[RE-wrenches] Assembly Protects Conductors?

[Brian Teitelbaum]

To at least partially answer Ray's question about Class-R fuses: Square-D
Heavy Duty fused disconnects, commonly used as DC discos for PV arrays and
which use Class-R fuses, are only rated for 80% duty. The non-fused HD
discos are 100% rated in PV systems. I'm not sure if that is because of
the fuse rating or the fuse holder rating though.

Brian Teitelbaum
AEE Solar



-----Original Message-----
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Ray Walters
Sent: Friday, August 22, 2014 6:14 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Assembly Protects Conductors?

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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