Interconnection

[Dave Sheldon]

Trying to determine how we use the 120% bus bar rule in this scenario. I have a 225 amp meter main that does not have a main breaker. There are two 125 amp breakers that feed the houses two branch panels. How do we figure out the amount of solar that can be added?

Our plan was to line tap the two SER feeder wires from AC combiner panel that we install. We will be adding 107amps of solar electric to this meter main via the combiner sub panel.

Any advice is much appreciated.

Thanks

 

[ggunn]

Trying to determine how we use the 120% bus bar rule in this scenario. I have a 225 amp meter main that does not have a main breaker. There are two 125 amp breakers that feed the houses two branch panels. How do we figure out the amount of solar that can be added?

Our plan was to line tap the two SER feeder wires from AC combiner panel that we install. We will be adding 107amps of solar electric to this meter main via the combiner sub panel.

Any advice is much appreciated.

Thanks

Are you planning to feed the output of the AC combiner to the meter main or one of the subs?

 

[jaggedben]

If you are doing a supply side connection, the amount of solar is limited to the ampacity of the service. (This was also answered in the most recent thread in this forum.)

If you are tying into one of the subs, the question depends on details you haven't supplied, such as the rating of that panelboard.

(Also, I could be wrong, but I think the existing setup is in violation for having two 125A breakers fed by 225 bus.)

 

[Dave Sheldon]

Are you planning to feed the output of the AC combiner to the meter main or one of the subs?

I am planning on line tapping the output of the AC combiner to the SER wire coming off of one of the 125 amp breakers in the meter main. The part I am unclear on is the calculation for having the two breakers in the meter main. Typically we would say 225 x 1.2= 270 minus main breaker(125)=145 amps of solar we can add. How do I calc with two main feeder breakers. (Note, there is no actual main breaker in the meter besides the two 125 amp breakers going to the branch panels.)

 

[jaggedben]

I am planning on line tapping the output of the AC combiner to the SER wire coming off of one of the 125 amp breakers in the meter main.

Do you mean tapping on the line side or load side of the breaker? It makes all the difference in the world.

Tapping conductors on the load side of one of the 125A breakers invokes the 120% rule. The feeders must be rated for (125A+xA)/1.2 where x is the size of your solar breaker. That's 229A if your solar breaker is 150A. It's doubtful that the feeders are oversized by the amount they would need to be. Moreover if the panel that's fed by the 125A breaker happens to be an MLO panel, then it must also be rated for 229A.

Now that I look at your numbers again, it is almost certainly the case that you cannot do this installation except through a supply side connection, aka tapping service conductors on the line side of the 125A breakers, or finding a place to install a new breaker on the 225A bus.

The part I am unclear on is the calculation for having the two breakers in the meter main. Typically we would say 225 x 1.2= 270 minus main breaker(125)=145 amps of solar we can add.

This type of calculation is not relevant because, as you have described it, the 225A busbars are not on the load side of the service disconnecting means. This type of calculation should only be used for conductors and panels on the load side of either 125A breaker.

How do I calc with two main feeder breakers. (Note, there is no actual main breaker in the meter besides the two 125 amp breakers going to the branch panels.)

Both of those 125A breakers are 'main breakers', aka service disconnecting means.

If you make your connection on the line side of both 125A breakers, then both are irrelevant to any calculations. You are limited to the ampacity of the service (which is presumably 225A).

If you can make your connection on the load side of one of the 125A breakers, then use the 120% rule on the conductors and panelboards that are load side of that breaker. The other 125A breaker and the 225A bus are irrelevant to any calculations.

 

[jaggedben]

Incidentally the relevant code section is 705.12 (or 690.64 in the 2008 code.)