EG4 18KPV backfed (?) breaker

[BJToepper]

In reading over EG4's 18KPV manual — in particular the backfed, partial load backup wiring diagram 4.1 on page 31 or thereabouts — I'm hoping to understand better why it references the 120 percent rule for the main panel. In particular, the diagram's notes state, "The sum of the main service breaker plus 125 percent of the inverter output rating cannot exceed 120 percent of the bus bar rating on the breaker panel." If I'm not mistaken, the 120 percent rule limits the amount of current that can be supplied to a panel using a backfed breaker. If I put a 70 amp breaker on the main panel and connected a large motor to it, I believe there would be no issue with the 120 percent rule. However, when I add a 70 amp breaker and connect it to the inverter's "GRID" input, then it seems correct to say that the inverter's GRID input may sometimes output power back into the main panel (?). If that's the case, is there any way to shut this off? I really just want to use the inverter to send 40 or 50 amps of grid power to the critical loads panel, and then let some batteries take over if the grid fails. Am I completely misunderstanding this?

Many thanks for any help.


EDIT: I've posted the page in question as an image.

 

[glandpuck]

I have the Luxpower version. I went through my manual. I could not locate the specific area of concern you are referencing.

My understanding of this rule is that if you say have 200 amp service into your panel and the buss bar is rated for 100 amps, and then you add in a PV source supplying to your main panel such as a grid tie inverter, then that amperage contribution should be added to your 200 amp mains. If that contribution exceeds 120 percent of the buss bar rating, you either need to limit the PV contribution to be in compliance or de-rate the mains down to 175 or 150 amps so as not to overload the buss bar.

Of course there are new main panels with buss bar ratings that are higher as well as new main panels where the PV input is on a separate buss bar from the local home circuits. This allows much larger PV systems to tie into the grid or provide local consumption. This eliminates worry over the 120 percent rule.

 

[BJToepper]

I have the Luxpower version. I went through my manual. I could not locate the specific area of concern you are referencing.

My understanding of this rule is that if you say have 200 amp service into your panel and the buss bar is rated for 100 amps, and then you add in a PV source supplying to your main panel such as a grid tie inverter, then that amperage contribution should be added to your 200 amp mains. If that contribution exceeds 120 percent of the buss bar rating, you either need to limit the PV contribution to be in compliance or de-rate the mains down to 175 or 150 amps so as not to overload the buss bar.

Of course there are new main panels with buss bar ratings that are higher as well as new main panels where the PV input is on a separate buss bar from the local home circuits. This allows much larger PV systems to tie into the grid or provide local consumption. This eliminates worry over the 120 percent rule.

Thanks...I've added the manual page as an attachment to my original post.

 

[BentleyJ]

The 120% rule only applies to AC coupled systems and/or if you are selling back excess PV production from an AOI Inverter.

 

[pvgirl]

If this system is going to be permitted or inspected, it would up to the inspector/AHJ if the software configuration of the inverter to disable export would be an acceptable enough control to not require the 120% rule to be enforced.

 

[BJToepper]

If this system is going to be permitted or inspected, it would up to the inspector/AHJ if the software configuration of the inverter to disable export would be an acceptable enough control to not require the 120% rule to be enforced.

Yes, I'd like to pass inspection. Do you know if there's a better inverter to accomplish a simple battery-only backup system for my house? My proposed setup is summarized on the attachment here.

 

[glandpuck]

I went thru the Luxpower manual and it does not have the same image you posted.

But if I understand you correctly, you want to have a 1 way grid tie, to the inverter, do not want to sell back to the grid and want to pass inspection.

The way to do this would be as follows:

Do not connect to the grid. Complete off grid inspection.

After inspection, connect to your main panel breaker (you mentioned 70 amps would be okay. I believe this would be 4 AWG size. A breaker will allow current flow in either direction. But you should be able to program the inverter to make certain no backfeed occurs. There is a setting on the BASIC tab menu: uncheck export to grid and check zero export and no selling will occur. The other option is to configure the MAX EXPORT TO GRID (in kW). Suppose your inverter is supplying a part of the property and the other part is still consuming say 2 kW an hour at all parts of the day. You could set the max export to grid to say 1 kW and this would be consumed locally and not travel out to the utility past your meter.

It might be a good idea to download the Luxpower manual for the inverter directly from their web site and compare settings.