Branch circuits in Subpanels [RE-wrenches]

[Jay Peltz, Peltz Power]

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Hi Allan,

The heat issue can be done with simple math, ie voltage drop.
So a 10% voltage drop at 120v would be 12v x  30 amps = 360 watts / #  
of feet of wire = watts per foot
I think you'll find that this number is pretty small, given the large  
wire, low resistance, and short runs.

As to the neutral.  As I understand it,  in a 240v configuration it  
only carries
the difference between the two hot legs.  So if the hot legs were  
each doing 30 amps,
then the neutral would be doing nothing.  However it only one leg  
were doing 30 amps,then the
neutral would be doing 30 amps as well.

Hope this helps,
jay

peltz power
On Oct 21, 2006, at 9:34 AM, 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?
>
> A


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