SW inverter to battery cable calculation [RE-wrenches]

[Dean T. Newberry]

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Hi Y'all,
I  have a paper on RF propogation from a solar inverter. A rather 
suprising amount of RF energy was being propogated from the DC lines by 
the inverter. It was one of the tall white cabinets, and did not have an 
FCC certificate listed on the cabinet. I'll send copys to J's. Berdner 
and Wiles. Suffice it to say the inverter was also a broad spectrum 
radio station, interfering with signal reception on a variety of 
consumer equipment in the area, so the wiring needs to have the ampacity 
to handle the rated as well as the transmitter loads.

Cool Huh??

cul  deant at dcn.org

Todd Cory, Mt. Shasta Energy Services wrote:

>Are we talking about the DC load on the batteries when the inverter is supplying
>AC power to loads, or the AC ripple on the DC fed to the batteries when the
>inverter is grid connected and acting as a battery charger?
>
>Todd
>
>
>Bill Brooks wrote:
>
>  
>
>>John and Joel and Wrenches,
>>
>>The 28 amps comes from both John W. and myself. The actual calculation is
>>found in my manual. The ripple current is very easy to read with a true RMS
>>clamp-on meter. 28-amps is actually a conservative number and increases with
>>increasing supply current. John B. does a nice job of explaining the issues
>>below.
>>
>>Although there is no currently accepted method on how to deal with the
>>issue, I put it in to get people thinking beyond the simple current flowing
>>at lowest battery voltage. Ripple is a much bigger issue at 12 and 24 volts.
>>My biggest question is not only what it does to the ampacity calcs on the
>>conductors, but what does it do to the battery--it has to reduce battery
>>life by increasing temperature at a minimum.
>>
>>Bill.
>>
>>-----Original Message-----
>>From: John Berdner [mailto:jberdner at sma-america.com]
>>Sent: Friday, November 28, 2003 12:02 PM
>>To: RE-wrenches at topica.com
>>Subject: Re: SW inverter to battery cable calculation [RE-wrenches]
>>
>>Wrenches:
>>
>>I "think" I know the answer Joel's question about where the extra 28
>>Amps comes from.
>>I would guess it comes from experimental data.
>>I will ask J. Wiles and forward his reply.
>>In the interim I will try to shed some light.
>>
>>I have discussed this issue many times with J. Wiles and the guys at UL
>>and the rational is as follows:
>>Normally we take the maximum ac output Watts / inverter efficiency at
>>full power to get the maximum dc input Watts.
>>Then we divide the maximum dc input Watts by the lowest battery voltage
>>to get the maximum battery current.
>>This current can theoretically be a continuous current so we need to
>>multiply this number by 1.25 to get the NEC required ampacity for the
>>battery cables.
>>The problem with this approach is that it does not accurately reflect
>>actual in field measurement of the current flowing in the battery
>>cables.
>>When you make RMS measurements of the current flowing in the battery
>>cables you usually see a higher number than you would get from the
>>straight dc calculation.
>>The difference is due to the substantial 120 Hz ac ripple you see on
>>top of the dc current when you measure the vast majority of inverter
>>chargers.
>>    
>>
>
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-- 

 
* Cooperative Community Energy* 	*Dean T. Newberry*

430 D Street, Davis CA, 95616
Tel: 530 758-6064
Fax: 530 758-3684
Email: deant at dcn.org
Web:  CCEnergy.com       
      Talbott Solar <http://www.dcn.davis.ca.us/go/deant/>

Contr. Lic. # B-667908

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