Re: O/T Electric wireing

Posted by KEB on March 29, 2006 at 19:51:50 from (207.94.26.1):

In Reply to: Re: O/T Electric wireing posted by VADAVE on March 29, 2006 at 15:26:26:

3-wire 120/240 volt wiring (two hots & a neutral) went out when grounded outlets came into being (or maybe the other way round). Anyway, as John says, normal house wiring is single phase, 240 volt, which is center tapped to make two 120 volt legs. I think the change to a separate safety ground conductor happened around 1960 or so. We had a house one time that was built in 1961, and it was originally built with grounded outlets in the kitchen & ungrounded outlets everywhere else.

VADAVE, there are instances where 3-wire 240 volt installations are still appropriate, as long as you don't try to tap 120 volts off of them. Dryers & ranges come to mind, where everything runs on 240 volts, and are wired with two hot legs and a safety ground. An air compressor with a 240 volt motor is another example.

The issue comes in when you try to connect to the center tap, which is actually what is grounded. When two conductor (ungrounded) outlets were commonly used, there was no separate grounding (safety ground) conductor, and the neutral (groundED conductor) and safety ground (groundING conductor) were in fact one and the same. Only two wires (hot & neutral) were run to the individual outlets.

With a three conductor grounded outlet, as currently required, a third wire is provided as a safety ground. This wire is not allowed to carry load current, only fault current, which is why it must be isolated from the neutral except at one point. If it were connected to neutral at any other point, then it would carry part of the load current and would not provide shock protection under certain fault conditions. This is also why the safety ground wire is allowed to be smaller than the current carrying conductors, as it only carries current in a fault condition.

As John pointed out, a four wire installation does not indicate three-phase power, although there are three phase installations that only use 4 wires. These are analogous to the 3 wire single phase installation, i.e., single phase 240 volt is 2 hot wires & safety ground. A delta three phase connection is 3 hot wires and a safety ground. A wye three phase connection is 5 wires, 3 hot plus neutral plus a safety ground. Note that 2 hot wires does not indicate two phases, but indicates only that neither of the wires is a neutral.

Maybe clarify things a little...

Keith

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