[RE-wrenches] Long MC Cable Mid String

[Larry Crutcher, Starlight Solar]

William,

That is exactly what I said in the first response to this thread,  
(quote)"You should calculate your voltage drop for the entire round  
trip distance of this circuit and size the wire accordingly. It does  
not matter where the wire is located in the series circuit, just the  
total length (of the circuit)." I believe this is what Dave was  
asking; "Will this extra long series wire cause problems?"

Sometimes our threads wander off track and don't stay focused on the  
substance of the question. In the interest of cohesive threads, could  
we please keep it on track or change the subject line? Thanks:)

Kindest Regards,

Larry Crutcher
Starlight Solar
(928) 941-1660
Renewable Energy Products, Service and Installation

Mailing Address (NO SHIPPING):
11881 S Fortuna Rd.
#210
Yuma, AZ 85367

Shipping and retail store (NO MAIL):
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Yuma, AZ 85365



On Apr 22, 2009, at 10:07 PM, William Miller wrote:

>
> Friends:
>
> We are missing some information.  We know the mid-string distance,  
> but what is the distance from the positive and negative ends of the  
> strings to the inverter?  This is all part of the entire string  
> circuit and must be analyzed.
>
> We have spent a lot of time researching voltage drop.  One of the  
> most interesting discoveries we made is when we considered the  
> duration of peak amperage conditions.  If one designs a PV circuit  
> for a target voltage drop at brochure Imp, one must consider how  
> often this value actually occurs.  We submit it is infrequent and of  
> short duration.  We took the hour-by-hour results available from PV  
> watts for an entire year and averaged all non-zero (daylight)  
> values.  The result was close to 50%.  This means that if you design  
> a system for 1% voltage drop at Imp (which is our standard), the  
> average Vdrop is actually 0.5%.  Knowing this, we sometimes allow  
> one size smaller wire when distance, cost, conduit fill or other  
> factors indicate this might make a design more pragmatic.
>
> For a free voltage drop calculator, look on our web site  
> (millersolar.com) in the Resources area.  We think it is superior to  
> most others because:  it allows one to use any voltage, not just  
> standard AC circuit voltages as on the SMA calculator,  it allows  
> one to view a range of wire sizes in one glance and it now  
> compensates for temperature and conduit fill.  Making this resource  
> available is one way in which we are trying to give back to the  
> industry..
>
>
> William Miller
>
>
>
> At 07:13 AM 4/22/2009, you wrote:
>> 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)
>>
>>>> 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
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