Backfed Breaker Sizing

climb_on

I am looking to install a 2nd array at my home and I am working out the 120% rule for backfed breakers, which I currently exceed. The 2nd system needs to be run through a separate inverter, meter & breaker, because of 2 different incentive programs.

First system, which is currently running, has a Solaredge SE10000-US inverter with a Max Output rating of 41a and a Max OCPD of 60a. With no plans for a 2nd array, I just rather blindly used a 60a breaker.  

I have a 225a rated bus and a 200a main breaker.  225a x 120% = 270a - 200a = 70a available to back feed.  

My proposed 2nd system will have a SolarEdge SE5000H-US inverter with a Max Output rating of 21a and a Max OCPD of 40a.

Do I need to size the breaker for the inverter's Max OCPD rating or for the Max Output rating?

However...IF I can size the breakers for the Max output of the inverters of 41a and 21a, respectively, with 45a and 25a breakers I would be right at 70a and satisfy the 120% rule.

oil pan 4

What kind of breaker panel do you have? 
A 25 amp tandem breaker may be possible but I don't know about a 45 amp.

You may just need to take some actual readings.

Also your breaker needs to only run at 80% of its rated capacity. Say you had a solar system that made 100 amps, you would need at least 120 amp breaker.

climb_on

Thanks.  I have an Eaton panel.  I'm not familiar with tandem breakers....will look into it.  If the breaker needs to be sized at 80% then I'm over the limit and would need to look at derating the main I think.

mcgivor

The purpose of an over current device is to protect the conductors, be it a branch circuit or a bus bar, let's use hypothetical values, the total current capacity  of all the branch circuit  breakers in the in the panel probably exceeds the value of the main breaker, rationale is, it would be highly unlikely to have all circuits are drawing maximum load at any one time. So let's put some numbers into the picture, say the load during the day is 100A, a high nominal figure, the grid tied inverters  are producing  70A, the loads would absorb the 70A and only 30A would be pulled from the grid to supplement the deficit. Now the loads decrease to 10A, the inverters are now pushing 60A into the grid, 10A to the loads, so the main breaker is in effect reverse current flow. As the inverters could never exceed the capacity of the grid, the main breaker is well within its limitations,  but now someone uses an auger to mount a post, accidentally severing the inverters supply conductors, the breaker protecting tthe conductors should be of an amperage rating less than that of the source of highest potential, the grid, but not exceeding that of the branch circuit conductors, 70A in this case.

The rated current carrying capacity of conductors is very conservative but remember the purpose of the over current device is to protect the conductors from the source of highest potential, so the feed in breaker could be of a higher value than the inverters could produce but not exceeding that the conductors used. Say in the case the distance between the inverters warrant a larger gauge to prevent voltage drop, it would be possible to increase the gauge but the OCP should not exceed the current carrying capacity of the conductors, obviously it could be less. Hope this makes sense
Edit. I should add the value of a branch circuit OCP should never exceed the value of the main breaker. 

climb_on

Thanks mcgivor that does make sense...mostly.  My current inverter has max OCPD rating of 60a and 41a max output x 120% = 49.2a.  So a minimum of 50a breaker to max of 60a breaker, depending on conductor size and distance (my run is about 10').  So 50a would be fine.

On the proposed 2nd system I would need a 25a (21a max output x 120% = 25a).

So 50a and 25a = 75a and is still over the max of 70a available to backfeed.  Looks like I either need to size a smaller 2nd system or derate the main.

How do I determine if I can derate the main breaker?  

mcgivor

Read this it will help explain a few things https://www.mikeholt.com/nec-conductor-sizing-and-protection.php there are many factors to sizing which are in the electrical code which I personally won't quote.

This thread will help you with the derating main breaker questions http://forums.mikeholt.com/showthread.php?t=168448

climb_on

Thank you sir.  I will take a look.

oil pan 4

You only need a tandem breaker for 240v inverter. But for that size it should be 240v.