Ray,
I can't follow you all the way down that path.
Is an insulated wire conductor permitted to be use at 100% of it's
rating under continuous operation? No. The ampacity is reduced and the
conductor must be up-sized. Hence, for a defined load (Isc * 1.25),
under continious operation, the conductor must be upsized to match
(defined load * 1.25). I don't see how this can be changed by simply
changing the nature of the OCP. I think the point of this exception is
to allow cost savings by allow assemblies and OCP devices with lower
ratings. The exception does not apply to every element of the circuit
(the conductors) but only to the 100% rated assemblies and OCP themselves.
What am I missing?
Thanks,
Mark
On 8/22/2014 6:14 PM, Ray Walters wrote:
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, ma...@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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