[RE-wrenches] DC wire sizing

jay peltz jay at asis.com
Mon Apr 5 08:09:31 PDT 2010


Since the topic of AFI's has come up I wonder if anyone has actually tested one, even an AC one.

From what I've read from some tests on the Mike Holt site for AC breakers is that they couldn't get them to work at anything less than standard ISC for the breaker.

thanks,

jay

peltz power


On Apr 4, 2010, at 10:47 PM, Kent Osterberg wrote:

> Ray,
> 
> The exception in 690.8(B)(1) reads: Circuits containing an assembly, together with its overcurrent device(s), that is listed for continuous operation at 100% of its rating shall be permitted to be utilized at 100% of it rating.   The field installed cable used to connect a breaker is not part of an "assembly that is listed for continuous operation at 100% of its rating" and as such the cable still has to be sized to carry 156% of Isc.
> 
> I agree with you about the need for DC arc fault protection.  I believe the 2011 code is going to require it for circuits over 80 volts.  But it seems unlikely that such a device will exist for a while.  It will have to have embedded electronics for detection that operates the contacts.  And at present we resort to 600 volt fuses because "simple" breakers rated for 600 volts dc are excessively expensive.  When dc AFCIs are available, they are sure to be expensive and are likely to add $100s to the cost of a combiner box.
> 
> Kent Osterberg
> Blue Mountain Solar, Inc.
> 
> 
> 
> R Ray Walters wrote:
>> 
>> And just to be clear, when using these 100% duty rated breakers, you only have to oversize the cable by 125%, not 156%.
>> 
>> Also, as William mentioned, even correctly sized wiring can have problems; we just recently had a short circuit that caused quite some damage in a junction box, and the breaker never tripped.
>> The 156% rule has its downsides too, as you end up with a breaker so oversized that array current will never trip it, even if you wish it could.
>> I'm now advocating for DC Arc Fault protection, as I have seen several cases of dangerous DC arcs that sustained for several minutes, but never drew enough current to trip a breaker.( but did create heat over 2000 degree F )
>> The standard thinking for safety in electrical systems assumes an infinite current supply that will trip a breaker, as soon as something goes wrong. (true for grid power, and battery systems) But on the PV source circuit, many bad things can happen that will not be stopped by the breakers. Also, high voltage DC has much more Arc potential than AC, so AFI protection is even more important.
>> Now, we just need somebody to make a reasonably priced UL listed DC AFI unit.
>> 
>> R. Walters
>> ray at solarray.com
>> Solar Engineer
>> 
>> 
>> 
> 

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