Ken Hall wrote:

Last time I got involved with it the neutral/ground/return bus in home
breaker boxes were all one bus. It was just brought to my attention
that today the neutral bus is separated from the ground, and 4 wires
are now used for 220v. In the old days 220v only used 3 wires.

When did this change and why?

Ken

It was just before 1970 when the NEC required the use of separate
neutral and equipment ground conductors. It was to provide a safe
grounding system to prevent life threatening electrical shocks from
equipment and appliances.

Henceforth, there were four conductors:

two 120 volt lines (line, hot- black & red, 180 degrees out of phase,
240 volts between them) source- utility company

a neutral (neutral, center-tap, white) source- utility company

a ground (equipment ground, earth, green) source- local ground rod, or
bond to water pipe. [NEC has whole list of "acceptable grounds", with
new plastic water pipes, many old installs no longer have a good ground]

At one point (and only one point) there is a bonding wire or bolt where
the eguipment ground and neutral are "bonded together", usually in the
"main disconnect enclosure". Removing the bond and testing for a fault
between the neutral and ground is usually part of the inspection
proceedures. The ground conductors terminate to a ground bar mounted
and bonded TO the enclosure. The neutral conductors to an insulated
neutral bar in the enclosure. (One of the bolts on this bar will be
marked "ground bond" and will go thru the bar and thread into the
grounded enclosure, effectively connecting the neutral and ground)

There are two notable exceptions to carrying a separate NEUTRAL and a
GROUND to ALL loads (incl. sub-panels).

1. Electric Range/stove. Only the two 120v lines and the neutral are
required to the classic 50 amp, 3 prong "stove" outlet.

2. Electric Dryer. ditto the above, except the outlet is 30 amp.

Also , NEUTRAL is not required if the load is a 240 volt only load. Has
no component that requires 120v.

I think the latest code has changed the dryer connection requirements to
4 conductor. All the stuff I do now is industrial and I don't run into
dryer outlets, all our 240v cords are 4 conductor. Also about 1980, the
grounding conductor size changed from a percentage of the current
carrying conductors to the same size. (ie 14/2 wg had a #18 ground, now
a 14/2 wg has a #14 ground)

-larry

ps: i remember the days of spool & tube too, fixed it, didn't run it ;-)