Branch circuits in Subpanels [RE-wrenches]

Ray Walters walters at taosnet.com
Mon Oct 23 08:48:35 PDT 2006


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Thanks everyone;

Your combined knowledge is an always astounding asset to the solar 
industry. It gets so that I sit around trying to dream up good questions 
for you all to shine on.

 I do indeed pass all neutral currents and hots through the same conduit 
nipple; the neutral currents are just being carried together combined in 
one larger wire. I'm sure there are some spurious eddy currents being 
generated (each separate hot will have a field around it, and the single 
large neutral will be closer to some hots and further from others) But I 
think its safe to say the conduit run is short enough, the diameter 
large enough relative to the wire sizes and currents, and the overall 
field generated at the conduit small enough (from the sum of hot 
currents roughly equaling the neutral current), that the heating effect 
can be considered negligible.

So this brings up another question: could you then use this same idea on 
longer runs. ie; several # 12 hots with a single oversized neutral and 
ground for branch circuit wiring, or PV circuits? (some how I recall 
there is a NEC prohibition on this)

Thanks again,

Ray

Allan Sindelar wrote:

>Dan,
>Your reply is clear and informative as usual; thanks. I did a GTWB system
>exactly as Ray described just two weeks ago, so the topic is relevant. The
>inspector had no issue with Ray's approach, asking only if the single
>neutral conductor between the two panels was sized correctly (that is,
>sufficient using #6 AWG for 50A as determined by the breakers on the backup
>systems input and output).
>
>So I agree that simply carrying a neutral conductor in the same nipple
>doesn't provide for canceling eddy currents when the system is in bypass
>mode; that is, when the utility feeds the backed up loads through the IBS or
>a manual transfer switch (which our local utility is currently requiring). I
>would be curious how much heat can be generated by a theoretical 30A
>(continuous) at 120 VAC.
>
>But to me the real question is this: If the common neutral conductor bonds
>the inverter AC input/output and both of the breaker panels, and passes
>through the same nipple, then the common neutral carries current equal to
>the sum of the currents on the branch circuit line conductors, and thus
>cancels out the eddy currents and resultant heating -- doesn't it?
>
>Allan at Pos E
>
>----- Original Message ----- 
>From: "Dan Rice" <danrice at scinternet.net>
>  
>
>>The issue with passing only the line conductors through the conduit nipple
>>between panels is induced eddy currents within the conduit nipple. The
>>changing magnetic field(s) in the conductor(s) passing through the conduit
>>nipple induces an electric current within the surrounding metallic
>>    
>>
>conduit.
>  
>
>>If both line and neutral of the given circuit pass through the conduit,
>>    
>>
>then
>  
>
>>the opposite effects of each wire's changing magnetic fields cancel one
>>another out, and there is no net effect on the surrounding conduit. This
>>    
>>
>is
>  
>
>>why all current carrying conductors of a particular circuit must remain
>>together inside metallic raceways. The problem is, the induced eddy
>>    
>>
>currents
>  
>
>>in the surrounding conduit can result in the conduit heating up, which I
>>believe is due to the conduit material's internal resistance to these eddy
>>currents, which can be rather high. I am sure that there are engineering
>>calcs that can be performed to ascertain how significant this heating
>>    
>>
>effect
>  
>
>>may be based upon the wire size, AC frequency, amperage, conduit size,
>>material type, length, phase of the moon and which side you part your hair
>>on.
>>    
>>
>
>  
>


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