[RE-wrenches] Assembly Protects Conductors?

[Ray Walters]

Most combiner boxes, DC rated breakers, and DC rated fuses and fuse 
holders we use today, qualify as assemblies for the 100% exception 
(Outback, Midnite, etc.....) so you only have to size the OCP by 125% of 
short circuit current rating for edge of cloud effects.
Since the conductor is sized based on the overcurrent protection, you do 
not need to size it to 156% of the short circuit current rating, if you 
take advantage of 100% duty rated breakers and combiners.  Also, 
recombiners are still basically combiner boxes, and so all the same 
rules apply.  The additional 125% is only for breakers that are rated at 
80%  (most AC breakers for example) .
For DC, the only time you would size for the full 156% would be if you 
were using something like an old QOU breaker, and those aren't rated for 
high enough voltage for today's arrays anyway.   I'm not aware of any 
other readily available DC rated breaker that is NOT 100% duty rated.   
Some RK5 fuse assemblies may also not be 100% rated. (Wrench Colleagues 
please correct me on this....)
Basically unless you are using surplus equipment from the stone age, (no 
offense to my fellow stone age wrenches)  you only need to oversize the 
conductors by 125%.  By the time you apply all the other applicable 
deratings for conduit fill, terminals rated at 75 C, and direct sunlight 
on roofs (table 310.15B3c)  and consider voltage drop, you really don't 
need to keep oversizing.

R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760

On 8/22/2014 2:34 PM, markf at berkeleysolar.com wrote:
> Folks,
>
> In looking at a circuit breaker based re-combiner for the first time, I am
> now pondering the meaning of the Exception below:
>
> NEC 2014 690.8 (B) Conductor Ampacity. PV system currents shall be
> considered to be continuous. Circuit conductors shall be
> sized to carry not less than the larger of 690.8(B)(1) or (2).
>
> (1) One hundred and twenty-five percent of the maximum
> currents calculated in 690.8(A) before the application of
> adjustment and correction factors.
>
> Exception: Circuits containing an assembly, together with
> its overcurrent device(s), that is listed for continuous operation
> at 100 percent of its rating shall be permitted to be
> used at 100 percent of its rating.
>
> Just to be clear, it is only the assembly that can be used at 100 percent
> of its rating. (Isc*number of strings*1.25) The conductors in the circuit
> still have to be sized for continuous operation (Isc*number of
> strings*1.25*1.25).
>
> I ask because a vendor with a 100% circuit breaker re-combiner states in
> their cut sheet that use of the device "allow
> smaller home run cable sizes from the string combiner to the inverter". Is
> anyone aware of a means by which this claim can be substantiated?
>
> Thanks,
>
> Mark Frye
> Berkeley Solar Electric Systems
>
>
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