10kw solar on detached garage (shed), electricion challenges

brainfog · December 2016

I'm having difficulty getting an electrician to wire to the PV and inverter within my shed. Even more trouble wiring it how I want it done.

Some background. I'm in MA. The shed is new with no electric service. I'm *mounting* the PV panels/rack with a permit. My house was build in 1963 w/100 amp fuse box and a sub-panel breaker box w/8 branch circuits in 4 slots - possible 60 amps. The inverter is Solar World 10000W single phase, 42A maximum output current, and yes I have 10.2kw on a 399 sf shed that looks like alot like a detached garage.

There are two options that I believe would be cost effective to get my PV system operational and grid attached. Are these code compliant options?

1) Don't touch the house wiring, add a supply side tap via a 60 amp disconnect switch between the meter and house panel.
Run ~140 ft of 4wire #6 or #4 to the shed in a 30 inch deep ditch created for this purpose.
Install 100 amp breaker box in the shed, w/60 amp main OCPD (120% rule)
Connect the inverter at the bottom location via a ~50 amp breaker. Label this breaker PV not to be moved.

I've been told that the supply side tap can only go directly to the inverter, not another breaker box. That would create two services (or something). Also told that even if I went to the inverter only, that house panel would still have to be updated.

2) Upgrade my house panel to 200 amp breaker box w/175 amp main OCPD (for 120% busbar rule compliance). At the bottom location of the breaker box, install a 60 amp breaker that connects to a 100 amp shed subpanel as described above. Label the PV breaker that its not to be relocated.

Both of 'my' iptions run a single line though the ditch, leave the inverter in the shed, and get a breaker box in the shed for expansion.

So far the only proposal I've received runs a pair of DC strings though the ditch to an inverter in the house which is supply side connected. For reasons unknown I'm told the house service still needs to be updated and there will be service added to the shed. And the cost is ~$10k - twice my expectations.

OTOH - if it is completed before the existing SREC program shaves 20% off the price (in 2 weeks) I still would be money ahead if I just pay the $10k and suffer w/non-optimal design.

Thank you!

BB. · December 2016

Will your electric company and local codes allow a supply side tap? SSTs seem to be difficult to get for commercial installations and hardly ever for residiential (from what I have read).

Also, if you end up with two meters--Your local Net Metering rules (and minimum cost of service) may make for some issues with billing. See what your electric companies billing and tariff rules are. Two separate meters/service drops may not work out well.

-Bill

mvas · December 2016

Option #1: Supply Side Tap (SST) typical interconnect ...
http://www.homepower.com/sites/default/files/articles/ajax/docs/1_HP150_pg109_CC..jpg
No, you typically do not add another "100 Amp breaker box in the garage"
because that new panel would be a secondary AC Service Entrance with a secondary shut-off point <- not good.
I do not understand why an SST would require the Main Service Panel to be updated
since the whole point of an SST is to make a direct Utility Connection without affecting the Main Service Panel.
Ask your AHJ, "WHY does a Residential SST require the Main Service Panel to be updated?"
The AHJ may be saying, "It is time to upgrade from 100 Amp Fuses to 200 Amp Circuit Breakers no matter what modification you do today."

Option #2: Dedicated Breaker in Main Service Panel ...
A "Dedicated Breaker" means dedicated to the PV Inverter
No you cannot use the Dedicated PV Breaker or the wire between the breaker and the Inverter for any other purpose.

Neither, option #1 nor option #2 gets you a new 100 Amp Sub-Panel in your shed.

The Sub-Panel for the shed will require a separate Breaker / Fuse in the Main Panel and its own wire in the ground.
Maybe, the AHJ is requiring the 200 Amp Service Panel upgrade to allow for this large Sub-Panel in the Shed.

You need to ask the AHJ for his reasoning to fully understand your options.

brainfog · December 2016

BB. said:

Will your electric company and local codes allow a supply side tap? SSTs seem to be difficult to get for commercial installations and hardly ever for residiential (from what I have read).

I might be using "supply side tap" incorrectly - as I'm talking about a connection between the utility meter and the breaker box - thus a 2nd meter isn't required.

Over all it would be the circuit for option 1 would be :

Grid -> utility meter -> AC disconnect -> split to path A and B
path A - to the existing house fuse or breaker box
path B - to a 50 or 60 amp fused outside service switch (commonly used with outside AC units) -> to new shed breaker box -> breaker to the inverter

The "SST" as I'm calling it is just something I've read about as a solution to the 120% (on the bus bar) and it was also proposed via the electrician.

Photowhit · December 2016

Not my end of the pool, but wouldn't the easiest way around to leave the house wiring alone and put a new main box at the shed and make the house a subpanel?

brainfog · December 2016

Thank you mvas

So far the AHJ (wire inspector) hasn't said yes or no to anything. I did verbally propose these options to him, but am waiting for his response. Its also possible that since the proposal was verbal, it won't be understood correctly. Not sure I can submit an understandable wire diagram (I'm not that kind of EE) - esp with the limited time. The electron is stating "this is the proposal" and I need to prove to him my ideas are workable. From what you're saying - they're not!

1) In all cases there would be a single AC disconnect point for the whole service behind the utility meter. My use of SST might be incorreect but my previous comment has the connection path listed in more detail.
2) The electrician's proposed SST places the inverter next to the existing fuse box. Possibility touching the wires between the meter and fuse box could possibly trigger the upgrade? The outside utility meter is actually directly behind the breaker box.....

I've found a paragraph by Mike Holt's that seems to be my option 2.

http://solarprofessional.com/articles/design-installation/nec-section-70512-and-utility-interconnections/page/0/2

"For example, consider a building that has a 400 A main distribution panel protected with a 400 A OCPD. That main distribution panel feeds a 100 A subpanel protected at 100 A. Inside the subpanel, a 20 A dedicated inverter breaker is installed. The 20 A inverter breaker satisfies Section 705.12(D)(2) because the subpanel busbar rating is not exceeded by more than 120%. The 400 A main panel can accept up to 80 A of additional power sources (400 A x 1.20 – 400 A). Section 705.12(D)(7) states that the first OCPD, the 20 A breaker in this example, is used for all upstream conductors and busbars. Previous interpretations of the Code required that you use the subpanel’s 100 A OCPD when calculating the upstream conductors and busbars, and would have prevented a small 20 A PV array from being connected to the subpanel in this example."

Possibly the breaker box in the shed can't be called a subpanel - but the only difference is the distance of the breaker box from the main panel.

mvas said:

Option #1: Supply Side Tap (SST) typical interconnect ...
http://www.homepower.com/sites/default/files/articles/ajax/docs/1_HP150_pg109_CC..jpg
No, you typically do not add another "100 Amp breaker box in the garage"
because that new panel would be a secondary AC Service Entrance with a secondary shut-off point <- not good.
I do not understand why an SST would require the Main Service Panel to be updated
since the whole point of an SST is to make a direct Utility Connection without affecting the Main Service Panel.
Ask your AHJ, "WHY does a Residential SST require the Main Service Panel to be updated?"
The AHJ may be saying, "It is time to upgrade from 100 Amp Fuses to 200 Amp Circuit Breakers no matter what modification you do today."

Option #2: Dedicated Breaker in Main Service Panel ...
A "Dedicated Breaker" means dedicated to the PV Inverter
No you cannot use the Dedicated PV Breaker or the wire between the breaker and the Inverter for any other purpose.

Neither, option #1 nor option #2 gets you a new 100 Amp Sub-Panel in your shed.

The Sub-Panel for the shed will require a separate Breaker / Fuse in the Main Panel and its own wire in the ground.
Maybe, the AHJ is requiring the 200 Amp Service Panel upgrade to allow for this large Sub-Panel in the Shed.

You need to ask the AHJ for his reasoning to fully understand your options.

brainfog · December 2016

Thank you mvas

So far the AHJ (wire inspector) hasn't said yes or no to anything. I did verbally propose these options to him, but am waiting for his response. Its also possible that since the proposal was verbal, it won't be understood correctly. Not sure I can submit an understandable wire diagram (I'm not that kind of EE) - esp with the limited time. The electron is stating "this is the proposal" and I need to prove to him my ideas are workable. From what you're saying - they're not!

1) In all cases there would be a single AC disconnect point for the whole service behind the utility meter. My use of SST might be incorreect but my previous comment has the connection path listed in more detail.
2) The electrician's proposed SST places the inverter next to the existing fuse box. Possibility touching the wires between the meter and fuse box could possibly trigger the upgrade? The outside utility meter is actually directly behind the breaker box.....

I've found a paragraph by Mike Holt's that seems to be my option 2.

http://solarprofessional.com/articles/design-installation/nec-section-70512-and-utility-interconnections/page/0/2

"For example, consider a building that has a 400 A main distribution panel protected with a 400 A OCPD. That main distribution panel feeds a 100 A subpanel protected at 100 A. Inside the subpanel, a 20 A dedicated inverter breaker is installed. The 20 A inverter breaker satisfies Section 705.12(D)(2) because the subpanel busbar rating is not exceeded by more than 120%. The 400 A main panel can accept up to 80 A of additional power sources (400 A x 1.20 – 400 A). Section 705.12(D)(7) states that the first OCPD, the 20 A breaker in this example, is used for all upstream conductors and busbars. Previous interpretations of the Code required that you use the subpanel’s 100 A OCPD when calculating the upstream conductors and busbars, and would have prevented a small 20 A PV array from being connected to the subpanel in this example."

Possibly the breaker box in the shed can't be called a subpanel - but the only difference is the distance of the breaker box from the main panel.

mvas said:

Option #1: Supply Side Tap (SST) typical interconnect ...
http://www.homepower.com/sites/default/files/articles/ajax/docs/1_HP150_pg109_CC..jpg
No, you typically do not add another "100 Amp breaker box in the garage"
because that new panel would be a secondary AC Service Entrance with a secondary shut-off point <- not good.
I do not understand why an SST would require the Main Service Panel to be updated
since the whole point of an SST is to make a direct Utility Connection without affecting the Main Service Panel.
Ask your AHJ, "WHY does a Residential SST require the Main Service Panel to be updated?"
The AHJ may be saying, "It is time to upgrade from 100 Amp Fuses to 200 Amp Circuit Breakers no matter what modification you do today."

Option #2: Dedicated Breaker in Main Service Panel ...
A "Dedicated Breaker" means dedicated to the PV Inverter
No you cannot use the Dedicated PV Breaker or the wire between the breaker and the Inverter for any other purpose.

Neither, option #1 nor option #2 gets you a new 100 Amp Sub-Panel in your shed.

The Sub-Panel for the shed will require a separate Breaker / Fuse in the Main Panel and its own wire in the ground.
Maybe, the AHJ is requiring the 200 Amp Service Panel upgrade to allow for this large Sub-Panel in the Shed.

You need to ask the AHJ for his reasoning to fully understand your options.

10kw solar on detached garage (shed), electricion challenges

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