[RE-wrenches] DC wire size

[R Ray Walters]

I'm beginning to picture a standard combiner box, but with some interesting additions to the output: supplementary breaker, DC AFI.....
I wonder how close to short circuit current you could suzette supplementary breaker, without it nuisance tripping? Normal operation should be operating the array at mpp current which would be around 90% of SC.


On Apr 6, 2010, at 9:29 PM, jfh at greenhousepc.com wrote:

> Ray,
> 
> Take a look at 240.10.  You can have "Supplementary Protection" at however low you want to go.  This might be one of those sections that makes more sense in residential wiring (most of my electrical experience), but I believe it applies here as well.
> 
> The idea is that you "know" what the maximum current "really" is, and so long as it is below the required ampacity, you add "supplementary protection".
> 
> For example, if the AHJ isn't requiring a disconnect between the combiner box output buss, and the conductors leading to the charge controllers or inverters (some jurisdictions are requiring roof-top disconnects for fire fighters), the OCPD you're adding as "supplementary protection" is the only OCPD.  See the exception in 690.9 (A) (b) that says you don't need a OCPD there in the first place.
> 
> So, you've sized your conductors per 690.8 (B), you get your exception in 690.9 (A) (b), then you apply 240.10 to add a supplementary protection over-current device to insure that power is interrupted in the event it goes outside the range you expect.
> --
> Julie Haugh
> Senior Design Engineer
> greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on Skype
> 
> 
> -------- Original Message --------
> Subject: Re: DC wire size
> From: R Ray Walters <ray at solarray.com>
> Date: Tue, April 06, 2010 9:21 pm
> To: jfh at greenhousepc.com
> 
> 690.8B1 mentions both the wire and OCPDs have to meet the 156% rule unless the breakers are rated for 100%, then 690.8A still prevails (125% of short circuit)
> 
> R. Walters
> ray at solarray.com
> Solar Engineer
> 
> 
> 
> 
> On Apr 6, 2010, at 1:46 PM, jfh at greenhousepc.com wrote:
> 
> Ray,
> 
> Code doesn't require that you oversize the OCPD, only that you oversize the conductor.  You are free to install circuit breakers that are SMALLER than the conductor ampacity, you just can't install one that's larger, and you can't install conductors that are smaller than required.  The reason you =want= to oversize the OCPD is if you know that you're going to see greater than 100% STC current due to having cold panels, edge-of-cloud, snow reflection, or other such increases in output.  If you know that you're never going to see those things happen, Code allows you to go right down to the absolute minimum for the string.
> 
> The places where OCPDs are required to be of a certain size are things like branch circuits, where Code specifies the ampacity of a branch circuit.
> --
> Julie Haugh
> Senior Design Engineer
> greenHouse Computers, LLC // jfh at greenhousepc.com // greenHousePC on Skype
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20100407/ae21b089/attachment-0002.html>