[RE-wrenches] DC wire sizing
Brian Crise
bcrise at nietc.org
Wed Apr 7 09:03:24 PDT 2010
Kent,
One of the things to take a look at and to make sure that your
inspector is aware of is the last paragraph of Section 90.4. This was
brought up at the last code meeting to allow new products to come on-
line and to implement new technology into the market. Due to the
timeliness of changes happening to the NEC, it might be perceived as
making the product go "primetime" before it is ready. In this case,
we looked at the possible consequences of a delay until 2014 and
thought that the industry with its current growth rate would be
impacted negatively by not having DC AFCI in article 690 to help
prevent a black eye for solar. Sometimes the code does drive product
implementation, but I would like to think that it is proactive rather
than reactive.
Brian Crise
Lead Instructor
NIETC
On Apr 6, 2010, at 10:15 AM, Kent Osterberg wrote:
> Ray,
>
> Considering that we design PV wiring to be efficient with voltage
> (and power) loss typically less than 2%, the wire size is nearly
> irrelevant to arcing issues. Essentially all the energy available
> from the PV array can be dissipated in the dc arc. And since the
> current is limited by the nature of the IV curve, breakers alone
> usually won't clear the fault. The best combiner breakers can do
> (if you have enough parallel circuits) is isolate the fault to one
> string in the PV array. With one string being 1 or 2 kW in many
> systems there is still the potential for a lot of heat.
>
> With the 2011 code just around the corner and no dc arc fault
> protection on the horizon, it looks like our industry is again going
> to have a code requirement that no one can fulfill.
>
> Kent Osterberg
> Blue Mountain Solar, Inc.
>
>
>
> R Ray Walters wrote:
>>
>> I agree that we don't want to create the code first, and try and
>> develop the product after. On the other hand, if a DC AFI can be
>> developed that could stop some of the problems I've seen breakers
>> not help, I'm installing them, and pushing for code requirements.
>> AC GFIs were gimmicky too at first, but now have gone on to save
>> countless lives; usually kids, but a few of us wet booted
>> contractors too.
>> As far as running things at 100%, I do agree with you, but I also
>> think that 156% over rating in many cases is too much. If the wire
>> and breaker are that oversized, it is less likely to trip when you
>> want it to.
>> My most recent damage I saw, the cable a few inches back was in no
>> way damaged, as it only saw array short circuit current, but the
>> connector that arced burned up a whole circuit board.
>> More oversizing would have only increased the arc potential, and
>> reduced the chances of a breaker tripping.
>> Proper sizing (not too big, not too small) is the way.
>>
>> R. Walters
>> ray at solarray.com
>> Solar Engineer
>>
>>
>>
>>
>> On Apr 6, 2010, at 9:16 AM, robert ellison wrote:
>>
>>> I have seen info from independent tests that convinces me that
>>> AFCI's probably don't work for AC and i hate to think what they
>>> would do for DC, if anything. Besides drive the costs up.
>>> This was a few years ago and maybe they have gotten it together by
>>> now.
>>>
>>> Anyone remember the original ground faults form Trace (?) after
>>> the code change requiring them in 96? Expensive and prone to
>>> catching fire comes to mind, if i remember correctly. Lets not
>>> encourage more if that type of experimentation in the industry.
>>>
>>> Just the same i am not a believer of running anything at 100%, it
>>> will always have a higher failure rate than something run at a
>>> lower capacity, be it a generator, lawn mower or a circuit breaker.
>>>
>>> Bob
>>>
>>>
>>>
>>> Mon, Apr 5, 2010 at 5:36 PM, William Miller <william at millersolar.com
>>> > wrote:
>>> Ray:
>>>
>>> It is my analysis that combiner breakers (if present) will protect
>>> only wiring upstream of the combiner -- that is, the individual
>>> string circuits. This protection would happen if there is a fault
>>> in one individual string (in the wiring or the modules) that
>>> allows current from other strings, in excess of the breaker
>>> rating, to be supplied through the breaker feeding the faulted
>>> string.
>>>
>>> There are two scenarios at play here:
>>>
>>> 1. Any fault between the combiner and the feeder destination will
>>> not trip any circuit breakers. The breakers are sized such that
>>> the current from each individual string is less than the breaker
>>> rating (by more than 1.56 times) and they will not open.
>>>
>>> 2. PV GFDI protection at the destination end of a feeder will not
>>> help. PV GFDI circuits will not remove power from a feeder and
>>> they will open the ground-to-grounding conductor bond.
>>>
>>> Analyzing this further: Fault conditions are made more likely
>>> given that PV string circuits are no longer protected by conduit.
>>> Faults are then more likely in individual string circuits (those
>>> circuits without conduit protection). This is most problematic at
>>> installations with two or fewer strings, where there is no
>>> combiner, i.e. residential installations. Statistically,
>>> residential installations offer greater exposure to electrical
>>> fires because: occupancy occurs for more hours per year, fire
>>> alarms and sprinklers are often not installed, children are more
>>> often present and standards are more lenient for residential
>>> wiring systems.
>>>
>>> These two facts are PVs dirty little secrets. Further innovation
>>> is needed...
>>>
>>> William Miller
>>>
>>>
>>>
>>> At 12:22 PM 4/5/2010, you wrote:
>>>> I think the 100% rating exception is an interpretation issue. I
>>>> consider the assembly to be defined as the breaker mounted in its
>>>> listed enclosure.
>>>> I agree that the AFIs would add cost, but they might actually
>>>> offer some protection too. (possibly one AFI unit could offer
>>>> protection for multiple circuits?)
>>>> I've never had a PV circuit breaker actually trip, except some
>>>> nuisance tripping due to faulty breakers.
>>>> PV breakers seem to only offer protection for very limited
>>>> situations ie, a short in a PV wire being backfed by enough other
>>>> PV circuits to trip the breaker.
>>>> It could happen, but I've never actually seen it. Even completely
>>>> shattered modules still have enough internal resistance to limit
>>>> the short circuit current to
>>>> a value below the breaker trip point.
>>>>
>>>> Ray
>>>>
>>>> On Apr 4, 2010, at 11:47 PM, Kent Osterberg wrote:
>>>
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