I have found inspectors agree that if it is labled "Solar only no Load
circuits" the main breaker protects this panel, or the sum of the breakers
does not exceed the busbar rating, the panel is protected without using the
120% rule
I agree with Kirk
________________________________
From: Kirk Herander <k...@vtsolar.com>
To: 'RE-wrenches' <re-wrenches@lists.re-wrenches.org>
Sent: Tuesday, June 26, 2012 3:53 PM
Subject: [RE-wrenches] combiners and the 120% rule
Jason,
In your email below you state:
“You DO need to observe the 120% rule for the combining subpanel, regardless of
whether there are loads present, at least in jurisdictions where I have worked.
I've heard that some inspectors will allow you to ignore it if it is labeled as
a PV combiner with "add no loads" notation.”
NEC 705.12 (D) states that the distribution equipment (in this case the
combiner panel, fed by multiple inverters and a utility source) must be
“capable of supplying multiple branch circuits or feeders or both” for (D)(1)
through (7) to apply. If you fully populated a combiner panel with inverter
breakers, leaving no slots for load breakers, it is not capable of supplying
branch circuits or feeders, and IMO the 120% rule does not apply to the
combiner buss or the conductors back to its point of utility interconnect. I
have argued this point as well as label combiners “load circuits prohibited”
(with or without available slots) and received AHJ approval.
You could also just lock shut a combiner that had spare slots as a deterrent to
adding load breakers.
Kirk Herander
VT Solar, LLC
dba Vermont Solar Engineering
NABCEPTM Certified installer Charter Member
NYSERDA-eligible Installer
VT RE Incentive Program Partner
802.863.1202
From:re-wrenches-boun...@lists.re-wrenches.org
[mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Jason Szumlanski
Sent: Tuesday, June 26, 2012 8:28 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] Enphase grid tie question
I'll email you off-list a 1-line diagram from a system with 164 microinverters
broken down into 8 strings in a 208V system. This particular system used two
subpanels to accumulate PV, but that was only because we had to backfeed two
existing subpanels due to the size of existing 480/208V transformers. You will
have to look at the utility service and all existing equipment.
Regarding the breakers in the subpanel, you will only need a maximum of a 20A
breaker for each string. The max inverters per string is 25 and the calculation
for OCPD is:
215W / 208V x 25 inverters / 1.732 x 1.25 = 18.65A
“You DO need to observe the 120% rule for the combining subpanel, regardless of
whether there are loads present, at least in jurisdictions where I have worked.
I've heard that some inspectors will allow you to ignore it if it is labeled as
a PV combiner with "add no loads" notation.”
Use a MLO panel with a fusible disconnect between the subpanel and the
interconnection point. If you use a 225A panel, you can feed it with 270A. With
eight 20A backfed PV circuits, you would need to protect the line side of the
panel with a 100A fusible disconnect. That probably isn't going to work. You
may be best off from a cost perspective using two 225A subpanels and two 60A
fusible disconnects. Anything larger than a 60A 3P disconnect and the price
skyrockets. It all depends on your circuit calculations and the existing
equipment. Of course, you would need two spaces for your interconnection point.
Jason Szumlanski
Fafco Solar
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