Big Wire? [RE-wrenches]

[Ray Walters]

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Thought I'd reply to my own email....

>Hey All;
>
>Iclassic 2% loss. I found the NEC table to be overly conservative and it 
>doesn't take into account what total % ampacity you are running the wire 
>at, or temperature.

Table 8 uses 75degC which accounts for the overly conservative numbers 
generated.  Since I'm talking about the main run from the combiner box 
often underground to the load center the temperature correction factor  in 
the fine print  yields closer to reality numbers.



>Next, instead of converting voltage drop to a percentage, I multiply it by 
>the current to get total watts lost. I multiply the watts being burned as 
>heat by the cost of an installed PV watt (say $6.00). This gives me an 
>actual dollar figure of what the volt drop is costing us. Basically, I 
>compare 2 to 3 likely wire sizes and costs, and pick the wire that is the 
>most cost effective ( of course I back check it for ampacity  against the 
>NEC book). Many times blindly sizing for a 2% volt drop just doesn't make 
>sense. PV costs are down, while copper prices are rising fast. Why spend 
>an additional $300 for larger cable when you may be only netting another 
>20 watts of power ($120 value)? Besides being more cost effective, this 
>sizing method reduces our use of  copper, which anyone who has seen a 
>copper strip mine knows is a good thing.
>Any comments or criticisms on this method?  Try the calcs yourself next 
>time, you'll be amazed how quickly the point of diminishing returns 
>approaches. Use retail installed costs for both the PV and the wire and 
>conduit. Remember that bigger wire takes longer to pull and connect.  By 
>the way this method assumes an MPPT controller, with a regular controller 
>your power savings from over sizing wire are even less.
>
>
>Ray Walters
Buried a little too deep in the books this week......

Ray again  

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