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Two [electrical] panels, one gateway

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Howdy! I'm having solar panels and Powerwalls installed. My installer thinks I'll need two gateways, whereas I think I don't. I need your help!

Here's what I have today: One 200A meter, housed in a 320A meter base, which feeds two 150A breakers, each protecting one of two 225A load panels. My house is large but fairly efficient, so I haven't yet exceeded 55A of total load (sum of both panels). If I were to run all air conditioning units, pool equipment, and dryer at the same time (water heater is gas), I would still be below 100A of total load.

I'm having 4 Powerwalls and 20.0 kW of panels installed. My installer saw the two load panels and therefore wants to install two gateways. This will not only cost me more, but will also lead to potentially unbalanced loads vs. backups in an outage. Not to mention the annoyance of tracking production and consumption for the two gateways separately in the app.

Since I only have a 200A meter, I think it makes sense to have the following: One 200A meter, housed in a 320A meter base, which connects to one gateway with a 200A breaker, which connects on the backup loads side to two 150A breakers, each protecting one of the two 225A load panels. Each load panel would keep their existing loads as they are, plus two new 30A breakers for two Powerwalls and one new 45A breaker for half the solar panels (all such new breakers on the opposite end of the feeder).

As far as I can tell, the two load panels would comply with the 120% rule (150 + 2*30 + 45 < 1.2*225). However, the installer is unsure whether the gateway supports connecting two separate load panels in parallel, especially in a scenario where the breakers protecting the two load panels add up to 300A whereas the gateway is only rated for 200A.

Does anyone know why this would or would not work? Does anyone have a similar setup at home, or has installed something similar for one of your customers?
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Here is a makeshift diagram of what I described above.

SLD.jpg
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
What size solar inverters? With the 200A rating of the Gateway and 4 Powerwalls, you can only have (200 - 4*30)/125% = 64A of continuous inverter output current, or @ 240V inverters adding up to 15.4 kW. But if you are under the 2020 NEC (or can get permission to use 2020 NEC 705.13 if under an earlier version), then I believe the Gateway can be configured as a Power Control System to limit the total export current to 160A for a 200A service, allowing additional solar inverters.

With that issue aside, your proposed layout would comply with NEC 705.12. As you observe, each 225A panel is OK under the 120% rule. Tesla's Gateway2 has a pair of load side lugs, can you can run separate 200A feeder segments to each 150A breaker. [Or if the distances are short enough, you could use the feeder tap rules and run 150A feeder segments.]

An alternative would be to have a separate 225A generation panel. That would introduce the complexity of having 3 feeder segments at the Tesla Gateway to connect, but only two sets of load side lugs. But that might be worth it if it is difficult to free up the 3 double pole spaces in each existing 225A panel, or if the two 225A panels are far apart and it would be desirable to have all the generation equipment in one place. In the latter case, if the generation panel is added next to one existing 225A panel, a single feeder segment from the Gateway to that location could be used for both the generation panel and load panel, with some additional attention to the details.

Cheers, Wayne
 
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SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
What size solar inverters? With the 200A rating of the Gateway and 4 Powerwalls, you can only have (200 - 4*30)/125% = 64A of continuous inverter output current, or @ 240V inverters adding up to 15.4 kW. But if you are under the 2020 NEC (or can get permission to use 2020 NEC 705.13 if under an earlier version), then I believe the Gateway can be configured as a Power Control System to limit the total export current to 160A for a 200A service, allowing additional solar inverters.

With that issue aside, your proposed layout would comply with NEC 705.12. As you observe, each 225A panel is OK under the 120% rule. Tesla's Gateway2 has a pair of load side lugs, can you can run separate 200A feeder segments to each 150A breaker. [Or if the distances are short enough, you could use the feeder tap rules and run 150A feeder segments.]

An alternative would be to have a separate 225A generation panel. That would introduce the complexity of having 3 feeder segments at the Tesla Gateway to connect, but only two sets of load side lugs. But that might be worth it if it is difficult to free up the 3 double pole spaces in each existing 225A panel, or if the two 225A panels are far apart and it would be desirable to have all the generation equipment in one place. In the latter case, if the generation panel is added next to one existing 225A panel, a single feeder segment from the Gateway to that location could be used for both the generation panel and load panel, with some additional attention to the details.

Cheers, Wayne
Thanks, Wayne, I really appreciate you taking a look!!! My inverters do add up to 66.5A but I am also under 2020 NEC. I would have no problem configuring the Gateway to export no more than 160A, especially since I am not under a time-of-use electric plan and therefore do not ever benefit from exporting power from the Powerwalls.

I do have enough space in the two 225A panels for all the breakers needed for the Powerwalls and inverters. I would only have ~4 slots left in each panel after that, but down the road I could always free up additional space in the two backed up panels by moving non-essential loads to the non-backup left side of the Gateway. The two 225A panels are also right next to each other, which makes installation somewhat easier.
 

arnolddeleon

Supporting Member
Jul 21, 2012
842
912
SF Bay Area
If they were using a gateway 2 couldn't they use the optional panel board as the generation panel? I don't recall how many slots it would have but if it has enough then I would that think is the cleanest option.
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
If they were using a gateway 2 couldn't they use the optional panel board as the generation panel? I don't recall how many slots it would have but if it has enough then I would that think is the cleanest option.
The internal panelboard uses one of the two sets of load side lugs in the Gateway2; it comes with precut jumpers to be field installed from those lugs to lugs on the panelboard.

The panelboard has 6 1" spaces, so it could could (2) BR3030 quads and (1) BR4040 (?) quad, if the inverters are only 7.68 KW each. I don't think there are 45/45 quad breakers.

Cheers, Wayne
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,072
Los Altos, CA
Would it be possible to use a Generation Panel as one backup lug and make one of the existing panels a sub of the other? Maybe have to reduce to 125A feeding the one with fewer loads, but the full 200A feeding the primary.
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Would it be possible to use a Generation Panel as one backup lug and make one of the existing panels a sub of the other? Maybe have to reduce to 125A feeding the one with fewer loads, but the full 200A feeding the primary.
The complexity here comes from the fact that the meter and two existing 150A breakers are on one side of the house while the two 225A load panels, Powerwalls, and inverter terminations are in the garage on the opposite side of the house. We're in Texas where houses are large, so "opposite side" is a very long distance away.

I would have to check that at least one of the two feeders connecting each 150A breaker with its respective load panel would support 200A of current across the very long length of the feeder. If that's the case, then I guess we could always use the remaining, now unused feeder to connect the Gateway on one side of the house to a newly installed generation panel in the garage.
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
Would it be possible to use a Generation Panel as one backup lug and make one of the existing panels a sub of the other? Maybe have to reduce to 125A feeding the one with fewer loads, but the full 200A feeding the primary.
On further reflection, with a Generation Panel, and more than one load panel, some segments of the feeder are likely going to need a rating above 200A, unless a 200A breaker (or smaller) is introduced between the two load panels and the two sources. The obvious thing that needs to be avoided is both panels drawing 150A simultaneously through a component rated only for 200A.

Cheers, Wayne
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
The complexity here comes from the fact that the meter and two existing 150A breakers are on one side of the house while the two 225A load panels, Powerwalls, and inverter terminations are in the garage on the opposite side of the house. We're in Texas where houses are large, so "opposite side" is a very long distance away.
Given the above, your proposed diagram would put the Gateway by the meter and the Powerwalls and inverters by the 225A load panels? There's a communications cable that connects the PWs to the Gateway, with a distance limit of 150ft. Is that enough for your "very long distance"?

It is worth determining what conductors are run from the 150A breakers to the 225A load panels, and how. If they are identical in basically all respects including length, you could combine them into a single 300A feeder. (Which, depending on other limitations of the equipment involved, you might only use for 200A). That would let you easily put all your new equipment by the 225A load panels. A picture of the equipment at the meter might help, along with any model numbers there. Are the 225A panels MLO or do they have a main breaker in them?

Cheers, Wayne
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Given the above, your proposed diagram would put the Gateway by the meter and the Powerwalls and inverters by the 225A load panels? There's a communications cable that connects the PWs to the Gateway, with a distance limit of 150ft. Is that enough for your "very long distance"?

It is worth determining what conductors are run from the 150A breakers to the 225A load panels, and how. If they are identical in basically all respects including length, you could combine them into a single 300A feeder. (Which, depending on other limitations of the equipment involved, you might only use for 200A). That would let you easily put all your new equipment by the 225A load panels. A picture of the equipment at the meter might help, along with any model numbers there. Are the 225A panels MLO or do they have a main breaker in them?

Cheers, Wayne

Good point, I hadn't thought about that. I estimate the communications cable will have to be about 130ft long, taking into account vertical drops, curves will take in the attic, etc. It may get close but it's unlikely to exceed 150ft.

The two sets of conductors between the 150A breakers and load panels are XHHW-2 2/0 with 90°C insulation. I saw you suggested in a different thread this would be enough to service a 200A circuit. From that I infer I can make a single connection from the gateway, to a 200A breaker, to the first 225A load center (MLO) , with perhaps a 150A breaker connecting to the second 225A load center (MLO; now a subpanel). I could then use the remaining set of conductors to create a connection between the gateway and a new load generation panel, also located in the garage.

Here are two pictures of the meter (sorry for potato quality). The meter itself is an Aclara L-200-C.

PXL_20210209_222639007.jpg
PXL_20210209_222653677.jpg
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
Good point, I hadn't thought about that. I estimate the communications cable will have to be about 130ft long, taking into account vertical drops, curves will take in the attic, etc. It may get close but it's unlikely to exceed 150ft.
Then as long as running that cable is feasible, I think your original idea is likely easiest. The alternatives only make sense to me if you can't run that cable or if it would end up too long. [Edit: or if there is a local requirement for an exterior disconnect for the inverters that would need to be grouped with the Gateway but can still be away from the meter and service disconnect.]

The two sets of conductors between the 150A breakers and load panels are XHHW-2 2/0 with 90°C insulation. I saw you suggested in a different thread this would be enough to service a 200A circuit.
Copper or Aluminum? The 75C ampacity of 2/0 copper is 175A and of 2/0 aluminum is 135A. Those would be the ampacity limits for use carrying inverter output current, or for use as a feeder that is smaller than your service size. However, if you put all of your house loads on just one of those feeders, then residential services/whole house feeders can use a 0.83 sizing factor, meaning that you can use 2/0 copper for a 200A service, or 2/0 aluminum for a 150A service.

This would not apply to your current setup as you have the loads split between two separate feeders. But if the conductors are aluminum, 135A is not a standard breaker size. So it is allowed to be protected at 150A, as long as the NEC load calculation for the connected loads does not exceed 135A. That makes me think there is a good chance they are aluminum, in which case you effectively have a 270A service at the moment.

As to model numbers, the model of the disconnects on either side of the meter, and of the meter base, are of more interest that than the meter itself.

Cheers, Wayne
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Then as long as running that cable is feasible, I think your original idea is likely easiest. The alternatives only make sense to me if you can't run that cable or if it would end up too long. [Edit: or if there is a local requirement for an exterior disconnect for the inverters that would need to be grouped with the Gateway but can still be away from the meter and service disconnect.]


Copper or Aluminum? The 75C ampacity of 2/0 copper is 175A and of 2/0 aluminum is 135A. Those would be the ampacity limits for use carrying inverter output current, or for use as a feeder that is smaller than your service size. However, if you put all of your house loads on just one of those feeders, then residential services/whole house feeders can use a 0.83 sizing factor, meaning that you can use 2/0 copper for a 200A service, or 2/0 aluminum for a 150A service.

This would not apply to your current setup as you have the loads split between two separate feeders. But if the conductors are aluminum, 135A is not a standard breaker size. So it is allowed to be protected at 150A, as long as the NEC load calculation for the connected loads does not exceed 135A. That makes me think there is a good chance they are aluminum, in which case you effectively have a 270A service at the moment.

As to model numbers, the model of the disconnects on either side of the meter, and of the meter base, are of more interest that than the meter itself.

Cheers, Wayne

Your logic is flawless and had me convinced I indeed had 2/0 aluminum. But no, 2/0 copper it is. The two disconnects are Eaton ECB225R with 150A breakers in them. The meter panel is Eaton but has no labels on the outside hinting as of the model (and it is also locked).

All of this makes me wonder why, with only a 200A meter, the builder didn't just wire everything via a single 200A breaker and a single set of conductors to the first 225A load panel, with the second load panel installed as a subpanel. The builder would have saved a boatload by avoiding a second, 130ft run of 2/0 copper.

So basically it looks like I have two options:

Option 1: My original idea, which is connecting the gateway to the two 150A disconnects in parallel, each of which leads to one 225A load panel, each of which would connect to two PWs and half the inverters.

Option 2: Connect the gateway to the two disconnects, one of which would have a 200A breaker instead. The disconnect with the 200A breaker connects via the first set of 2/0 copper conductors to the first load panel, with the second load panel installed as a subpanel. The disconnect that still has the existing 150A breaker connects via the second set of 2/0 copper conductors to a new load generation panel housing breakers for all PWs and all inverters.
 

wwhitney

Member
Nov 2, 2017
907
1,193
Berkeley, CA
On option (2) you'd be limited to 175A of inverters by the 175A ampacity of the 2/0 Cu conductors. The breaker in the disconnect could be increased to 175A, but with (4) PWs you'd be limited to 240V * 55A / 125% = 10.56 kW of PV inverters. You could do 13 kW DC of PV on a 10 kW inverter.

Cheers, Wayne
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
Thanks everyone for all your help! I thought I had made good progress with my installer with all the suggestions offered here, until we got to the load calculations.

First, one question about the optional load calculations for additional loads. Am I right that there is either a mistake or a loophole in 220.83? 220.83(A) seems to apply when there is no additional HVAC equipment being installed (my case), but the load calculation does not explicitly include any HVAC equipment (unless HVAC equipment is considered an "appliance", in which case it would be subject to a 40% load factor). HVAC equipment is explicitly listed in 220.83(B), but it only seems to apply to situations where "additional" HVAC equipment is to be installed (not my case).

Second, are PWs considered loads? My calculated load under the optional method is around 180A. If PWs indeed count as loads, four PWs would take me over 200A of calculated loads. I have never seen calculated loads discussed as a constraining factor to the number of PWs installed. Instinct tells me there is something about the Gateway being a PCS or preventing PWs from charging from the grid that would be relevant here. But I can't find anything in 2020 NEC or Tesla documents making it clear one way or another. Or are PWs not loads only if segregated to a generation-only electrical panel?
 

CrazyRabbit

Member
Apr 21, 2020
415
126
Fort Worth TX
powerwalls are not loads, they are generation devices. you will probably have a generation panel landing inverters and powerwalls. when charging from grid, tesla understands that the gateway will have to manage the load on the grid, same with grid services.

alternate method worked for me, plus i got electric resistance heat. i will move spa tub and evse to non backup panel.
 

SolarGardener

Member
May 19, 2021
15
7
Dallas, TX
powerwalls are not loads, they are generation devices. you will probably have a generation panel landing inverters and powerwalls. when charging from grid, tesla understands that the gateway will have to manage the load on the grid, same with grid services.

alternate method worked for me, plus i got electric resistance heat. i will move spa tub and evse to non backup panel.
But can't the Powerwall draw power from the grid during Storm Watch? If so, wouldn't the Powerwall be a load at least some of the time?

The issue is my installer is claiming the Powerwalls have to be included in the load calculation. If that's not correct, I'd like to point him to something specific in NEC or Tesla documents that says otherwise.
 

BGbreeder

Member
Jun 19, 2020
297
190
Bay Area
But can't the Powerwall draw power from the grid during Storm Watch? If so, wouldn't the Powerwall be a load at least some of the time?

The issue is my installer is claiming the Powerwalls have to be included in the load calculation. If that's not correct, I'd like to point him to something specific in NEC or Tesla documents that says otherwise.
Your installer is right.

Powerwalls are both a load and a source, and the NEC calculations need to reflect that.

All the best,

BG
 

miimura

Well-Known Member
Aug 21, 2013
6,365
6,072
Los Altos, CA
Gateway 2 has PCS (Power Control System) that will prevent the Powerwalls from overloading the grid feeder with Storm Watch charging. How this fits with NEC load calcs, I have no idea.
 

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