Big Wire? [RE-wrenches]

[Ray Walters]

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Hey All;

I have been studying for the NABCEP exam, and for voltage drop 
calculations, they dictate using Table 8 of the NEC book, with the classic 
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. For PV array to load center wire sizing, I use a formula that 
uses circular mils and has  a lower resistance factor if the wire is being 
run at less than 50% of its ampacity. This formula seems to correspond well 
with actual field measurements also.  What are my fellow Wrenches using 
these days?

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

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