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
As far as I know, the neutral and ground are seperated when using a sub-panel. Otherwise if you have just 1 main panel, they are usually tied together on the same buss.
The two should be bonded together in the main panel. The only time you will need 4 wires for a 240 volt circuit is if you also have some component of the circuit feeding a 120 volt load. Then you will have neutral current as well as 2 hots and a ground. An example of this would be a 240 volt stove with a 120 volt convenience outlet on it, or a subpanel with 120 and 240 volt circuits in it.
Stretch
And do you know about when this separation of neutral from local ground was added to the code requirements?
Ken
That panel is not the main disconnect. Those four wires go back to a main disconnect panel where the neutral and ground finally come together.
Where is that ma> Last time I got involved with it the neutral/ground/return bus in home
From my memory only. I know for sure that in 1996 the NEC required four wires for dryer and ranges for example. The change might have been made in 1993 but I am fuzzy about that.
Manufactures make residential service panels with two busses because some authorities require the grounds to be separate from the neutrals. The ground and neutral are connected by some means dependant on the manufacture. Commercial/industrial services have always been separate.
You can still run 3 wires to a motor and meet the NEC. As long as your load does not require a neutral for anything.
The concept is...... using the ground as a neutral has always been a violation of the NEC, it was just ignored in the residential market for a long time.
I think there is one exception left -- a feeder circuit that serves as the service to a separate building (but not a mobile home.) Either 3 or
4 wires would be equally correct unless there are water, gas, CATV, telephone, etc. connections between the buildings.Best regards, Bob
Thanks much for the info.
With the exception of devices using one of the 120 legs for something, it's still not clear to me the reason for this rule.
Ken
Ken, If you use a ground wire for neutral current and a ground fault happens, It may not be big enough to carry both ground and neutral currents. Also, it is more likely for such a combined wire tooverheat/corrode/oxidise and end up with a high resistance connection somewhere allong its length. Then it wouldn't work right and someone would be dead! (Maybe You!!) The code is there as a safety code to protect people and property, not just to keep inspectors employed. It may save your life someday, in fact maybe it already did, you just don't know it. It also keeps the lawyers from having too much work. :-)
Stretch
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 companya 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).
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 ;-)
Oh Crap, I got "the book", the new one, out of the truck-
both of the old exceptions are gone
since 1999, the nec requires neutral and ground on everything, no stove and dryer 3wire outlets.
(100 times on the chalk board - i will not post unless i have the ref in front of me)
-larry
larry wrote:
Nonsense.
In no case would the current flowing through the ground conductor be greater than the current flowing through the hot conductor. If the breaker is properly sized for the conductors, that's not a problem - and if the breaker is *not* properly sized for the conductors, then it's a problem anyway, even *without* a ground fault.
Really? Please explain.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
Definately nonsense -- the neutral has to be the same size as the hot conductors, so the current capability has to be there with the neutral alone.
The issue with grounding something with the neutral is that since the neutral carries a current, and since conductors are not "perfect" (i.e.: they do not have zero resistance), a voltage drop will occur. Depending on the load current and the length of the cable run, any neutral-bonded piece of equipment will not be at ground potential, but a few volts away from ground. In the right circumstances, contact with this and something that is at ground potential could be very dangerous.
BZZT Wrong. The neutral can be sized for only the worst case of current imbalance which is the failure of one leg of a split single phase service or of one phase of a three phase service. Loads that run at the end to end voltage of a single phase service or phase to phase of a multi phase service need not be included when calculating the required ampacity of the grounded current carrying conductor (neutral).
The real risk of bonding the exposed conductive frames of appliances to the neutral is that if the neutral goes open the voltage on the frame of the affected appliance/s will rise to the voltage to ground of the supply source.
-- Tom H
Q and what happens if that neutral wire should fail open for whatever reason?
A you get 120 V on the cabinet.
Yes, that is a definate and very serious reason for bonding equipment with a separate, non-current-carrying, conductor.
However, the frequency of this occurring is relatively small (but not insignificant). While the possibility of a neutral-bonded chassis sitting at a volt or two above ground is somewhat higher (as in, it will be true when the item is drawing a load, as well as when something else on the same circuit is drawing a load). For the most part, this will not likely pose a dangerous situation, but the potential is there -- it only takes a few milliamps through the heart to be fatal.
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