wire sizing at Higher voltages [RE-wrenches]

[Joel Davidson]

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Ray,
Interesting discussion. From the SMA web site:
What size cabling should I use between the PV panels, and the inverter?
The size of the cabling on the DC side depends on the distance between the
PV array and the Inverters, as well as the short circuit rating of the PV
string. SMA America recommends sizing the PV wired to keep the voltage drop
below 1%. Please look at table 310-16 of the NEC code for cable sizing
requirements. Use 125% of parallel module rated short circuit current. In
any case, the DC terminals in the Sunny Boy Inverter will only accept up to
10 AWG wire.
Best regards,
Joel Davidson

----- Original Message ----- 
From: "Ray Walters" <ray at solarray.com>
To: <RE-wrenches at topica.com>
Sent: Monday, April 19, 2004 5:14 PM
Subject: Re: wire sizing at Higher voltages [RE-wrenches]




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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