[RE-wrenches] DC wire sizing

Joel Davidson joel.davidson at sbcglobal.net
Wed Apr 7 16:09:01 PDT 2010


Hello Ray,
I agree with Bob-O. Smaller wires between the inverter and the grid may meet 
ampacity requirements and your I2R design goal, but high grid-tie voltage 
has been a problem. Also, 1% of lost power over the life of a PV system adds 
up especially when grid power continues to go up in price. I agree your 
reasoning, but I also agree with Feed-In Tariff PV system operators who want 
every kWh possible and are willing to pay the up-front cost. I'm sticking 
with less than less than 1% wire loss between the inverter and the grid and 
less than 2% overall wire loss unless the customer explicitly states that he 
wants more loss.
Joel Davidson

----- Original Message ----- 
From: "Bob-O Schultze" <bob-o at electronconnection.com>
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Sent: Wednesday, April 07, 2010 1:50 PM
Subject: Re: [RE-wrenches] DC wire sizing


> Ray,
> Sorry, I'm not buying that argument over a 25+ year design lifespan. 
> Also, -and perhaps something many folks don't consider- the NEC requires a 
> MAX loss of ≤5% over the ENTIRE circuit. That means all the way to the 
> mains panel. Best practices would require no more than 1.5% VD between the 
> inverter and the mains so as to avoid any potential of overvoltage 
> disconnection by the inverter. As it tries to push the current into the 
> grid, it also pushes up the voltage on that line and on it's own sensors. 
> Obviously, the larger the current flow and the higher the VD, the worse 
> the situation could be. Bear in mind that the grid is not held to the same 
> over and under voltage specifications as your inverter is. 3% + 1.5% is 
> pushing it just a bit too close for comfort and I still think wasting 
> watts in wire losses is bad design. We agree that orienting PVs to an 
> azimuth other than 180° and a bit less than latitude elevation is less 
> than ideal, but you "run what you brung" in terms of the orientation and 
> roof pitch of a structure. Better to take a hit on production if we're 
> talking about the backasswards method of incentivizing by the installed 
> watt, than not installing PV at all. That said, we DO have control over 
> wire sizing. IMO, throwing away watts forever just to cheapen it up a very 
> little bit upfront is poor economy and as the price of gird supplied power 
> increases over the years, the waste and lost revenue is even more 
> acerbated.
> Best, Bob-O
>
> On Apr 6, 2010, at 6:04 PM, R Ray Walters wrote:
>
> Just run the numbers sometime. Compare the cost difference of #6 vs. #4 
> wire say, and then look at how many more watts you're actually saving, 
> then multiply that additional wattage by the installed cost per watt.
> Very simply, once you've satisfied code requirements, there is a point at 
> which it is cheaper to add more panels than use bigger wire.
> Also, that point is a moving target that fluctuates with PV and wire 
> costs. I've found recently for our projects, that that point is falling at 
> about 3% loss.
> I also include a cost factor for oversizing the conduit, extra labor 
> (bigger wire is harder to handle), and any connectors needed to land the 
> larger wire.
> I've got very well designed systems working for decades, using this 
> method.
> This is how large commercial systems are designed as well.
> You can't simply pull 1%, and then call us bad designers because we 
> actually do an economic analysis for each wire run.
> It used to be unheard of to install PV facing anything but due south at 
> latitude tilt, but now we know to add a few more modules. Same concept.
>
> R. Walters
> ray at solarray.com
> Solar Engineer
>
>
>
>
> On Apr 6, 2010, at 1:37 PM, Bob-O Schultze wrote:
>
>> Ray,
>> A 2% wire loss is the generally accepted metric for battery based systems 
>> with relatively low PV voltage input (<150Voc). It's just plain bad 
>> design to accept more than a 1% VD on higher voltage systems. PVs ain't 
>> THAT cheap.
>> Best, Bob-O
>>
>> On Apr 6, 2010, at 11:44 AM, R Ray Walters wrote:
>>
>>
>> Once I have fulfilled NEC min. requirements, I use a spreadsheet to 
>> analyze the cost of larger wire vs. the cost of power lost. Going under 
>> 2% is usually not worth it, if copper prices are high, and PV cost is low 
>> enough (current market). Sizing for under 2% was good economics a few 
>> years back, when PV was high, and copper was low, though.
>>
>> For NEC 2011, I agree: while I readily welcome development of DC AFI, 
>> implementing code before the technology is ready, is a bad idea. But that 
>> may be the only way to get the technology in place.....
>>
>> Ray
>>
>> On Apr 6, 2010, at 11:15 AM, Kent Osterberg wrote:
>>
>>> Ray,
>>>
>>> Considering that we design PV wiring to be efficient with voltage (and 
>>> power) loss typically less than 2%, the wire size is nearly irrelevant 
>>> to arcing issues.   Essentially all the energy available from the PV 
>>> array can be dissipated in the dc arc.   And since the current is 
>>> limited by the nature of the IV curve, breakers alone usually won't 
>>> clear the fault.  The best combiner breakers can do (if you have enough 
>>> parallel circuits) is isolate the fault to one string in the PV array. 
>>> With one string being 1 or 2 kW in many systems there is still the 
>>> potential for a lot of heat.
>>>
>>> With the 2011 code just around the corner and no dc arc fault protection 
>>> on the horizon, it looks like our industry is again going to have a code 
>>> requirement that no one can fulfill.
>>>
>>> Kent Osterberg
>>> Blue Mountain Solar, Inc.
>>
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