[RE-wrenches] DC wire sizing

[R Ray Walters]

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




On Apr 2, 2010, at 4:35 PM, Allan Sindelar wrote:

> Yes, they are 100% duty rated. Also, a little-known fact about the CBI breakers used by Midnite and Outback, gleaned from conversations with Robin Gudgel: because of this hydraulic/magnetic construction, all are capable of controlling both AC and DC, even if not listed as both. 
> 
> Allan Sindelar
> Allan at positiveenergysolar.com
> NABCEP Certified Photovoltaic Installer
> EE98J Journeyman Electrician
> Positive Energy, Inc.
> 3201 Calle Marie
> Santa Fe, New Mexico 87507
> 505 424-1112
> www.positiveenergysolar.com
> 
> 
> robert ellison wrote:
>> 
>> I believe that the breakers from CBI, Midnite and Outback are hydraulic / magnetic and may be operated at 100% While the derating applies to all others that are Thermal / magnetic,
>>  
>> Bob
>> 
>> On Fri, Apr 2, 2010 at 11:14 AM, Kent Osterberg <kent at coveoregon.com> wrote:
>> Erika,
>> 
>> You should refer to NEC 690.8 this information.  There are two issues that contribute to the 1.56 factor.  First is that the maximum current from the PV array is considered the short circuit current multiplied by 1.25 to account for higher than standard irradiance such as cloud edge effect, high altitude, and high operating temperature.  The second multiplier of 1.25 is because PV current is considered to be continuous.  All conductors (and breakers) operating continuously (over three hours I believe) that are subject to NEC rules are limited to 80% of the ampacity in Table 310.16.  The two 1.25 factors equals 1.56.
>> 
>> For normal operation, wires selected under this criteria are conservatively rated.  But it is also the objective of the NEC to design power systems in a manner that they will remain safe under abnormal circumstances.
>> 
>> Kent Osterberg
>> Blue Mountain Solar, Inc.
>> 
>> 
>> 
>> 
>> Erika M. Weliczko wrote:
>>> 
>>> To my understanding the 156% on PV source and output circuits is related to the ability of PV to deliver more than rated and be continuous.
>>> Therefore, the wire has to be able to carry this current, so now the temperature and fill corrections are applied to find the wire capable of the 156%.
>>>  
>>> I am in a debate where the question is why correct for temp and fill on 156% of ISC and spend all that extra money when the normal operating is at Imp. Or why correct the 156% but why not correct the Isc or Imp.
>>>  
>>> I am going to stick to the fact that the circuit has to carry the 156% under all conditions…
>>> Thoughts?
>>>  
>>> Erika
>>> 
>> 
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