Parallel String Brain Teaser [RE-wrenches]

[Joel Davidson]

I use a 15-amp fuse on 2 parallel 4-module series strings of SP75 solar modules
because 4.75 amps Isc x 1.25 = 6 amps x 2 parallel strings = 12 amps. Is this
incorrect?

Bill Brooks 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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