Connect solar combiner to sub panel

[gar]

110617-1458 EDT

jaggedben:

Others contributed much more than I did. I was simply asking questions to ferret out the road blocks. iwire provided the specific code parts that set the limitations.

I have some more questions. Are the feeder wires from the main panel to the subpanel adequate for the 60 A backfeed? The worst case I believe is with a capacitor on the subpanel drawing 60 A of reactive current, and the inverter providing 60 A of unity PF current. This results in an 84 A current in the feeder. No information has been provided on the feeder wire rating, maybe something is implied about its rating.

Can the feeder wire to the subpanel be moved from a load breaker on the main panel to a line side tap thru an inexpensive breaker box?

All these questions inter-relate economically with the cost of a separate line from the inverter to the main panel area.

When DTE was doing their Solar Currents program they required the inverter to pass thru its own meter back to their feed. I do not know if DTE requires this on all the grid-tied inverters to their system, or just for the Solar Currents program.

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[jaggedben]

Just a note option 1 will never be complaint alone. You must do so in combination with portions of option 2. ....

I hope I did not confuse the situation further as I am not trying to.

You're right, and I knew that but forgot to include it. It's that second sentence in 2008 690.64(2) or 2011 705.12 (7)

In systems with panelboards
connected in series, the rating of the first overcurrent
device directly connected to the output of a utilityinteractive
inverter(s) shall be used in the calculations for
all busbars and conductors.

And in option 1 the breaker feeding the subpanel can't be less than 60 amps, either, which is, like, duh.

Wish we could edit posts after more than 15 minutes.

 

[jaggedben]

110617-1458 EDT

jaggedben:

Are the feeder wires from the main panel to the subpanel adequate for the 60 A backfeed? ... No information has been provided on the feeder wire rating, maybe something is implied about its rating.

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When I hear there is a 100amp subpanel, I take that to imply that the wires feeding it are properly sized for 100 amps. And I didn't want to go on and on about that in the earlier post. I agree that any such detail that might have been left out by the OP could change things.

60 A of reactive current, and the inverter providing 60 A of unity PF current.

Solar breakers are required to be sized at 125% of the inverter max output current, for continuous load. I assumed that when the OP said 60 amps he meant that the breaker feeding the solar subpanel was a 60amp breaker, properly sized. The actual solar current would then be 48 amps or less. Given his wording, this assumption of mine is probably less warranted than the one about the 100 amp subpanel wiring. He might actually have 60amps of solar, requiring an 80amp breaker.

 

[jaggedben]

One more thing...

The concern is does the load on the sub exceed 40 amps. If so this option is not available.

If the solar were tied in to the subpanel, then we know the 'load' on the sub exceeds 40amps, because the solar is 60amps. That 60amps (nominal) has to backfeed through whatever breakers are feeding the panels it's connected to. This is why I am embarrased that I even suggested the possibility that the subpanel breaker could be downsized below 60 amps. Typing before thinking.

I hope I did not confuse the situation further as I am not trying to.

I don't think you did, but I think I did. :ashamed1:

 

[jaggedben]

Here is the edited version, a.k.a. what I should have said the first time:

I am assuming that the breaker feeding the solar subpanel is 60amps.

Option 1: Do load calculations for the subpanel to determine if the breaker feeding it can be replaced with a 60amp breaker. If yes, you may install the 60amp solar breaker in the subpanel, at the opposite end of the busbars from which they are fed, with a label not to move the solar breaker. To do this, you must also do option 2.

Option 2: Do load calculations on the whole house to see if the main breaker can be downsized to 175amps. (You may also need to verify that you can do this in the main panel without violating its listing.) If yes, you may install the 60amp solar breaker in the main panel, at the opposite end of the busbars from which they are fed, with a label not to move the solar breaker. Or, you may do option 1 if the subpanel breaker is positioned in the same way. (If you can downsize the main breaker to less than 140amps, the location of the breakers in the main panel does not matter.)

Option 3: Line-side tap. As you can see, options 1 and 2 are tall orders.

 

[gar]

110618-1607 EDT

wwilkins:

More information is required.

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