[RE-wrenches] DC Rating of Disconnects

[Peter Parrish]

The only thing I am aware of wrt to fused DC discos is the notion that PV
runs that penetrate a building shell need over current protection. The City
of Santa Monica used to (and may still) have this requirement.

 

The (non) issues as I understand them:

 

(1)   PV modules should be treated as any other independent source,
regardless of the fact that they are inherently current limited. It would be
straightforward to determine whether or not the wire used has the adequate
ampacity.

(2)   Inverters may be capable of back-feeding the PV array . Manufacturers
that continue to sell and market a specific inverter model, are usually
willing to write a letter to the effect that back-feeding will not occur.
The only mfgr that I know of that demurred is Sharp wrt their JH-3500, and
it is no longer sold in the US.

 

Of course the use of an unfused disco makes good sense, prior to penetrating
building shell. 

 

- Peter

 

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com 

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of ASAP POWER!
2
Sent: Friday, November 21, 2008 10:47 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] DC Rating of Disconnects

 

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:

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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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