Line Side Tap & Islanding

[heimdm]

Has anyone done a line side tap and also done islanding? I am thinking of down the road doing the Tesla powerwall. I am just curious what the looks like from a design perspective.

[jflorey2]

Has anyone done a line side tap and also done islanding?

A line side tap is a method of connecting to the feeds between the service entrance and the load center, thus bypassing the load center and its limitations on maximum current. It is usually not compatible with an islanding system.

If you want to do islanding the conventional way, then use a subpanel with all your critical loads that you want to back up. The straightforward way to do it is via DC coupling, where the inverter sits between the main panel and the subpanel. This gives you a lot of flexibility and control over the system. The main panel has all your heavy loads (resistance heaters, air conditioning etc) and the subpanel has the critical loads. The charge controller ensures the battery is correctly charged.

If you want to do AC coupling that's a little harder. You will either have a disconnect for the entire panel (i.e. an external transfer switch or disconnect) or again use a subpanel with critical loads. In a well integrated system the inverter will communicate with the battery (and even the loads) to ensure the battery is not overcharged, and will disconnect loads if the battery is discharged too far. If the external transfer switch/disconnect is between the line side tap and the service entrance (which is unusual) that might work, but it's not common AFAIK.

[Mike90250]

A line side tap will not accommodate any off grid operation BUT a simple Grid Tie would work, when the grid goes down - your inverter goes down.

To use any battery, you need to be able to isolate parts of your house from the grid. Many hybrid inverters have small transfer relays in them, but if they are feeding a line side tap, no go, unless you add a new sub panel and move your loads to it

[Will792]

One line diagram for my system with PWs. CT requires external switch to isolate solar and batteries from the rest of the house.
BBE15726-5A6C-4358-89FD-8FFD18AD627A.jpeg

[heimdm]

What are you using for batteries?

[Will792]

3 PW2s. The man motivation to get batteries was for extended blackouts since I cannot get NG service and propane/gas distribution is commonly disrupted after hurricanes. Now I started using batteries to shift load a bit since I have net metering it is not really important.

[heimdm]

This is the current design. We are building out the patio structure as we speak. We have 320A service, so that complicates the Tesla gateway. I am guessing my only option would be to put the Tesla gateway between the meter and each of the 200 amp panels in the house. What I would like ideally is to put the batteries/powerwalls with the inverters. What I don't want is 2 gateways with 2 banks of powerwalls.

[heimdm]

ats-theortical.png

ats-convential.png
As I continue looking at this and the available tesla documentation. I really see 2 days to do this.

Option 1: Theoretical Drawing
- Would need to replace the manual service disconnect with an automatic transfer switch, that could also flip/trigger the breakers in main breaker panel. Does anyone know if they make a transfer switch/controller that can have remote switches?

Option 2: Conventional ATS.
- Leave the existing manual service disconnects in place
- Put at ATS (automatic transfer switch) before the each of the main breakers that come into the house.

If I could find the right device, option 1, would be easier from a wiring perspective.

[Will792]

Can you have just one breaker panel’s load with battery backup? In this case it becomes regular 200A service option for Tesla gateway connected to one 200A load center and generating devices panel, as long as all PWs and solar system combined nominal power does not exceed 200A.

I am almost certain the each PW can be connected only to one Tesla Gateway so if you place GW for each load panel you would need 2 PW banks, something understandably you want to avoid.

Since I decided against upgrading my 200A service to 400A I did not have to go through the same difficult design process. Unfortunately Tesla GW is 200A device and its ATS and communications bridge from PW to internet are essential so the only option is to create 200A branches in the load current flow.

I do not believe your second drawing design will pass review. Combined breakers rating for all panels connected to the meter 600A exceeds rating of your supply line.

[Ampster]

....

I do not believe your second drawing design will pass review. Combined breakers rating for all panels connected to the meter 600A exceeds rating of your supply line.

Can a load calculation or derating of those load panels help pass review? I have two 100 Amp subpanels fed by 70 Amp and 60 Amp breakers. The sum of all the breakers in both panels exceeds my 200 Amp service.

[Will792]

Can a load calculation or derating of those load panels help pass review? I have two 100 Amp subpanels fed by 70 Amp and 60 Amp breakers. The sum of all the breakers in both panels exceeds my 200 Amp service.

Sum of breakers inside panels is almost irrelevant. Sum of all main breakers of all panels connected to the meter cannot exceed rating of the supply line. It does make sense. Supply line does not have current limit device on the pole side so the design should not create a possibility of drawing more than supply line allows.

[Ampster]

..... Sum of all main breakers of all panels connected to the meter cannot exceed rating of the supply line. ......

So the solution to pass design review would be to derate the sub panels by feeding them with smaller breakers. Other than that, the design should work?

[heimdm]

I updated the diagram to reflect using a outside rated Automatic Transfer Switch. Based on previous conversations it sounds like, I would need to put a 400 amp disconnect after the meter, to provide overcurrent protection for the underground service entrance conductors.1Line-Enhanced-v2.png

[Will792]

I think breaker after a meter would handle protection for the supply line. The downside of the last design is PWs becoming backup sources only, activated by ATSes only. BTW I do not See why you need external fused disconnect. Regular disconnect will be fine since Tesla GW has already 200 breaker.

Most likely this design will not be allowed in states that require external house disconnect from all generating sources. The line from Tesla GW to ATSes does not have external disconnects. My guess is that this requirement is for firefighters use to make sure that power does not flow into the house if main supply line is disconnected. I do not know if the requirement is to have only one disconnect. In this case you will need to have external disconnect between Tesla GW and generating sources panel. I do not believe you need a disconnect between 320A cabinet and Tesla GW.

[Will792]

So the solution to pass design review would be to derate the sub panels by feeding them with smaller breakers. Other than that, the design should work?

Let’s do simple math. Supply line is 320. Size of the main breaker in Tesla GW should be 200A, based on max current from PWs and solar (current design has it more than 200A but since main breaker protects the buss most likely it is acceptable) so it leaves only 120A for both panels.

I am surprised that electric company would allow residential system of this size. CT(my state) has 20KW limit for residential systems. Interestingly enough combined size of my PWs and solar is more but since PW are not expected to feed the grid while solar is active it passed electric company review. I now see that it is theoretically possible for batteries and solar to feed into the grid at the same time since Tesla has a program with electric companies where Tesla sells electricity from customers batteries, with 80% of payment going to customers and 20% to Tesla.