Do I really need a line side tap?

[Davis White]

A licensed electrician who probably didn't know that much about solar (not that I do!) told me I needed a line side tap. My residential service is single phase, 125A busbar with 100A overcurrent protection. The mains are underground and the transformer is not that clo$e. :>( I would prefer not to derate.

I am installing an Outback Radian 4048A rated at 4000W, with 16.7 Aac output @ 240 volts.

Seems to me I have 50 Amps of excess overcurrent protection available and the highest amperage backfeed would be 16.7 X 2 X 125% = 41.7 amps. So they could put a 50 amp breaker in the main panel for the critical loads subpanel and I am on my way.

My service panel cannot be easily tapped because it has a mechanical link from the meter to the busbar, and not a wired one....

I asked another solar electrician who is "looking into it." Maybe I am oversimplifying this?

I would be very grateful for an insight you can give me.

 

[RCinFLA]

You are close. I think I remember as 20% over rating of panel bus rating with PV and Grid pushing their breaker amperage. Section 690.64(2)

 

[Hedges]

Looks exactly correct to me. 120% of 125A busbar = 150A
150A - 100A main = 50A allowed PV breaker at end of busbar.

Line side tap would only be needed if you wanted more than 50A breaker for PV.
Or, if you want to feed grid directly into your inverter, and be able to switch the whole house main panel to output of inverter, using it as UPS for the entire house.

Simple way to go is what you were thinking. 50A breaker feeds Outback. Outback feeds critical loads panel.
If you put an interlocked main & backfeed breaker in the critical loads panel, then you can flip switches to bypass Outback and power critical loads from main panel if Outback fails.
If you put an interlocked main and backfeed breaker in the main panel, in the event of a power failure you can flip switches to power any/all household loads from Outback. (while in this condition, Outback can't automatically switch back to grid when power returns because main breaker to grid is off.)

"16.7 X 2 X 125% = 41.7 amps."

That math looks like a red herring to me.
Unless you put 16.7A 240V through a transformer to get 120V, nothing is multiplying current by 2.

 

[Ampster]

I am glad there are a couple of solutions. Five years ago I installed a Radian but never had is connected to solar because the AC coupling was not developed yet. I since sold the Radian and moved to another house and AC coupled a GT system to and Outback Skybox which was designed from the ground up to be grid interactive. I do have a lot of respect for the service and support staff at Outback.

 

[Davis White]

You are close. I think I remember as 20% over rating of panel bus rating with PV and Grid pushing their breaker amperage. Section 690.64(2)

Right! I was thinking of circuit breaker sizing vs. continuous loads.

 

[Hedges]

Line-side tap can also be done (not that you want to), with an adapter below the meter. PG&E allows that but limits it to 40A.

 

[Davis White]

Thanks! IF I had to go the lineside tap route, are any of you familiar with what Pacific Gas Electric calls Green Meter Adapter? San Diego has something similar: Renewable Meter Adapter. As I understand it, they attach it to the meter and it takes solar current and moves it upstream of the main breaker, like a lineside tap. Costs about $1300 and they have to install/own it. Takes about 1/2 hour and you can keep your old service panel.

Do I have that right? Would it be appropriate if I ever want to upgrade?

 

[Davis White]

Line-side tap can also be done (not that you want to), with an adapter below the meter. PG&E allows that but limits it to 40A.

Thanks, Hedges! I asked about that as you were posting. I appreciate your confirming it.

 

[RCinFLA]

Line taps are often a pain in the ass. The meter has to be pulled to do the tap. It can also restrict some of your future options that may need isolation from grid. It is almost as bad as putting in a large generator ATS transfer switch.

Often you will find there is aluminum wire from meter to main breaker input. If you do the line tap and have aluminum wire take the opportunity to change it out to 2/0 copper.

 

[schmism]

1 - I was under the impression there was no provision in the NEC for labeled breaker sum vs main disconnect rating. eg you can put 10, 20a breakers on a 100a main panel.
2 - How is inverter produced amps seen at the main disconnect? If the house is pulling 99.9 amps and the inverter is supplying 50a. Does the meter (main disconnect) not see 49.9 amps?

 

[Ampster]

Yes but the calculation is about the main buss bar. That is as much as I know except I have seen the example of the house putting 140 Amps of load on the buss bar and the main disconnect seeing only 90 Amps

 

[Hedges]

1 - I was under the impression there was no provision in the NEC for labeled breaker sum vs main disconnect rating. eg you can put 10, 20a breakers on a 100a main panel.
2 - How is inverter produced amps seen at the main disconnect? If the house is pulling 99.9 amps and the inverter is supplying 50a. Does the meter (main disconnect) not see 49.9 amps?

2) correct.

1) Maybe not for loads, because if the only source is the main breaker, no part of the busbar will ever carry more current than the main breaker.
However, if I have a 200A main and I connect two, 100A branch circuit breakers adjacent to the main and feed them with 100A each from PV, then 200A flows into the grid. If I put 400A of loads further down the busbar, they can draw 200A from the main plus 100A each from the two PV circuits. That's 2X the current, 4X the power dissipation the busbar was designed for. Probably won't hurt the busbar much, but it'll melt or burn the breakers and the plastic insulator holding the busbar.

Yet, if we put two, 100A breakers at the far end of the busbar and fed PV into both of them, nothing would go wrong. We could even feed branch circuits totaling 400A when there was enough sun. (not enough sun and you would trip the main breaker.) Nothing wrong with doing it except the NEC guys decided not to allow it. After discussion, they acknowledged it works and allowed us to have main plus PV up to 20% over busbar rating (to make PV reasonably doable), but didn't allow more because some day some idiot would move the breakers to the wrong end.