Parallel String Brain Teaser [RE-wrenches]

Sat May 18 20:44:40 PDT 2002

Wrenches:

The UL series fuse rating is to protect the module from current feeding
back into a fault in the series string. 
The module is tested by running a fault current at the sure rating back
into the module.
This is done until a failure occurs or until thermal stability is
reached.
I believe this makes it a continuous rating so you only need to consider
the 1.25 safety factor for high irradiance but not the second 1.25 for
continuous duty. 

Now, 690 has an exception that says you do not need to provide fuses if
you can show that all the conductors are rated for more than the
available fault current.  UL has said you should think of the UL series
fuse rating as the Ampacity of the module. 

So, if you have two strings (PV source circuits) at 4.75 * 1.25 = 6 Amps
for a total of 12 Amps.
This would require a minimum 15 Amp (12* 1.25) fuse in the PV output
circuit to meet NEC.

If you have a fault in one sting then the current can only come from the
other strings plus any contribution from the inverter and / or battery.
If you have a 15 Amp fuse in the PV output circuit then the total
available fault current in one string is 15 Amps (back feed from
inverter / battery) PLUS the other 6 Amps from the second string.
As Bill pointed out this is more than the UL series fuse rating of the
module so you are out of luck and need to have fuses for each string.

The minimum fuse size for each PV source circuit is 7.5 Amps 
(4.75 * 1.25 = 6, 6 * 1.25 = 7.5)
The maximum fuse size is 15 Amps per the UL Listing for the module. 

If you have any questions, or if I can be of further assistance, please
do not hesitate to contact me.

Best Regards,

John Berdner
Phone: 530.273.45595
Fax: 530.274.7271

-----Original Message-----
From: Bill Brooks [mailto:billb at endecon.com] 
Sent: Friday, May 17, 2002 9:01 PM
To: RE-wrenches at topica.com
Subject: RE: Parallel String Brain Teaser [RE-wrenches]

Joel,

If a battery or any other inverter than a SunnyBoy is on the other side
of
that fuse, you have violated the 15-amp rating of the module as I
pointed
out below. You would actually need a 21-amp fuse rating (15+6=21).
Siemens
goofed if they intended to do what you suggest. However, they were
brilliant
if you consider three strings connected to a SunnyBoy--in this case no
fuses
are necessary.

One string per fuse for you. Sorry for the bad news.

Bill.

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Friday, May 17, 2002 3:46 PM
To: RE-wrenches at topica.com
Subject: Re: Parallel String Brain Teaser [RE-wrenches]

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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