solar interconnect via feedthrough lugs

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SceneryDriver

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Electrical and Automation Designer
Solar array with 31.25A design current and 35A fuses in fused disconnect (PoCo requirement to see "open blades") currently connected as a line-side tap to a clapped-out 150A service.

Proposed replacement:
200A meter/main with 200A main breaker and (8) breaker spaces and feedthrough lugs.

(2) 125A breakers feed: main panel in house, and critical loads panel (eventually to be generator backed). Single pole breakers to feed panel service receptacles and basement lights (to have power and light if work on any panels is necessary). 200A panel in garage is fed via the feedthrough lugs to accommodate multiple EV chargers in the future.

Solar is backfed through a 40A breaker installed at the opposite end of the bus from the main breaker in the meter/main. My interpretation is that this would meet the 120% rule. Is that correct even when using the feedthrough lugs? The bus stops at the lugs, yes?

Thoughts?


SceneryDriver
 
Does the garage subpanel have a main breaker?

What year NEC is the project subject to?

Are 200A panels available with subfeed lugs at the main breaker end, rather than feed thru lugs at the far end?

Cheers, Wayne
 
wwhitney said:
Does the garage subpanel have a main breaker?

What year NEC is the project subject to?

Are 200A panels available with subfeed lugs at the main breaker end, rather than feed thru lugs at the far end?

Cheers, Wayne
Click to expand...
2017 code cycle. I haven't seen any meter/mains with subfeed lugs on the supply-end of the bus.


SceneryDriver
 
wwhitney said:
Does the garage subpanel have a main breaker?

What year NEC is the project subject to?

Are 200A panels available with subfeed lugs at the main breaker end, rather than feed thru lugs at the far end?

Cheers, Wayne
Click to expand...
Only thing I've seen close was what I'll call a 2 main panel, it had 1 that simply had breaker feeder (no buss), the other breaker onto a buss that would allow 4 spaces and feed thru lugs. Both breakers were buss fed from the single meter side. Wasn't what I was looking for so forgot the details of the mfg.
 
SceneryDriver said:
Solar array with 31.25A design current and 35A fuses in fused disconnect (PoCo requirement to see "open blades") currently connected as a line-side tap to a clapped-out 150A service.

Proposed replacement:
200A meter/main with 200A main breaker and (8) breaker spaces and feedthrough lugs.

(2) 125A breakers feed: main panel in house, and critical loads panel (eventually to be generator backed). Single pole breakers to feed panel service receptacles and basement lights (to have power and light if work on any panels is necessary). 200A panel in garage is fed via the feedthrough lugs to accommodate multiple EV chargers in the future.

Solar is backfed through a 40A breaker installed at the opposite end of the bus from the main breaker in the meter/main. My interpretation is that this would meet the 120% rule. Is that correct even when using the feedthrough lugs? The bus stops at the lugs, yes?

Thoughts?


SceneryDriver
Click to expand...
Your feed through lugs are at the opposite end of the bus, therefore your 40A PV breaker isn't. So that's a code violation. And it's a real overcurrent protection problem, too, because your 200A panel can draw 232A.

One solution would be to put a 200A main breaker in the sub, which is why Wayne asked. If the feeder were also rated 240A then that would help with certain interpretations.

Another solution, if it's not too far away, would be to move the PV breaker to the opposite end of the subpanel busbar.
 
jaggedben said:
Your feed through lugs are at the opposite end of the bus, therefore your 40A PV breaker isn't. So that's a code violation. And it's a real overcurrent protection problem, too, because your 200A panel can draw 232A.

One solution would be to put a 200A main breaker in the sub, which is why Wayne asked. If the feeder were also rated 240A then that would help with certain interpretations.

Another solution, if it's not too far away, would be to move the PV breaker to the opposite end of the subpanel busbar.
Click to expand...
I had not considered the lugs as being the end of the bus. Good point.

New idea:
Downsize the feeder to the garage and place it on a 125A breaker in the meter/main (125A is the largest breaker the meter/main will accept). Solar connects to the feedthrough lugs, protected by the 35A fuses in the fused disco.

Meter/main:
milbankworks.com

Milbank - U5168-XL-200-BL-NE

milbankworks.com milbankworks.com



SceneryDriver
 
jaggedben said:
This wouldn't really take care of the problem.
Click to expand...
How's that?

Having the lugs between the main breaker and the PV breaker would let the PV breaker be at the opposite end of the bus as the main breaker. And it avoids the possibility of actually overloading 1 of the 2 busses if there's a breaker next net to the PV breaker plus feedthru lugs protected downstream at no more than the bus rating.

Cheers, Wayne
 
jaggedben said:
Yup that works.
Click to expand...
Or, to preserve the 200A capacity to the garage panel for future EV charging, the solar feed could be connected via Polaris blocks to the 200A feeder going to the garage panel. That would definitely place the solar at the opposite end of the bus from the main, including the lugs.


SceneryDriver
 
SceneryDriver said:
Or, to preserve the 200A capacity to the garage panel for future EV charging, the solar feed could be connected via Polaris blocks to the 200A feeder going to the garage panel. That would definitely place the solar at the opposite end of the bus from the main, including the lugs.
Click to expand...
I agree that works, but be aware that by moving the interconnection downstream (away from the grid), you are increasing the number of places that have multiple points of supply. So you have to check both the feeder and the service panel under 705.12(B)(2)

The feeder interconnection is subject to 705.12(B)(2)(1). That will be fine with 200A conductors as long as the garage panel has a 200A main breaker (I don't think you've answered that question). Note that the conductors from the 35A fused disconnect to the Polaris blocks will need to be rated at 200A or satisfy the tap rules, as their only grid side OCPD is the 200A service disconnect.

The panel bus still has to satisfy 705.12(B)(2)(3), which it does under subsection (b).

Cheers, Wayne
 
wwhitney said:
I agree that works, but be aware that by moving the interconnection downstream (away from the grid), you are increasing the number of places that have multiple points of supply. So you have to check both the feeder and the service panel under 705.12(B)(2)

The feeder interconnection is subject to 705.12(B)(2)(1). That will be fine with 200A conductors as long as the garage panel has a 200A main breaker (I don't think you've answered that question). Note that the conductors from the 35A fused disconnect to the Polaris blocks will need to be rated at 200A or satisfy the tap rules, as their only grid side OCPD is the 200A service disconnect.

The panel bus still has to satisfy 705.12(B)(2)(3), which it does under subsection (b).

Cheers, Wayne
Click to expand...
The solar conductors will satisfy the outdoor unlimited tap rule, and they'll be #8 conductors in conduit anyway. The garage panel would have a 200A main breaker, and the short stubs of conductor between the feedthrough lugs and the Polaris taps will be sized for 125% of 200A, or 250MCM - kind of silly, considering they'll be about 6" long, but it is what it is.



SceneryDriver
 
SceneryDriver said:
the short stubs of conductor between the feedthrough lugs and the Polaris taps will be sized for 125% of 200A, or 250MCM
Click to expand...
That 125% factor is not necessary, for the same reason it's not necessary to have a 250A bus in your meter main. Both those conductors and the bus have two sources of supply, but they are at opposite ends, so the currents don't add up.

That's in contrast to the conductors from the Polaris block to garage subpanel. Those either need to be protected by the 200A MB in the garage panel (an allowance I consider a type of tap rule), or they need to be sized for 200A + 125% of the maximum inverter output current.

Cheers, Wayne
 
wwhitney said:
That 125% factor is not necessary, for the same reason it's not necessary to have a 250A bus in your meter main. Both those conductors and the bus have two sources of supply, but they are at opposite ends, so the currents don't add up.

That's in contrast to the conductors from the Polaris block to garage subpanel. Those either need to be protected by the 200A MB in the garage panel (an allowance I consider a type of tap rule), or they need to be sized for 200A + 125% of the maximum inverter output current.

Cheers, Wayne
Click to expand...
4/O everywhere and a 200A main in the garage it is. And it'll keep me from looking foolish, asking for 1ft of 250MCM at the supply house. :)


SceneryDriver
 
wwhitney said:
How's that?

Having the lugs between the main breaker and the PV breaker would let the PV breaker be at the opposite end of the bus as the main breaker. And it avoids the possibility of actually overloading 1 of the 2 busses if there's a breaker next net to the PV breaker plus feedthru lugs protected downstream at no more than the bus rating.

Cheers, Wayne
Click to expand...
Just that the size of the feeder and/or main breaker in the sub is 90% of the expense and difficulty, and that applies no matter where the feed through lugs are. Admittedly on the 2017 code this is vague but on the 2020 it is not.

Also I think you're comment about 1 of two busses is wrong. I can't make any sense of that.
 
jaggedben said:
Also I think you're comment about 1 of two busses is wrong. I can't make any sense of that.
Click to expand...
OK, let me elaborate, maybe my understanding of bus geometry on single phase panels is wrong. If the bottom most space on the left side of a panel is the "B" leg, is the bottom most space on the right side "B" leg, or "A" leg, or it varies by manufacturer?

If it's "A" leg on the bottom right, then the PV breaker on the bottom left is connected to the "A" leg with both the bottom right breaker's A leg and the subfeed lugs downstream (gridwise) from it. So the bit of A bus between the second to last breaker space and the last breaker space could carry current from both the grid and the PV, and supply it to both the breaker on the right and the subfeed lugs. That could overload that short piece of bus.

If it's always "B" leg on the bottom right and the breakers share a common bus "post" (not sure of the proper term), then I guess this problem can't occur. That post will either be carrying current from the vertical bus or to the vertical bus, and in the latter case the vertical bus doesn't carry any of the load from the bottom right breaker.

Cheers, Wayne
 
wwhitney said:
OK, let me elaborate, maybe my understanding of bus geometry on single phase panels is wrong. If the bottom most space on the left side of a panel is the "B" leg, is the bottom most space on the right side "B" leg, or "A" leg, or it varies by manufacturer?

If it's "A" leg on the bottom right, then the PV breaker on the bottom left is connected to the "A" leg with both the bottom right breaker's A leg and the subfeed lugs downstream (gridwise) from it. So the bit of A bus between the second to last breaker space and the last breaker space could carry current from both the grid and the PV, and supply it to both the breaker on the right and the subfeed lugs. That could overload that short piece of bus.

If it's always "B" leg on the bottom right and the breakers share a common bus "post" (not sure of the proper term), then I guess this problem can't occur. That post will either be carrying current from the vertical bus or to the vertical bus, and in the latter case the vertical bus doesn't carry any of the load from the bottom right breaker.

Cheers, Wayne
Click to expand...
I can't tell one way or the other based on the photo of the Milbank meter/main I linked to earlier, but I wouldn't want to get into an argument with the inspector over something so trivial. I think the photo shows the breaker stab and the bus as separate but who knows. Practically in the real world there's probably no difference, but one is code compliant and the other isn't.

A pair of Polaris taps preserve the 200A feed to the garage for future EV charging, and prevent any argument with the inspector. Done and done as far as I'm concerned.


SceneryDriver
 
wwhitney said:
OK, let me elaborate, maybe my understanding of bus geometry on single phase panels is wrong. If the bottom most space on the left side of a panel is the "B" leg, is the bottom most space on the right side "B" leg, or "A" leg, or it varies by manufacturer?

If it's "A" leg on the bottom right, then the PV breaker on the bottom left is connected to the "A" leg with both the bottom right breaker's A leg and the subfeed lugs downstream (gridwise) from it. So the bit of A bus between the second to last breaker space and the last breaker space could carry current from both the grid and the PV, and supply it to both the breaker on the right and the subfeed lugs. That could overload that short piece of bus.

If it's always "B" leg on the bottom right and the breakers share a common bus "post" (not sure of the proper term), then I guess this problem can't occur. That post will either be carrying current from the vertical bus or to the vertical bus, and in the latter case the vertical bus doesn't carry any of the load from the bottom right breaker.

Cheers, Wayne
Click to expand...
It's always the same leg on the left and right in any row, and the part that sticks out is called a stab.
 
jaggedben said:
Your feed through lugs are at the opposite end of the bus, therefore your 40A PV breaker isn't. So that's a code violation. And it's a real overcurrent protection problem, too, because your 200A panel can draw 232A.

One solution would be to put a 200A main breaker in the sub, which is why Wayne asked. If the feeder were also rated 240A then that would help with certain interpretations.

Another solution, if it's not too far away, would be to move the PV breaker to the opposite end of the subpanel busbar.
Click to expand...
JaggedBen,
Let me ask in this context, this was a year ago, electrical engineer didn't approve the method, so I could not use the lug kit method, he said the breaker in sub panel can be only 70A max. is this true? what if I use 400A load center with 200A main breaker.
another question
Also per 2020 code , the Lug kit has to be just below the main breaker?? can you please elaborate this code??
 

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