Branch circuits in Subpanels [RE-wrenches]

[Allan Sindelar]

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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