wire sizing at Higher voltages [RE-wrenches]

[Ray Walters]

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Hey Geoff;

No insult taken. I wasn't quit sure, and I thought the group might be tired 
of me always questioning our normal ways of doing things. (You guys can 
tell me) As far as NEC expertise, Bill Brooks, John Wiles, John Berdner, 
and others will give you far better info than I. I'm liable to lead you 
astray from the "good book" from time to time.
On the PV wire sizing thread: I found a very interesting thing. While the 
2% volt drop is overkill and not cost effective on 12 vdc and even 24vdc 
systems, it is pretty close for 48 volt systems at current wire and PV prices.
Now for the big bomb: higher voltage systems (especially over 100 vdc) can 
benefit from sizing below 1/2% volt drop! I ran the numbers , and the 
watts, while seemingly insignificant as a percentage of the whole system, 
are there to be had for $2/ watt or less.
Consider a 50 ft run from array combiner box to inverter at 20 amps. a #10 
wire will drop 41 watts at ambient temp.  A # 6 wire will only drop 16 
watts. So regardless of voltage, you can buy 25 more watts for maybe 
another $30 of wire. The #6 would provide 6.7% volt drop for a 12 volt 
system, but for a 144 volt system you are only  at a 1/2% volt drop. My 
point once again is that volt drop isn't as important as  power drop. Those 
watts are still worth going after; especially in this example they're only 
costing $1.20 / watt!
John Berdner or someone with more grid tie experience, can you confirm the 
cost benefit of over sizing wire beyond the 2% rule at high voltages? What 
is the largest wire the Sunnyboy lugs can accommodate? I know for a 3000 
watt array another 25 watts doesn't seem like much, but isn't it still 
worth it if the wire cost is less than the PV per watt cost?

Always asking too many questions,

Ray

  

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