The "Carter
System"
The so-called
"Carter" system of multi-location lighting circuit
control is one wherein two or more three-or four-way switches
are connected to both sides of the line. The advantage of the
Carter system is the saving of wire. The disadvantage is it's
a violation of the Code to switch the white grounded conductor.
(380-2b)
The Carter system
was in question with the Code even back in 1920 when it was being
used.
The example below
shows two buildings with a 3-way switch in each building controlling
the light on one building which also has a duplex receptacle
which is hot all the time. To correctly wire this circuit it
would require four circuit conductors and an equipment grounding
conductor. With the Carter system the duplex receptacle is hot
all the time and the two 3-way switches will control the light.
Only three wires are used instead of four. With the Carter system
the duplex receptacle is hot all the time and the two 3-way switches
will control the light. Only three wires are used instead of
four.
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