Supply side tap downstream from 400A safety disconnect at detached building

[pclausen]

So I have a question about doing a supply side tap at a detached building. I have 400A service at the main dwelling. The 400A meter base is connected to a 400A safety disconnect. The load side of the disconnect feeds both the main dwelling as well as my shop located 200ft away.

Here's what the setup at the main dwelling looks like:



The tall box on the left is a 400A automatic transfer switch that feeds a pair of 200A sub panels inside the main dwelling. This is for diesel backup generator.

Anyway, here's what the inside of the transfer switch in the middle looks like:



As you can see, the shop feed exits at the bottom and shares a 300A fused feed with the house. Inspector made me change from 400A fuses to 300A fuses since I'm "only" using 300 MCM Cu wiring between the meter base, disconnect and transfer switch. The 300 MCM feeding the shop is spliced to 500 MCM Al as soon as it exits the cabinet.

At the shop 200 ft away, that 500 MCM comes up into a 400A meter base as seen here:



The inspector made me separate Neutral and Ground since I have a 4 wire feed from the main dwelling. The meter base does not readily accommodate this, so I have to pull the neutral and splice it to 3/0 Cu that is being feed to the 200A subpanel in the shop. In the above pic, you can see how I propose doing a side tab for the PV subpanel.

Here's a look at the sub panel inside the shop:



As you can see, my thought is that I'll pick up Neutral for the PV subpanel from the Neutral bar here.

So will that fly, or should I pick up the Neutral for my side tab from the splice in the meter base instead? Inspector doesn't want me to land 2 Neutrals under the same screw, so I would like have to get a different splice than what I have right now.

Final question is how to ground the PV subpanel. Can I just ground it using a split bolt at the shop ground rod?

The whole reason for having to do this PV subpanel at the shop is the 120% rule. I'm doing a 20kw system which will have 6 x 20A breakers as seen in the above pick, so that's 120A worth of PV breakers, way exceeding the 120% rule.

The only way around needing a dedicated PV subpanel at the shop would be if I was able to convince the inspector that the panel there has a 225A bus bar, which this label would seem to indicate: (upper left corner)



And I would have to swap out the 200A main breaker for a 150A one. Doing so would get me 225 * 1.2 = 270A. 270A -150A = 120A

But, alas, my shop loads add up to more than 150A going by breaker ratings alone. I suppose if I look at the motor labels on the compressor and lift, they are likely less and I can probably also remove recepts that I never use, to get down below 150A.

And then there is this 2nd label inside the panel that states 200A max:



So getting a 150A breaker would be a gamble I would think.

Which direction would you go?

Thanks!

[Mike90250]

I think your transfer switch, is simply a disconnect. I don't see any contacts or inputs for a generator feed, no relays or control wires.

I also think your PV string (wrong name, PV strings are DC and don't belong in AC panel) They are PV Microinverter feeds .

[SunEagle]

I am curious as to why you have a second POCO meter at your shop.

Also if you backfeed the shop panel & meter you then backfeed the POCO Meter at the house. I presume that both those meters would have to changed out by your POCO so they are able to measure power going both ways for a Net Metering system.

[pclausen]

I think your transfer switch, is simply a disconnect. I don't see any contacts or inputs for a generator feed, no relays or control wires.

It is. The transfer switch is next to it. Here's a shot of the wiring between them:



And here's how the generator will be wired:

I also think your PV string (wrong name, PV strings are DC and don't belong in AC panel) They are PV Microinverter feeds .

Noted. I'll be sure that put that on the actual labels.

[pclausen]

I am curious as to why you have a second POCO meter at your shop.

I used to have seperate 400A services to the house and shop. POCO would not let me do PV to shop only and credit my house account. So they dropped the shop feed. I dug it up and ran it to the house instead. I kept the shop 400A meter base and purchased my own Net Meter for it, primarily to allow current flow through that meter base. The base and meter belongs to me and the POCO is ok with that.

Also if you backfeed the shop panel & meter you then backfeed the POCO Meter at the house. I presume that both those meters would have to changed out by your POCO so they are able to measure power going both ways for a Net Metering system.

The official POCO Net Meter will be at the main house with the disconnect and transfer switches next to it. The shop "meter" just happens to be inline with the feed to the shop sub-panel.

Here's an overall diagram:

[foo1bar]

But, alas, my shop loads add up to more than 150A going by breaker ratings alone. I suppose if I look at the motor labels on the compressor and lift, they are likely less and I can probably also remove recepts that I never use, to get down below 150A.

It is fairly common for the branch circuits to add up to more than the main breaker for a panel.


BTW - I don't know if having 6 PV breakers on the panel is OK under the 120% rule.
My impression was it was 1 breaker at the end of the bar - not 2 at the end, 2 at the next closest location, etc.
And anytime you want multiple backfeed breakers you do a subpanel for the backfeed, and connect that at the end location.
But I am not an electrician, nor a licensed engineer or installer, so do not take my word on it.
It's possible that your inspector could allow multiple backfeed breakers, but I would check with them beforehand.

Also, from your pictures it's not clear to me whether the wires from your shop's meter to it's panel are sufficiently protected for their size.
It looks like they decrease in size in the meter base, but there is no fuse/breaker for their smaller size. (Probably the breaker panel is just on the other side of the wall and has a small enough breaker that it's protected that way. And there's probably something in the code to allow that type of thing. But if it were me, I'd check)

As for the 225A bus bar - I think you should find the model number for the panel and look it up on their website.

Lastly, my understanding is that the 120% rule applied to a subpanel then gets applied to it's parent, and then that parent and on up.
I think you are going to have 300A fuses, then panel using 120% rule, then dedicated PV panel.
I don't know how the 300A fuses and the 120% rule on the subpanel interact - I didn't have to look into that.
If it were me, I'd probably draw a 3-line diagram showing the fuses, the subpanels, etc. And review that with the plan reviewer for the building dept.
Then any issues are hopefully resolved on paper instead of after an inspection.

[SunEagle]

I used to have seperate 400A services to the house and shop. POCO would not let me do PV to shop only and credit my house account. So they dropped the shop feed. I dug it up and ran it to the house instead. I kept the shop 400A meter base and purchased my own Net Meter for it, primarily to allow current flow through that meter base. The base and meter belongs to me and the POCO is ok with that.


The official POCO Net Meter will be at the main house with the disconnect and transfer switches next to it. The shop "meter" just happens to be inline with the feed to the shop sub-panel.

Here's an overall diagram:

I understand. Just make sure that meter you purchased at your shop has the ability to "reverse" or subtract what your pv system supplies. Otherwise you will not get good data from it.

[inetdog]

The way the NEC defines things, your "safety disconnect" contains fuses and so is actually your "service disconnect". As a result everything downstream of it would be considered a load side, not a supply side connection.
The 120% rule and the feeder tap rules would apply to any connections made on the load side of that first disconnect.

The second meter downstream of the first meter and disconnect seems highly irregular to me, since all power going through the second meter will have passed through the first meter along the way.

If the second meter were in series with the inverter output only I could see its justification as a total production meter, suitable for SREC determination or other POCO purposes. But if it really is a NET production meter which measures the excess of the PV production over the shop load, it does not make sense to me.
If it is just a customer owned meter, not read by POCO at all, you can have it measure anything you want it to measure.

The only other safety concern is that the PV connection must be to the POCO side of the transfer switch, which it is. That way the PV will not try to reverse feed the generator during an outage.
That does not make it a "supply side" connection though.

[pclausen]

The second meter downstream of the first meter and disconnect seems highly irregular to me, since all power going through the second meter will have passed through the first meter along the way.

Yes is it highly irregular. That meter base was already on the side of the shop from when it had its own service feed. Rather than removing it and having to wrestle with those 500 MCM cables, it was much easier just leaving it in place and pick up my own meter. The main purpose of that meter is just to pass current through to the sub-panel in the shop. I could have used some old door hinges, but that would have been even more irregular. I went down to the county inspection office, showed them pics and the above riser diagram, and he said that would be fine, as long as I separated ground and neutral in the meter base, which I did by lifting neutral and splicing a 3/0 Cu to carry Neutral into the sub-panel in the shop. That said, the meter I picked up from eBay is in fact a 2 way meter so it will measure flow in both directions. Sure, I'd have to add back the power consumed by the shop equipment to get the true value of what is being generated, but that is immaterial since my solar gateway tracks the exact production, and the POCO NET meter at the house does the same.

If the second meter were in series with the inverter output only I could see its justification as a total production meter, suitable for SREC determination or other POCO purposes. But if it really is a NET production meter which measures the excess of the PV production over the shop load, it does not make sense to me.
If it is just a customer owned meter, not read by POCO at all, you can have it measure anything you want it to measure.

And that is exactly what it is. Just a pair of fancy door hinges.

The only other safety concern is that the PV connection must be to the POCO side of the transfer switch, which it is. That way the PV will not try to reverse feed the generator during an outage. That does not make it a "supply side" connection though.

Correct. I used the wrong term when I titled this thread "Supply side tap". I should have called in "Load side tab before the first sub-panel" or something like that.