[RE-wrenches] DC wire sizing

[robert ellison]

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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