Series fuses/combiner boxes [RE-wrenches]

[Ryan Mayfield]

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Gary,

Bill actually posted a very good explanation to the parallel fusing
issue (pasted to the bottom of this post) awhile back. This started a
good thread if you'd like to sift through the archives, titled Parallel
String Brain Teaser, May 2002. The example he used actually shows the
rare occasion you can use 3 parallel strings without fusing. But, I have
found that for the majority of the modules out there once you go to a
third parallel string, fusing is a must.


Bill,

You mentioned inverters being listed for no backfeed but a lack of
documentation. Without proper testing procedures and documentation how
are the designers and installers to know who has such listings and where
to find them? I've had multiple manus tell me their inverters have such
listings, some with very detailed and compelling explanations, and
others say no such listing exists. With the influx of inverters hitting
the market how are we to know? Can you or any manufacturers shed some
light on this for me?


Darryl,

Can you direct me to the 1.25 safety requirement for voltage in Code? In
690.7 I see the temperature correction factors but no mention of an
additional 1.25. Is this one of those times where I have to refer to 7
sections before finding the right answer? Or is it because it is Monday
morning and my eyes are still adjusting?

Thanks to everyone in advance.


Ryan Mayfield
Energy Outfitters


Wrenches,

Since we have been talking about parallel strings and max series fuse
ratings--let's have a little more fun with this.

The Siemens SP-75 has a max series fuse rating of 15 amps and a Isc of
4.75 amps. The code in 690-8 says that                               the
Isc of the series string will be 4.75 x 1.25 or 6.0 amps.

Question: What fuse size is allowable for two strings of modules in
parallel?

Answer: There Ain't None

Why?: Total allowable input to a series string of modules is 15 amps.
Take the output of one string plus the string
fuse and you get 9 amps (fuse) plus
6 amps (parallel string of modules) equals 15 amps. Why can't we use 9
amps for the fuse size--because 2 (source circuits) times 6 amps (per
circuit) equals 12 amps which will blow the 9 amp fuse quite nicely on
any sunny day.
So who screwed up? I don't know, but some at Siemens claim that you can
put two sets of modules to a 15-amp fuse and they are clearly not
accounting for the current from the other string of modules.

Question: Is there a case where two sets of modules can be wired in
parallel?


Answer: YES, in fact you can do three in parallel!

Why?: If the strings are going into a source that can provide no
backfeed current (e.g. a SunnyBoy inverter that has been listed for zero
backfeed current or a direct drive DC motor), we can parallel up to
three sets of SP-75s in 
parallel since the maximum current is only 12 amps from the other two
modules strings.



See how much fun PV can be! Oh the wonderful things that Mr. PV can do.

Bill.

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