[RE-wrenches] Long MC Cable Mid String

[William Miller]

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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