[RE-wrenches] Adding on to existing PV System

Thu Sep 29 13:50:12 PDT 2011

OK, this subject has come up before and this is a gray area in the Code.
What is the definition of "the output of the inverters."

I have sucessfully argued that the output of the inverters is the last point
in the circuit where the circuit is free of premise load circuits. In this
case it would be the 40A breaker connecting the dedicated PV load center to
the busbar of the main panel.

So...it is between you and you inspector. Tell him you are going to put a
label on the subpanel that says something to the effect of "Dedicated
inverter output accumulation subpanel. Add no additional circuits."

I think this is a much better interpretation and method that preserves a
clean and functional load side connection topology. I think most reasonable
inspectors would agree as well.

Good Luck.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.com  

  _____  

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 1:33 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Adding on to existing PV System

I understand what you are saying about the 40A breaker, but according to
690.64(B)(2), ".with panelboards connected in series, the rating of the
*first* overcurrent device connected *directly* to the output of the
inverters shall be used in the calculations for *all* busbars and
conductors." The way I have seen that interpreted is that the main
distribution panel needs to be rated to support 120% of the two inverter
circuits plus the main breaker, regardless of the fact that there is a 40A
backfeed breaker in the main panel.

200+30+15 > 200 x 1.2

I know it doesn't make much sense, but that's the way it's been interpreted
around here. It stands to reason that this applies only to the PV subpanel
busbar and the conductors between the panels, and that the calculation for
those should be:

(30 + 15 + 40) / 1.2 = 70.83A required ampacity

.and the calculation for the main distribution panel should be:

(200 + 40) / 1.2 = 200A required ampacity

. but that's not what I am being told.

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 Mark Frye
Sent: Thursday, September 29, 2011 4:19 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Adding on to existing PV System

Jason:

SB 5000 = 21A @ 240V, 21A x 1.25 = 26.3A needs 30A breaker

M215 = 0.9 A @ 240V, 0.9A x 9 = 8.1 A, 8.1 x 1.25 = 10.1A needs 15A breaker.
(Or that rounded to 10A and use a 10A breaker)?

26.3A + 10.1A = 36.4 A needs 40A breaker

So, get a new 100A main lug subpanel. Connect it to a 40A breaker in the
existing main panel. Move the existing 30A breaker into the new subpanel and
install the new 15A breaker into the same.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.com  

  _____  

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 12:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Adding on to existing PV System

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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