[RE-wrenches] Adding on to existing PV System

[Jason Szumlanski]

That's not a bad idea, except that 690.64(B)(2) requires that the
breaker directly connected to each inverter(s) must be used for the
calculation.

 

I have an existing SB5000 with a 30A breaker and a new string of nine
Enphase M215's requiring a 15A breaker. If you combine these in a 100A
or higher rated subpanel with a 40A main breaker, you still calculate
the total current supplying the busbar in the main distribution panel
based on 200A + 30A + 15A = 245A (with 30A and 15A being the overcurrent
protection directly connected to the inverters). This exceeds the 120%
rule by 5A. It also requires that the ampacity of the conductors between
the two panels be 85A / 1.2 = 70.83A (assuming a 40A backfeed breaker in
the main distribution panel). If your interpretation is different,
please let me know!

 

If my interpretation is correct, I like the idea of just adding a
fusible disconnect and using #10 conductors all the way to the tap. That
would be the most cost effective solution I believe, and since there is
no foreseeable further system rating increases, the advantage of a
subpanel is reduced.

 

 

Thanks for all the other suggestions provided. Unfortunately I can't put
in smaller main breaker, and the busbar in the main panel is definitely
rated 200A.

 

Jason Szumlanski

Fafco Solar

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Glenn
Burt
Sent: Thursday, September 29, 2011 2:40 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Adding on to existing PV System

 

How about feeding both systems into an AC combining subpanel before
feeding the main load center?

Often the calculations required will allow you to do more backfeeding
than the simple backfeeding a load center with individual breakers.

 

-Glenn

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jason
Szumlanski
Sent: Thursday, September 29, 2011 11:49 AM
To: RE-wrenches
Subject: [RE-wrenches] Adding on to existing PV System

 

We've been contracted to add on to an existing PV system in a residence.
The existing system has a 30A 2P Breaker on a 200A rated panel with a
200A main breaker. The new system would require a 15A breaker, which
would exceed the limit in NEC 690.64(B)(2).

 

The existing system has a fusible AC disconnect mounted next to the
meter. This was originally intended as the utility external disconnect.
The utility no longer requires a disconnect, but if we do a supply side
connection, we would need a fusible AC disconnect anyway for the new
system. I would collocate the new disconnect with the old.

 

Does anyone see any problem with doing a supply size connection for the
new system? Any labeling snags that I might need to consider?

 

Jason Szumlanski

Fafco Solar

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