Big Wire? [RE-wrenches]

Thu Apr 15 07:24:28 PDT 2004

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Ray,

Here is an additional argument for veering from a strict 2% criteria 
-- All the times that the array output is LESS than peak, so is the % 
loss.

We have a very handy wire sizing system on our web site that is 
universal for any voltage you choose. Go to:

http://www.dankoffsolar.com/cgi-bin/siteman/page.cgi?g=Detailed%2F26.html&d=1

It is based on wire resistance data from which more typical tables 
are derived. We first published this chart in Home Power in 1988 and 
it has been confirmed and accepted ever since.

If anyone can suggest how a factor for applying it to 3-phase, please do.

Windy

>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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