[RE-wrenches] combiners and the 120% rule

[Darryl Thayer]

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 <kirk at vtsolar.com>
To: 'RE-wrenches' <re-wrenches at 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-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at 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
 
_______________________________________________
List sponsored by Home Power magazine

List Address: RE-wrenches at lists.re-wrenches.org

Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org

List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org

List rules & etiquette:
www.re-wrenches.org/etiquette.htm

Check out participant bios:
www.members.re-wrenches.org
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20120626/87c86ec5/attachment-0003.html>