[RE-wrenches] 3-phase wire run considerations

[Ray Walters]

I'm with William on this: wire sizing for volt drop or power loss is not 
the same as max current safety calculations.
After first sizing for NEC requirements, you have 2 issues you're 
possibly over sizing for:
1) reducing the power losses on the line to improve overall system 
efficiency.
2) minimizing volt drop so that the inverter doesn't disconnect from an 
over voltage issue.

For the power loss issue, I created a spread sheet (and William has one 
too) that takes into account power losses, not just volt drop, and you 
can allot a cost per watt for the system and then compare that to the 
cost of over sizing the wire.
In this case, it doesn't make sense to save 30 watts, if the wire costs 
$1000 more.
As William pointed out, you don't really want to size for max, but 
instead for normal operating conditions. Even with the array at max 
output, your inverter efficiency is going to reduce that by at least 5%, 
and that would probably only occur for brief periods.
On issue #2 which is more important, the volt drop to avoid inverter 
shut down is going to depend more on the utility.  Is the utility 
voltage stable, and if so what is the grid voltage usually there? If 
it's already on the high side all the time (>250 on a 240), and your 
line losses add another 10 volts, you might have regular shut downs 
(>264 @ inverter).  If grid voltage tends to be low (<230), and has low 
voltage occurrences (<210), grossly over sizing the wire will only 
encourage low voltage shut downs (<211 @ inverter).

Ray Walters


On 11/16/2011 10:49 PM, William Miller wrote:
> Luke:
>
> Maximum inverter output will occur rarely.  Most of the time the 
> inverter(s) will be running at lesser output amps.  One approach is to 
> size wiring for a certain percentage of the average operating amps.  
> What is the average operating amps?  One way I have found to calculate 
> this is use PV Watts and invoke the tool that gives a data set of hour 
> by hour results.  This includes night time hours.  Delete all values 
> with a zero output and average the rest.  I found a daily bell curve 
> that is about 50% of full power.  Your results may vary.
>
> Good luck,
>
> William Miller
>
> At 07:51 PM 11/16/2011, you wrote:
>> 1: Given the significant cost of the AC cabling for this situation, 
>> what is the most appropriate number to use for current in running 
>> voltage drop calcs? The wiring is going to be significantly oversized 
>> to achieve a ~1% VD at this length. I could use the max rated output 
>> current of the inverter (120A), but that seems like it might be 
>> overly conservative. Opinions?
>>
>
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