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[TowerTalk] 220VAC circuits

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Subject: [TowerTalk] 220VAC circuits
From: (Robert Hummel)
Date: Tue, 23 Mar 1999 10:48:58 -0500
When using non-metallic cable (NMC), such as Romex(tm), for 110/115/117 VAC
circuits, use black=hot, white=neutral, green/bare=ground.

The ground wire and neutral wire in a residential electrical
system should be connected together at one -- and only one --
point in the system: where the service enters the house. Thus,
if you have a 220V service, your service entry panel will have
two hot wires and a neutral coming from the utility company.
The panel will be grounded to earth. At that point, the
neutral is connected to ground.

Connecting the neutral and ground together anywhere else but
the entry service will result in a potential difference being
impressed across the ground wires throughout the system in
case of a short -- a dangerous situation.

3 vs 4 WIRES?
As wiring is distributed, to a sub-panel for example, the
neutral and ground must be carried seperately. So a subpanel
with 100A service would use #2 cable with 3 conductors and a
seperate ground wire.

Do not use the ground wire as a neutral. In general, it is a smaller wire
and does not have the current carrying capability of the main conductors.

If you want to run both 220 and 110 off a single wire, you must use 12/3 +
ground. This gives you two hot wires (one for each phase) and a third to
function as the neutral return for the 110V circuits. Any pure 220V
connections on the line will ignore the neutral wire and connect between
the two hot wires. If you don't run 12/3, you can't "split" the circuit.

For example, I run 12/3+G to a quad box near my table saw. The box has a
220V (across the hots) outlet and two split 110V (break the bonding bar on
the duplex outlet) outlets. An overcurrent condition in any of them will
trip the 220V bonded breaker.

The function of circuit breakers is to protect the wiring, not the device
plugged in. You must rate your circuit breaker to protect the smallest wire
in your circuit. If you run all #12, for example, but have a short section
of #14 to an overhead light, you must install a 15A breaker to protect that

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