Big Wire? [RE-wrenches]

Ray Walters ray at solarray.com
Fri Apr 16 11:00:30 PDT 2004


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>Hi Joel;

I think if you relook at my original email, you will see I am comparing the 
one time cost of the wire vs. the one time cost of the PV modules. The end 
result is to maximize the energy output over the lifetime of the system for 
the lowest total initial cost. If installing more PV modules is cheaper 
than oversizing the cables, then everyone (including the environment) is 
better served.
No amount of oversizing is going to reduce wire losses to zero. Each size 
increase buys you a few more watts. The point is that we should stop 
increasing the wire size when the price per watt saved costs more than the 
price per watt of the PV modules.

Ray

>Hello Ray,
>PV costs are down, but wire is still cheaper than voltage drop. Even a small
>percentage of energy lost over the life of a PV system can add up whereas
>wire is a one-time cost. As a percentage of system cost, wire is not a place
>to economize. Also, old copper wires are recycled. Sometimes, sad to say,
>new copper wire is stolen and recycled.
>Best regards,
>Joel Davidson
>
>----- Original Message -----
>From: "Ray Walters" <ray at solarray.com>
>To: <RE-wrenches at topica.com>
>Sent: Wednesday, April 14, 2004 1:55 PM
>Subject: Big Wire? [RE-wrenches]
>
>
>
>
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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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