front.
Thats why it can accept up to 100a solar
instead of the 50a on a 200a bus
Listed and accepted by pge

otherwise, are there any 200a meter main combos with 225a buss?
-Gotta go 320 class for that but they are pretty big
Small house w 2 EVs and a spa, that's basically it.

Really should just buy a powerwall BB inverter/ critical load panelboard
and dont worry about the size of the bus

MC2442S1200SC

 

[mopowr steve]

One interconnect requirement is testing to make absolutely sure the solar generator doesn’t backfeed onto the lines and potentially get someone killed.

I would hate to know I connected an installation illegally and without the POCO knowledge, and without any regard for the lives of the guys working on the lines.

one can bet lawsuits will be many for the homeowner, electrician, and or solar company...

If I’m not mistaken,
Unless your system can work in Island mode it won’t back feed power without the power coming from the power company to begin with. As the inverter needs the 60Hz to sync with for output.

And as my gears grind, I would think that in theory a solar unit would need to be a maximum of only half the size of a service. As if a load draws unity among the two different systems with a max input of say 200 amps then the most a solar would offer is 100 amps (half current supplied by the POCO and half current from solar)