wire sizing at Higher voltages [RE-wrenches]

Wed Apr 21 22:41:17 PDT 2004

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You have to add the incremental cost of the larger conduit, conduit fittings,
and the additional labor to handle the heavier wire run. Here in 
California, it
might even bump you up to the next higher workers' comp bracket. Where
the PV system gets a rebate, the net installed cost may be under $4/watt.
Then you have to derate the losses to account for the less-than-full load
periods (maybe the average loss is 75% of peak power rate).

As an asside, the past few days have been clear an bright and I've been
visiting numerous customers. Many of the systems were exceeding their
ratings significantly. Today from 11 AM through 3 PM the system at the
house I was at consistently reported over 4,900 watts on the sunny boys
(36 Sharp 165 = 4,900 CEC AC watts). The highest I saw was 5,100 Wac.
On Sunday, I saw 5,100 Wac on two west-facing 5.0 kW BP systems
and the same over-production for a Kyocera system.

/wk

>If your going 100 ft at a current of 15 amps, #10 will have a power loss 
>of 15x15x 200ft x 1.02Ohms/ 1000 ft = 54 watts.
>a #6 wire will have a power loss of 15 x15x200 ft x .403 ohms/ 1000 ft 
>=  18 watts. We just saved 54 -18= 36 watts.
>At $6/ watt for PV, that was worth $216.00.  If #6 is 30 cents more per 
>ft. that's $60 extra for 36 watts or $1.66/ watt .
>
>a #4 wire will have a power loss of 11 watts, therefore we could save 
>another $42 of power. So if #4 is less than 21 cents per ft more expensive 
>than the #6, it might be worth it. At higher currents it becomes an easier 
>choice.
>On PV its power drop, not voltage drop.
>
>Ray
>
>
>>Hi Ray,
>>
>>I'm curious about just how far are you trying to run this wire? #4 using a
>>SMA?
>>
>>And what voltage drop are you trying to get?
>>
>>thanks,
>>jay
>>
>>Peltz Power
>>
>>----- Original Message -----
>>From: "Ray Walters" <ray at solarray.com>
>>To: <RE-wrenches at topica.com>
>>Sent: Tuesday, April 20, 2004 5:05 PM
>>Subject: Re: wire sizing at Higher voltages [RE-wrenches]
>>
>>
>> >
>> >
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>> > HI Joel, Ron, & all;
>> >
>> > Seems like the terminals should go up to a #4 Ga, if SMA's recommendation
>> > is to size for less than 1%. The reason I asked in the first place is that
>> > extra splices to land an oversized wire on small lugs cost extra money and
>> > eats into our savings from over sizing in the first place.
>> > Ron, I'm glad I'm not the only one sizing for power drop. I got into it
>> > years ago when the cost of keeping to a 2% volt drop on 12 volt systems
>>was
>> > getting outrageous. (200 watt array on 2/0 cable??) I had to convince
>> > myself before I could convince my customers that it was worth it.( It
>> > wasn't) I have some arrays on non MPPT controllers and we're letting volt
>> > drop go as high as 10%. It just wasn't cost effective, and without MPPT
>>you
>> > don't really see that extra power anyway.
>> > Another funny thing that came out of the calculations I was doing: it
>> > didn't matter how short the run was. If you could save 1 watt on a 5 ft
>>run
>> > by oversizing it, it was worth it. (Not of course if the lugs or conduit
>> > can't take it)
>> >
>> > Sincerely,
>> > Ray
>> >
>> >
>> >
>> > >Ray,
>> > >Interesting discussion. From the SMA web site:
>> > >  SMA America recommends sizing the PV wired to keep the voltage drop
>> > >below 1%.
>> > >  In
>> > >any case, the DC terminals in the Sunny Boy Inverter will only accept up
>>to
>> > >10 AWG wire.
>> > >Best regards,
>> > >Joel Davidson
>> >
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