Supply Side Tap at Main Panel in Garage?

[shyamster]

I have a home with a meter and service entrance panel outdoors. The service panel is rated 200 AMPS and has couple of breakers for outdoor lights that I am not planning to tie into my solar. The "Main Panel" is in the garage with a 200 AMP breaker that supplies the house - well pump, EV charger, two electric dryers and a Mitsubishi heat pump. The service panel and the main panel are about 40 ft away from each other and the line is buried underground between the two.

If I install a EG4 18K in the garage with batteries, I believe I have to do a supply side tap since my main panel in the garage is rated only for 200 AMPS and I might have trouble derating it to 175 with all the loads. Can I simply tap the inverter to the line that's coming from the service panel outside into my panel in garage just before the 200AMP breaker? Is that still considered supply side tap? Or something else? Or even allowed?

 

[hwy17]

Is your goal grid export or just self consumption?

 

[zanydroid]

My gut feeling is there is a not terrible way to do this, that will not require swapping MSP or swapping feeder to larger size.

Share some pictures please. There is some ambiguity of terminology.

Wiring diagram stickers of both your service and what you call a “main”. I think what you call a main is nothing but a subpanel with the bulk of your house loads.

What is your electrical experience?

What are the size of the feeders? How long are they and are you willing to increase? You will most likely have to increase the feeder with a tap as you described, this is covered in the 705.12 section on feeding into taps.

Did you do a load calculation of all subpanels? 175 is quite a lot for a house.

 

[zanydroid]

Shooting blindly from the hip you can add a 200/225A subpanel or breaker holder next to your service, give it a 200A main breaker. Wire new subpanel to service with like 3 feet of 300A wire. Put the solar tap you want right there on that 200A conductor (you will need yet another subpanel with the 100A breaker you probably need for a 18kW inverter).

The advantage of this approach is that it has upsized the feeder being fed into to guarantee handling a fault before the OCPD on the 200A subpanel main. And the 200A existing feeder to your distribution subpanel for your house is immediately protected at 200A

 

[shyamster]

Is your goal grid export or just self consumption?

Self-consumption. Actually I want to start with just batteries to backup critical loads and add solar later. Solar will never be more than what I consume since I have tall trees all around

 

[zanydroid]

How will the transfer when grid goes down work in this setup? Are you going to make a new critical loads panel and hang it off the 18k?

 

[shyamster]

My gut feeling is there is a not terrible way to do this, that will not require swapping MSP or swapping feeder to larger size.

Share some pictures please. There is some ambiguity of terminology.

Wiring diagram stickers of both your service and what you call a “main”. I think what you call a main is nothing but a subpanel with the bulk of your house loads.

What is your electrical experience?

What are the size of the feeders? How long are they and are you willing to increase? You will most likely have to increase the feeder with a tap as you described, this is covered in the 705.12 section on feeding into taps.

Did you do a load calculation of all subpanels? 175 is quite a lot for a house.

I'm a beginner, yes. I don't plan to do the work myself BUT I believe in knowing my options before I have someone do the work.

You are right, the panel in the garage is a subpanel and there are no other subpanels. Just the panel near the meter (main/service panel) and the one in the garage.

I don't want to touch anything at the meter or the service panel. I don't want to upgrade the wiring between the service panel and the sub panel in the garage. I'd simply like to add a critical loads panel in the garage, batteries, and some solar at a later date. The batteries and a small solar will only support the critical loads when grid goes down. I will have a smallish generator to charge batteries (through the inverter) when the grid is on extended outage.

The subpanel in the garage has the following 240V loads (30A well pump, 50A EV Charger, 2 x 30A Electric Dryers) and 120V loads (22 x 20A, 6 x 15A circuits). I'll move the well pump and some of the 20A and 15 A circuits to a critical loads panel.

 

[shyamster]

How will the transfer when grid goes down work in this setup? Are you going to make a new critical loads panel and hang it off the 18k?

Yep, and a 10KW generator to recharge batteries to handle extended outage

 

[zanydroid]

The subpanel in the garage has the following 240V loads (30A well pump, 50A EV Charger, 2 x 30A Electric Dryers) and 120V loads (22 x 20A, 6 x 15A circuits). I'll move the well pump and some of the 20A and 15 A circuits to a critical loads panel.

Ok I can totally see the load calc hitting close to 175. The heavy hitters are ofc the 240v loads.

You can put the EVSE on load management to help a lot with this. Like Wallbox pulsar + RS485 connection to energy monitor monitoring the current on the feeder. That will delete the EVSE from the load calc. Hardware is about $900 for this I believe, pulsar and energy monitor being the heavy cost hitters.

———

Another approach to consider is to modify the bit subpanel feed so that you can feed EG18K as:

MSP - subpanel - EG18k (from a new subpanel holding a 100A solar breaker to it, probably 100A main as well)

In that order exactly, either reusing the feeder or splicing a little bit more on the end, 200A ampacity.

I assume that subpanel has a 200A main on it. If not then things get a little complicated.
This approach should not require derating. You also don’t need to put the EVSE on energy management

 

[zanydroid]

The reason the latter approach works (module some neutral loading thing that hedges will probably pop in to talk about, but NEC appears to not mind when it sanctions this approach in 705.12 ) is that the solar and distribution panel current sort of cancel each other out and will never add when they interact on the existing 200A feeder to your distribution panel.

If you put the solar and the service on the same side of the 200A feeder relative to the distribution panel then there are many scenarios where the current will add instead of cancel.

You of course cannot end feed the EG4 18K on the distribution subpanel because that blows way past the 120% rule for the busbar unless you derate

 

[shyamster]

Ok I can totally see the load calc hitting close to 175. The heavy hitters are ofc the 240v loads.

You can put the EVSE on load management to help a lot with this. Like Wallbox pulsar + RS485 connection to energy monitor monitoring the current on the feeder. That will delete the EVSE from the load calc. Hardware is about $900 for this I believe, pulsar and energy monitor being the heavy cost hitters.

———

Another approach to consider is to modify the bit subpanel feed so that you can feed EG18K as:

MSP - subpanel - EG18k (from a new subpanel holding a 100A solar breaker to it, probably 100A main as well)

In that order exactly, either reusing the feeder or splicing a little bit more on the end, 200A ampacity.

I assume that subpanel has a 200A main on it. If not then things get a little complicated.
This approach should not require derating. You also don’t need to put the EVSE on energy management

I don't quite understand the second option. It seems like you are suggesting adding another 100A subpanel to my 200A subpanel. If that's the case, I might as well swap out my current 200A subpanel to a 225A capable one to give me enough room for load side 70A breaker?

 

[zanydroid]

No I’m recommending a more advanced config where you have two subpanels hanging off the feeder in a specific order. The new subpanel does not hang off your existing subpanel. The central conceit is that hanging off feeder directly bypasses the busbar rating rules. I have not done the math but you should easily be able to do a 200A solar breaker (though I’d stick with something smaller because that may require the tap splices to be rated some hard to find current level, and it’s pushing the envelope on a NEC clause that isn’t super well tested).

A picture is probably essential for explaining but I am busy this weekend installing my own system so I’m not able to draw one for a few days vs vomiting text in a spare moment.

If 70A is sufficient to pass inspection on EG4 18k (I don’t know it’s AC nameplate), sure go do it that way. Another advantage is easier to explain to installer and permit office.

 

[shyamster]

No I’m recommending a more advanced config where you have two subpanels hanging off the feeder in a specific order. The new subpanel does not hang off your existing subpanel. The central conceit is that hanging off feeder directly bypasses the busbar rating rules. I have not done the math but you should easily be able to do a 200A solar breaker (though I’d stick with something smaller because that may require the tap splices to be rated some hard to find current level, and it’s pushing the envelope on a NEC clause that isn’t super well tested).

A picture is probably essential for explaining but I am busy this weekend installing my own system so I’m not able to draw one for a few days vs vomiting text in a spare moment.

If 70A is sufficient to pass inspection on EG4 18k (I don’t know it’s AC nameplate), sure go do it that way. Another advantage is easier to explain to installer and permit office.

Appreciate the explanation. I'm going for minimalism here. My main goal is to have a 2-3 day battery backup for critical loads that will charge from the grid. At some point in the future I might have some solar panels to charge the battery. Where my house is, I am surrounded by 100 foot trees and I probably have a bit of sun between 11:00 AM and 3PM. So I am looking for a sufficient enough solution

 

[zanydroid]

I think adding a cap on the line side of your distribution panel is the most minimal. In a DIY perspective it would only require a few feet of feeder going to new subpanel for holding the backfeed breaker. This can be as small as a 4 space spa panel, so like $50 and easy to find wall space for.

Add about $100 for three splicing devices for L L N and $50 for breakers.

Derating to 175A would require buying like a $100 main breaker, and depending on whether your service disconnect is hot bus combined with your distribution panel is lug only, maybe paying someone for sure to disconnect that (bad for DIY). Plus you may have functional and or code compliance problems with being underpowered.

Swapping to 225A subpanel is a lot of labor to move all the breakers plus $130 for the subpanel. Maybe a bit of patching of the wall if the subpanel is bigger than old one.

Is your distribution panel main lug only or does it have its own main breaker? This affects the details of several of the options. That can be hacked around by buying a breaker holder for a main breaker.

The calculus is different when hiring out of course. IMO if materials and time are lower then there is more room to negotiate.

 

[zanydroid]

Distribution panel that is main lug only and non convertible significantly limits quick patch options

In that case there is value to convert to 225A bus with 200A main breaker since it may be the only viable option that does not involve derate and also gives a few more config options down the line to scale up.

 

[zanydroid]

As “engineering” goes, strategically patching in a few subpanel, busbar, and feeder upgrades for your scenario is far from rocket science.