Parallel String Brain Teaser [RE-wrenches]

Bill Brooks billb at endecon.com
Fri May 17 12:59:06 PDT 2002


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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