[RE-wrenches] combiners and the 120% rule

William Miller william at millersolar.com
Wed Jun 27 00:21:44 PDT 2012


Daryl:

I apologize for my ignorance, but I can't seem to find this exception in 
the 2008 code.  Can you cite the section?

Unless you can physically prevent the addition of a load breaker in your AC 
combiner panel, 690.64 does not apply.  Any panel is capable of supplying 
branch circuits.  I think this disqualifies 690.64, so I am looking for the 
section that allows a PV only panel.

Thanks,

William






At 06:44 PM 6/26/2012, you wrote:
>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
>
>
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Miller Solar
Voice :805-438-5600
email: william at millersolar.com
http://millersolar.com
License No. C-10-773985
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