MIXED PANEL ARRAY'S [RE-wrenches]

[Travis Creswell, Ozark Solar]

Drake,

Thanks for the heads up.  I'd forgotten the Solar Boost 50 wasn't UL
approved yet.

next issue.. (we just can't seem to quit beating this dead horse)

>Eight Siemens SP 75s wired in series-parallel for 24 volts, have a short
>circuit rating of 19.2 amps (4.8 X 4) .  Multiply that times 1.25 twice (to
>comply with code) and you get 30 amps.  A pair of number 10 THWN-2 wires
>should satisfy any inspector.  (* Note:  The rating of THWN-2 comes on many
>THHN conductors).


I'm going to hold my ground on this series/parallel issue.  The wording of
code is nice but lets do the math as code specifies and observe how the
manufacturers rate their panels.  I will swallow a big old bucket if pride
if you guys can prove me wrong on the below math and code interpretation!

I have rarely worked with modules that have "max series fuse" ratings that
would allow more than one module in parallel.  Also, after you de rate the
wire by 1.56 and allow for temperature and conduit fill you are looking at
#10 THHN having a max ampacity of 11.9 amps.  Double check my math but here
is how I do it.. (all according to 99 code)

Using table 310-16 #10 THHN/THWN-2 starts out with an ampacity of 40 amps.

First apply the 1.56 de rate which comes from 690-8 (a)(1) and 690-8 (b) 

40/1.56 = 25.64 amps

Now apply the temperature de rate factor of .58 for temps of 61c-70c from
the bottom of 310-16.

25.64 x .58 = 14.87 amps

Now lets apply the conduit fill factor (table 310-15 (b)(2)(a)) de rate
assuming we have 2 source (sub array) circuits for a total of 4 wires and
that is .8.

14.87 x .8 = 11.9 amps is the max current you can put through #10 THHN being
used as a source circuit for solar panels.

Again I am going back to the "Max series fuse rating" on the back of the
module.  This must be more than the short circuit current of the number of
modules you want to parallel.  The typical modules rating is almost always
less than Isc x 2 which means that you can't parallel even two of those
modules.  The exception that I know of is Solarex which often has max series
fuse ratings of 15 or 20 amps as I remember.  Even when you can parallel
code only allows 11.9 amps on #10 wire anyway.  Who wants to step up to
number #8 copper in solar panel J-Boxes.

Regardless of the details of code I think we can agree that there are
troubleshooting and performance benefits to not paralleling modules.

Lets see if we get some agreement on this.  I'd like to this to be on go
onto grounding!  Won't that be fun.

Travis Creswell

- - - - - - -
To send a message:
 RE-wrenches at topica.com

The archive of previous messages: 
 http://www.topica.com/lists/RE-wrenches/

To unsubscribe send a message to: 
 RE-wrenches-unsubscribe at topica.com

To check out the other RE-Wrench participants:
 www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine: 
 www.homepower.com

For info contact list moderator by email:
 michael.welch at homepower.com
___________________________________________________________
T O P I C A  The Email You Want. http://www.topica.com/t/16
Newsletters, Tips and Discussions on Your Favorite Topics