[RE-wrenches] DC wire sizing

[William Miller]

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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