[RE-wrenches] Long MC Cable Mid String

Dave Click daveclick at fsec.ucf.edu
Wed Apr 22 07:13:26 PDT 2009


Drake, it's as quick as V=IR. So taking the 11A from your evergreens, 
and a #10 interconnect wire at 1.24Ohm/kft (NEC Table 8):

Vdrop = (11A) (0.00124Ohm/ft) (36 ft) = 0.5V. If the modules are hot, 
you're looking at a Vmp around 265V- so your worst-case Vdrop% in that 
jumper wire is just 0.5V/265V = 0.2%.

You're not using the voltage of the entire array to figure the drop on 
the interconnect; you're just using the string current. You use the 
array voltage just to see whether the voltage drop is significant (as 
David Katz wrote).

That answer your question?
DKC (just to add another Dave to the thread)


-------- Original Message --------
Subject: Re: [RE-wrenches] Long MC Cable Mid String
From: Drake Chamberlin <drake.chamberlin at redwoodalliance.org>
To: dkatz at aeesolar.com, RE-wrenches <re-wrenches at lists.re-wrenches.org>
Date: 2009/04/22 10:15

> I have scratched my head on this one before, because it seems that two 
> terminals bonded together by a wire are essentially the same terminal.  
> The theoretical voltage difference on a terminal is zero.  How do we 
> figure a voltage drop on a wire that has near zero volt potential 
> between the ends of the wire.  Can we really use the voltage of the 
> entire array to figure the drop on an interconnect between two 
> modules?   If so, why can we do this?
> 
> Thanks,
> 
> Drake
> 
> 
> 
> At 07:49 PM 4/21/2009, you wrote:
>> David,
>> There will be less than a 1 volt drop in 36 feet of 10 AWG at the 11 
>> amps you get from the Evergreen modules.  I don't think anyone will 
>> notice.
>> David
>>
>> David Katz
>>
>>
>> David Palumbo wrote:
>>> Does the following series string wiring cause a problem? With  (18) 
>>> Evergreen ES-A 205’s (low voltage modules 18.4Vmp) in series feeding 
>>> a grid tie inverter with a long series connector in the middle of the 
>>> string.  (2) Top Of Pole 9 module mounts with the series connection 
>>> from module #8 on the first pole to module #9 on the second pole. 
>>> This MC cable connection would be, about, 36 feet in length to go 10’ 
>>> down the pole below grade 18” for 13’ and back up the second pole 10’ 
>>> (in conduit all the way) .
>>>  
>>> Will this extra long series wire cause problems?
>>>  
>>> I could use other modules of course but that would raise the cost per 
>>> watt by 70 cents.
>>>  
>>> Dave
>>>  
>>> /David Palumbo, President
>>> Independent Power LLC
>>> 462 Solar Way Drive
>>> Hyde Park, VT 05655
>>> /NABCEP Certified
>>> / www.independentpowerllc.com <http://www.independentpowerllc.com>
>>> / 
>>>
>>>
>>>
>>>
>>>  
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> 
> Drake Chamberlin
> Athens Electric
> OH License 44810
> CO License 3773
> 740-448-7328
> 740-856-9648
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