[RE-wrenches] DC Rating of Disconnects

Jeff Blick jeff.blick at sunlightsolar.com
Fri Nov 21 11:12:00 PST 2008


If the inverter will not backfeed currents from the grid, and if you have
less than 3 strings I do not see why you would need to use fuses in the DC
disco (assuming 15 amp series fuse rating).  If you are at more than three
strings it seems to me that the fuses in the combiner would take care of the
fusing requirements.

 
 
Jeff Blick
PV installer
Sunlight Solar Inc.
4 NW Franklin
Bend, OR 97701
direct: 541-306-4196
fax: 541-322-1911
jeff.blick at sunlightsolar.com
www.sunlightsolar.com
 
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Subject: RE-wrenches Digest, Vol 1, Issue 349

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Today's Topics:

   1. Re: DC Rating of Disconnects (ASAP POWER! 2)


----------------------------------------------------------------------

Message: 1
Date: Fri, 21 Nov 2008 10:47:12 -0800
From: "ASAP POWER! 2" <info at asappower.com>
Subject: Re: [RE-wrenches] DC Rating of Disconnects
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Message-ID: <CB4DA178641F4621A120418402C034B3 at 1AU>
Content-Type: text/plain; charset="us-ascii"

In 2008, we have had to replace our standard Sq. D HU361RB used since our
beginnings, with the Sq. D H361RB (U stands for unfused).   Was there some
sort of code change NEC or CBC maybe that made most jurisdictions in So Cal
suddenly start requiring fused DC disconnects?  Typically, we fill the fused
Sq. D. box with 15A 5" fuses.
 
Peter D.

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of William
Miller
Sent: Friday, November 21, 2008 9:43 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] DC Rating of Disconnects


Peter:

I am resending, the first version inexplicably deleted some text:

An arc is created in the process of disconnecting any current under load.
DC circuits have an enhanced ability to sustain that arc.  PV circuits
exacerbate this problem as a short or load is disconnected because they can
sustain a short but the voltage rises drastically as a circuit is
disconnected.

This all justifies the added expense of a DC rated disconnect.  We use the
Square D HU361 without hesitation.  There are few alternatives available and
none with any significant cost savings.  In selecting a disconnect, a
design-build contractor must use the highest Voc possible. Make sure you are
entering the 690.7 table with the record low temperature for the location.

If we used a disconnect without the correct rating we would open ourselves
to unacceptable liability exposure and, worse yet, would not experience that
pleasant feeling one gets when you know you've done the job right.

William Miller


At 09:11 AM 11/21/2008, you wrote:



  _____  

From: Peter Parrish [mailto:peter.parrish at calsolareng.com] 
Sent: Friday, November 21, 2008 9:10 AM
To: 'RE-wrenches'
Cc: 'Peter Parrish'
Subject: DC Rating of Disconnects
 
I am looking into using the PV Powered PVP2000 inverter for some small, low
voltage designs. The string Voc will be around 200 Vdc, worst case.
 
Since the PVP2000 doesnt come with a DC disconnect, I am wondered what
other wrenches who install the PVP2000 have been doing about this. I hate to
spend $150 on a 600 V AC/DC disco such as the SD HU361RB to provide the
means of disconnection. I believe that the 240 V AC discos such as the SD
DU221RB have no DC rating.
 
>From an academic point of view, I wonder why the smaller discos are not
dual
rated, since we all know that the real-time voltage difference in a 240 V AC
system can be 41% greater than 240 V.
 
In any event, any Code-compliant solutions would be greatly appreciated.
 
- Peter

Peter T. Parrish, President

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