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[TowerTalk] outlet ground problems and shack grounds updatinginfo needed

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Subject: [TowerTalk] outlet ground problems and shack grounds updatinginfo needed please.
From: "Paul Christensen" <>
Date: Mon, 26 Jul 2010 09:54:18 -0400
List-post: <">>
> "However if a 3 wire plus ground #12 cable is run then two separate 120 
> volt
circuits can be used with a common neutral and the 240 volt circuit can also
be used. The two hots also need to be on a connected breaker (double pole)."

Gary's last sentence should be emphasized.  If one were to use 4-conductor 
cable (3-wire + Gnd) through a pair of single-pole 20A breakers on a common 
neutral, then its possible to connect both breakers to poles on the same 
distribution phase.  If that happens, a 20A load on each 120V circuit can 
result in 40A on a #12 neutral conductor.  The worst part is there's no 
overload protection from this wiring fault.  40A of current could continue 
on that #12 conductor until the wring and the home burns to the ground 
without either breaker opening.

When using a *connected* breaker as Gary points out, the two line conductors 
are served from opposite phases.  So, even if each line conductor draws 20A 
on a connected breaker, the neutral conductor's current cancels to zero 
since the neutral current is the vector sum of the two phase currents.

Also, check local codes on this.  I have to believe where primary use is for 
two 120VAC in a residential installation, the practice of splitting phases 
on a common neutral is prohibited in some areas.

Paul, W9AC


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