120% rule, help!

[wwhitney]

[Backup Gateway 2 already has a set of extra lugs in it that you can use for this wiring topology, so that would make it simple to do. I forget their rating.]

OK, I looked up the those lugs, they are rated for #2 wire maximum, meaning they would only be good for a 100A feeder segment to a non-backed up load panel. So if you really want more than 100A for the non-backed up loads, you would still need to do a separate splice. If there's room in your 200A interior panel for Polaris-style connectors, you could put them there, that way you wouldn't have to repull the conductors from the exterior disconnect.

Cheers, Wayne

 

[Vines]

Here is what I would do in your case. You are limiting your non backup loads to 100A max, but that is likely fine.

You still have 200A from the grid, but the possible downside is that more loads than you wish will be on the backup side, as that is the side with more capacity.

 

[wwhitney]

Hi Vines,

Nice drawing, it shows schematically what I was describing with (perhaps too many) words.

One caveat with using those "up to #2" lugs on the line side of the Gateway: they would be the start of a feeder tap, unless the utility side breaker plus the sum of 125% of the inverter output currents is less than the ampacity of the conductors (very unlikely). So the conductors need to land on a breaker within 25' of the Backup Gateway, meaning that if the non-backed up panel is farther than 25' away, a separate disconnect with OCPD would be required within 25'.

And for a large system, the highest ampacity conductors you can use are #2 Cu with an amapacity of 115A, so when the utility breaker plus 125% of the inverter output currents exceeds 345A, (e.g., with a 200A main breaker, (4) Powerwalls and PV over 20A inverter output current; or (3) Powerwalls and PV over 44A inverter output current), the distance becomes 10', because the 25' tap rule no longer applies.

I'm a bit surprised Tesla didn't provide lugs that accept up to 250 kcmil like all the other lugs for ungrounded conductors..

Cheers, Wayne

 

[Vines]

Hi Vines,

Nice drawing, it shows schematically what I was describing with (perhaps too many) words.

One caveat with using those "up to #2" lugs on the line side of the Gateway: they would be the start of a feeder tap, unless the utility side breaker plus the sum of 125% of the inverter output currents is less than the ampacity of the conductors (very unlikely). So the conductors need to land on a breaker within 25' of the Backup Gateway, meaning that if the non-backed up panel is farther than 25' away, a separate disconnect with OCPD would be required within 25'.

And for a large system, the highest ampacity conductors you can use are #2 Cu with an ampacity of 115A, so when the utility breaker plus 125% of the inverter output currents exceeds 345A, (e.g., with a 200A main breaker, (4) Powerwalls and PV over 20A inverter output current; or (3) Powerwalls and PV over 44A inverter output current), the distance becomes 10', because the 25' tap rule no longer applies.

I'm a bit surprised Tesla didn't provide lugs that accept up to 250 kcmil like all the other lugs for ungrounded conductors.

Cheers, Wayne

Thanks, its my tool I created to discuss these complicated SLD. What you saw is about 1/3 of it.

The reason for the lug size in my opinion is because otherwise people would treat this as a 200A connection In my opinion it should have been a 200A connection.

However I am confused by your interpretation of 220.21.B.2 I think this is more correctly applied by using the largest of the OCPD sources, rather than the addition of them. Item 1 of part 2 for up to 25' explicitly says to use the over current rating protecting the conductors.

I could see your point if someone strictly interprets this, but that is not appropriate for a 1741 listed inverter source in my opinion. Only the 200A grid breaker is otherwise an unlimited source. I could see the argument for a generator circuit in a tap conductor being the same category as a grid source but those usually have their own rules.

 

[wwhitney]

However I am confused by your interpretation of 220.21.B.2

See (2017) NEC 705.12(B)(2)(2), it specifically address this situation.

The reason for the lug size in my opinion is because otherwise people would treat this as a 200A connection In my opinion it should have been a 200A connection.

Yeah, that's what I meant, a 200A lug and a 200A busbar to it.

Cheers, Wayne

 

[Vines]

Thanks again for the schooling Great find, and totally correct. Not everything allowed in 705.12 is 1741 listed.

Edit: So basically you need to keep these subs within 10' for the size sytems we typically install, or outdoors. Also, either a 100A integrated breaker on the GW2 non backup side, or fully rated lugs would be ideal.

 

[wwhitney]

So basically you need to keep these subs within 10' for the size sytems we typically install, or outdoors.

Or just put a 100A disconnect with OCPD with 10'.

Also, either a 100A integrated breaker on the GW2 non backup side, or fully rated lugs would be ideal.

If the internal panel board is not being used for generation (would 6 spaces even be enough for 150A of generation breakers?), you can use it for a 100A backfed breaker and come off the bottom lugs of it to your non-backed up panel.

Cheers, Wayne

 

[Vines]

Or just put a 100A disconnect with OCPD with 10'.


If the internal panel board is not being used for generation (would 6 spaces even be enough for 150A of generation breakers?), you can use it for a 100A backfed breaker and come off the bottom lugs of it to your non-backed up panel.

Cheers, Wayne

Agreed about the 100A breaker panel only, though the usual reason that you don't put the non backup sub right there by the MSP is its an eyesore, a single MB only panel would be the same. If you had to put something near the MSP it might as well just be the loads panel itself in most cases.

I don't see any cases for the internal panelboard being extended though its lugs, though it would be legal. If you needed more slots for non backup loads than the internal panelboard allows its better to just go directly to another panel with your 100A feed, unless you have a weird case where a 100A main cannot fit in the existing panel or backfeed the bus.

With 6 slots only 3-2 pole breakers or 6 quads will fit. Quads max at 50/50. Certainly you could max out the 200A internal sub, before you run out of breaker spots but then usually a subpanel to combine either the PV or Powerwalls would be added. Those allow running right up to the edge of the 200A limit, without running out of slots first in the internal panelboard.

One thing i just considered is that I do not believe the lugs on the internal panelboard will accept 250 kcmil. Not even sure they will accept 3/0, as they are quite small. They are designed to use the factory supplied 105C rated jumpers from the GW load side lugs. I will have to take a look at this tomorrow, and see what is the largest size wire the 200A internal sub will take in the lugs.

Good thing to notice, and great conversation Wayne.

 

[Vines]

Just looking at the wiring in the gateway 2, and there are no concerns about the internal panelboard wiring as I had mentioned yesterday. The lugs on the internal panelboard will take #1 AWG to #300 kcmil

 

[roofsolartexas]

Thank you everyone, and thank you for the diagram. I’m confused though, if that diagram is for me, what was the reasoning for downgrading it to 100 amps?

My electrical system will be activated before Tesla even comes out. My electrician said he probably wouldn’t be able to feed a 200 amp sub panel from the main 200 amp panel. I don’t know if it’s not possible to take two 100 amp breakers, and interconnect them?

I did speak to Tesla, they said I could get the 6 circuit panel board inside of there. My question here:

1. I have a 200 amp disconnect breaker outside, so I need one on the panel for the entire thing? This would shut power off to both back up and non back up?

2. With the Gateway, if I upgrade the 125 Amp sub panel to a 150 amp sub panel, would there be two separate 200 amp breakers in the sub panel?

Tesla said the generation panel is not used or needed for power wall installations. Thanks!

PS, the rep from Tesla I use, it’s constant emailing. I email to ask a question, and he ignores it or bypasses it. For example, I asked specifically to write out how the power would be routed to help me understand, and he then says the design team won’t do any of that until things are finalized. I didn’t ask about the design team, I asked him...then I email back again telling him he has failed me, and he did not answer my question.
Thankfully some awesome person on here drew a diagram and that answers my question.

Power flow:
1. Meter
2. 200 amp exterior disconnect
3. Tesla Gateway 2(they confirmed it will be a gateway 2)
4. From the gateway, it will feed the sub panel and main panel.


Thank you everyone, you’re a great help!

 

[mongo]

Thank you everyone, and thank you for the diagram. I’m confused though, if that diagram is for me, what was the reasoning for downgrading it to 100 amps?

My electrical system will be activated before Tesla even comes out. My electrician said he probably wouldn’t be able to feed a 200 amp sub panel from the main 200 amp panel. I don’t know if it’s not possible to take two 100 amp breakers, and interconnect them?

I did speak to Tesla, they said I could get the 6 circuit panel board inside of there. My question here:

1. I have a 200 amp disconnect breaker outside, so I need one on the panel for the entire thing? This would shut power off to both back up and non back up?

2. With the Gateway, if I upgrade the 125 Amp sub panel to a 150 amp sub panel, would there be two separate 200 amp breakers in the sub panel?

Tesla said the generation panel is not used or needed for power wall installations. Thanks!

PS, the rep from Tesla I use, it’s constant emailing. I email to ask a question, and he ignores it or bypasses it. For example, I asked specifically to write out how the power would be routed to help me understand, and he then says the design team won’t do any of that until things are finalized. I didn’t ask about the design team, I asked him...then I email back again telling him he has failed me, and he did not answer my question.
Thankfully some awesome person on here drew a diagram and that answers my question.

Power flow:
1. Meter
2. 200 amp exterior disconnect
3. Tesla Gateway 2(they confirmed it will be a gateway 2)
4. From the gateway, it will feed the sub panel and main panel.


Thank you everyone, you’re a great help!

If you need a 200A pass through panel. Check out mobile home style panels.
This one has 8 spaces for your non-backed up loads.
Square D QO 200 Amp 8-Space 16-Circuit Outdoor Main Breaker Load Center with Feed-Thru Lug and Cover-QO1816M200FTRB - The Home Depot

 

[wwhitney]

Thank you everyone, and thank you for the diagram. I’m confused though, if that diagram is for me, what was the reasoning for downgrading it to 100 amps?

The Backup Gateway 2 has provisions for attaching a feeder to a non-backed up load panel, but the lugs for that are limited to 100A.

As to your questions (1) and (2), let me skip them for now and address some of your other comments. Then if something's still not clear, please ask again.

Tesla said the generation panel is not used or needed for power wall installations. Thanks!

Because you can use the internal 6 space panelboard for that purpose.

PS, the rep from Tesla I use, it’s constant emailing. I email to ask a question, and he ignores it or bypasses it.

Your questions are too technical for that person. It'd be like asking a dentist's receptionist if you need a root canal. And Tesla doesn't like to let you talk to their designers. 2 years ago I was able to get one to call me by asking a technical question and asking the rep to pass it on to the designer. But that's only after your project has gone to their designers.

Power flow:

Schematically, it's like this (and all panels have a main breaker, except perhaps the Generation Panel)

Service 
Disconnect ----------  Non Backed up Panels
              |
              |
            Backup
            Gateway
            Transfer
            Switch
              |
              |
Generation    |
Panel -----------------  Backed up Panel

Physically, it could be like this:

Service Disconnect
|
|
Backup Gateway ----- PV and Powerwalls
|   |
|   ------- Non Backed Up Panel
|
Backed Up Panel

And given what you have already, and your interest in 200A for the Non-Backed Up Panel, it could physically be like this:

Service Disconnect
|
|
--Non Backed Up Panel
|
|
Backup Gateway --- PV and Powerwalls
|
|
Backed Up Panels

where the -- in front of "Non Backed Up Panel" means use a 3-way splice on the 200A feeder, housed within the 200A non-backed up panel.

Cheers, Wayne

 

[Yinn]

Thanks for the article. I read that, but I’m not sure how the line side tap would work. Specifically, how would they tap into the wires?

They would install a junction box and using really big taps. Those would go after meter, prior to the PV disconnect.

Top Left to Meter
Top Right to PV Disconnect
Bottom Left to Main Panel

This setup is a valid method of interconnect but requires additional approvals from the POCO. If you set up the PV this way, the gateway would install as you normally would. You can have a critical loads panel or backup the entire main panel.

 

[M--J]

OK, I looked up the those lugs, they are rated for #2 wire maximum, meaning they would only be good for a 100A feeder segment to a non-backed up load panel. So if you really want more than 100A for the non-backed up loads, you would still need to do a separate splice. If there's room in your 200A interior panel for Polaris-style connectors, you could put them there, that way you wouldn't have to repull the conductors from the exterior disconnect.

Cheers, Wayne

Sorry for the very late reply, but I'm curious about your 100A number. It looks like 2AWG THHN copper comes in at 130A, unless I'm missing something?

 

[wwhitney]

Sorry for the very late reply, but I'm curious about your 100A number. It looks like 2AWG THHN copper comes in at 130A, unless I'm missing something?

#2 Cu has a 90C ampacity of 130A. But equipment will have a termination rating of at most 75C, limiting the use of #2 Cu to an ampacity of 115A. The 130A figure is primarily useful as a starting point for derating due to ambient temperature or number of conductors in a conduit or cable.

Cheers, Wayne

 

[Yinn]

Sorry for the very late reply, but I'm curious about your 100A number. It looks like 2AWG THHN copper comes in at 130A, unless I'm missing something?

Ampacity Calculator

The purpose of this calculator is to determine the ampacity of conductors in conduit, cable, or directly buried. All calculations are based on the National Electrical Code®.

www.cerrowire.com

Also take into consideration voltage drop, length of run, location, etc. The amp capacity charts are a good start but not the only factors. I've upsized a DC wire run for voltage reasons before.

 

[charlesj]

Ampacity Calculator

The purpose of this calculator is to determine the ampacity of conductors in conduit, cable, or directly buried. All calculations are based on the National Electrical Code®.

www.cerrowire.com

Also take into consideration voltage drop, length of run, location, etc. The amp capacity charts are a good start but not the only factors. I've upsized a DC wire run for voltage reasons before.

When I installed my system, I split the 17 Enphase inverters into two branches to minimize the IR drop and upgraded to a 10ga drop from the roof to subpanel. I couldn't upgrade the 12ga connecting the inverters, that was a given cable.
It add up over time. 10th years and counting.