Parallel String Brain Teaser [RE-wrenches]

Drake Chamberlin solar at eagle-access.net
Fri May 17 15:32:20 PDT 2002


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One thing to consider with series fuses is this interesting statement from 
690-4 (C)

"Supplementary overcurrent devices used for the exclusive protection of the 
photovoltaic modules are not considered as overcurrent devices for the 
purpose of this section."

I think series fuses are therefore not required to use the same multipliers 
as overcurrent devices classified as overcurrent devices.

Also, we can consider an arrangement of modules one source circuit from 
690.4 (C), so long as the circuit is rated under 50 volts.

"sets of modules interconnected as systems rated at 50 volts or less, with 
or without blocking diodes, and having a single overcurrent device shall be 
considered as a single-source circuit.

Remember that series fuses are not considered as overcurrent devices.

You Said:

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.

Also, I am confused as to why we can't use 15 amps for the series fuse size 
instead of 9.  I haven't been following this thread, so forgive me if it 
has been covered.

Thanks,

Drake


At 12:59 PM 5/17/02 -0700, you wrote:
>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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