14-3 shared neutral

I'd like some feed back on this one. When I built my house, I had a very
honest Father and Son contractor who were jack of all trades. They did the
wiring also. Sloppy but functional. A few years later, while making some
changes, I noticed they used 14-3 wiring to run 2 seperate circuits. These 2
circuits shared the neutral. I questioned them on it and they said that is
perfectly ok. I shared my concerns with them so they asked an electrical
inspector and he confirmed their beliefs. It's ok to do that. I understand
the load is such that it doesn't tax the neutral but what happens years down
the road when the homeowner throws an extra load here and an extra load
there on these circuits until the neutral is carrying more amperage than it
is designed for? The breakers don't trip because the hots are carrying just
under 15 amps each. 14.5 + 14.5 = 29. The 15 amp neutral can't carry 29
amps. I know it's probably a little far fetched but it is possible. I
have...no had 2 circuits like this in my house. Do I have OCD or what?
Residential electricians...what do you think?

 

The neutral carries the DIFFERENCE of those currents, not the SUM.
If the phase currents are equal, the neutral has I=0.
Wade

 

Right, so long as the two branches are on different phases. And in
office buildings where there are three separate phases of 120, that
used to be true, too. But PCs and monitors have switching power
supplies that pull a high current spike at the peak of the AC line
cycle, and the spikes from the three phases don't align in time, so
they don't cancel, and fires have resulted from the huge neutral
currents.

New directives regarding line harmonics (requiring unity-power-factor
power supplies) fix this issue.

John

 

This is true so long as someone competent does the work. I have seen
situations where someone added so much stuff on to a circuit that they
tied another breaker on to the kluged end of their mess probably
because the smaller gauge and resistive connections caused so much line
drop downstream. It took two breakers to turn off this mess! Was I
swearing. I saw this twice last week.
I also saw one where someone had replaced a 220 AC outlet with a
standard outlet. The lady wondered why her Christmas lights were the
brightest on the block and why they kept blowing out so much!

Richard

 

Err? Different 'Legs' might be a better term?
The point about peak currents and being close to unity power factor seem
very valid comments.

In an industrial setting (or in a large apartment building) they MAY happen
be two 'phases'. But in most individual residence situations the two wires
which have 230 volts between them are, sort of, plus 115 volts and minus 115
volts to neutral. They are (Not quite true because we are talking AC here)
usually the two outer ends of a 230 volt 'single phase' which has a centre
tapped neutral to create the two 115 volt 'legs'. These legs are often
mistakenly called 'Phases'. They are typically Leg A (Say Black) and Leg B
(Say Red).
Thus if there is load on both 'legs' it will tend to cancel out any load in
the neutral middle conductor. However; think about this ................;
if there is load on only one side (leg) of the circuit, the neutral will
carry the same current as the hot lead on that side; and the other hot lead
will be carrying nothing; right?
It is a common mistake to think of the neutral as NOT carrying current; but
all our basic circuit training tells us that current has to flow from a
supply, through a load and return!
That return IS the neutral conductor and it better be intact and in good
shape! The fact that a good neutral is almost or only a volt or two above
ground potential means that it is doing it's job of returning the current to
the low voltage (neutral) side of the supply panel with little loss due to
the resistance of the conductor.
An open neutral is bad news and may results in electrical current trying to
return to the supply panel through whatever is available.
For example: Those so called GFIs (Ground Fault Interupters) actually work
on the currents in the hot lead and the neutral being 'balanced'. If
unbalanced due to defective neutral or a fault to ground the GFI disconnects
the outlet/s to potentially save life.
It is also bad to think of the ground wire as "Being the same as a neutral".
It is not.
The ground wire is there for safety in case something goes wrong. I would
not like to have the grounded frame of my fridge being used as the return
wire for the electric current operating the fridge compressor!
Have fun, safely!

 

Beeper: I have duplex outlets in my kitchen wired this way. the upper socket
is wired, say to the red, and the lower socket of the duplex to the black.
This is not to get 230 volts between upper and lower but does allow two 115
volt loads to be plugged in, effectively doubling the current capacity of
the outlet. With 14 AWG that circuit should be fused/breakered at 15 amps. A
double pole breaker should be used to disconnect both sides 'legs' of the
supply to that circuit simultaneously.
Terry

 

Well, I'm an electrical engineer, and we don't refer to "legs". By
"different phases" I meant that the voltages are, well, not in phase.

Zero degrees and 180 degrees are different phases to me. Zero and 120
are also different phases, by my standards. Zero and zero are the same
phase. A good test is to short them together and see what happens.

Unless you're an engineer, in which case they are often correctly
called 'Phases.'

But definitions are personal property, so call them what you will.

John

 

But with a 240V center-tapped transformer, they are exactly in phase -
simply opposite polarity, with respect to the center-tap.

Cheers!
Rich

 

Yes, they are 180 degrees out of phase "in phase."
Tom

 

As I said, to see if they're in phase, merely connect them together
and see what happens. Report back and I'll interpret the experiment
for you.

John

 

But they're the same signal! How can a signal be out of phase with itself?
Picking up a voltage from opposite ends of a transformer winding does
not introduce any phase shift, nor does center-tapping that winding and
grounding the center-tap.

If you think they're out of phase, please show me the component that
delays the signal by 1/120 second.

Sorry.
Rich

 

Terry, you just said what I was hoping to hear. It is OK if the two seperate
circuits are mechanically connected(if you will) If one trips, the other one
trips also. This was not true in my case and that's why I questioned it. How
about everyone else who knows residential code. Mechanically connected? yes
or no?

 

If you run two separate circuits with 14-3, I don't see the neutral sharing
aspect. Each 14-3 leg has it's own neutral.
They should have used 12-3 anyway. And if you add circuits, run new lines
from the breaker panel with a new breaker.. Don't add to existing wiring.

 

Then why does the power company run them in on separate wires?

It can't.

Take the A and B signals from a residential 120/240 circuit. Use a
time-interval counter to measure the delay from identical points and
slopes on the A and B signals, both referenced to N. You will find the
delay to be very close to 8.333 milliseconds.

Repeat experiment with a phase meter. Repeat with an oscilloscope. All
will indicate a 180 degree phase difference.

The only sensible way to refer to such a power system is "two phase."

Sorry.

John

 

On Thu, 24 Feb 2005 09:00:16 -0500, Beeper top-posted (see below for
context)

They should be mechanically connected (ganged) if you will be using the
two hots for a 220 (240) load down the line. If you don't have anything
like that, and are absolutely certain that you never will, then
independent breakers should be fine. The most the neutral will ever have
to carry will be the current to the one side; otherwise they subtract,
as Terry has pointed out.

If each circuit has its own neutral conductor, then ganging breakers
doesn't make any sense in the first place - they're completely independent
circuits.

Hope This Helps!
Rich

[Context]:

 

By this logic, the positive pole of a battery is 180 degrees out of
phase with the negative pole.

Thanks,
Rich

 

No, phase is meaningless for DC.

John

 

---
No, they're not.

If you use one end of the transformer secondary as a reference, the
center tap will be in phase with the other end of the winding, (that
is, one positive-going peak will occur at the same time as the other
positive-going peak) but if you use the center tap as the reference
the ends of the winding will be 180° out of phase with each other.
That is, the positive-going peak of one will occur when the
negative-going peak of the other occurs.

 

If two circuits are run in a single 14/3 cable, the breakers feeding
that cable _must_ be mechanically interlocked, so that they will
switch on or off together, whether or not you are using them as a 220
V supply. The reason for this is so that if you turn off one circuit
to work on it, you won't get burned by the other, which will terminate
in the same box.