[RE-wrenches] calculating DC voltage drop

[R Ray Walters]

I use actual operating current, without multipliers. I also use actual temperatures, not the worst case temperatures that many use.
I created a spread sheet that uses the NEC adjustments for wire temperature, (see NEC chapter 9, table 8, FPN 2)
It makes quite a difference!
Volt drop and the accompanying loss of energy production needs to be looked at realistically under normal operating conditions. Adding worst case multipliers is necessary for sizing cables and breakers for safety, but it isn't appropriate when calculating energy losses. We shouldn't do an energy loss analysis for a situation that might never occur, or for only a very small % of the time. 
A correct analysis will reflect actual losses over the lifetime of the system, so that an informed economic decision can be made. Especially on larger systems with long runs, its worth it to you and the client to use real world numbers, not blind over sizing.
I got into this over a decade ago, when I noticed that actual measured volt drop was much less than calculated, now my calculations match real measurements. 

R. Walters
ray at solarray.com
Solar Engineer




On Jul 22, 2010, at 1:22 PM, Marco Mangelsdorf wrote:

> When calculating the voltage drop from a PV array to the inverter, what’s the most common accepted practice?  To use the Isc X 1.25 or 1.56 as the multiplier?
>  
> Thanks,
> marco
>  
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