Are residential electrical outlets in the U.S. wired as a
ring, or as a straight line from a supply to an end-point?

19 responses total.

There are several separate circuits, separately fused or with separate circuit
breakers.  I know that the wiring goes from the fuse box or breaker panel to
the first outlet, from there to the second, etc.  So I think 'ring' is the
answer.

There are a few different configurations possible, but one common one 
seems to be to daisy-chain from one outlet box to the next.  (Calling 
it a "ring" would be a misnomer because the last outlet doesn't need a 
wire going back to the electrical panel.)  That's within a circuit, of 
course.  Each circuit branches off from the main electrical panel, 
star-style.  There are rules about how many outlets you're allowed to 
have on one circuit, how many wires can be spliced inside a box of a 
particular size, etc.

Since an electrical cord has two or three wires in it (one is a ground) the
electricity actually runs along one of the hot wires to the last outlet or
appliance in the circuit and then back along the other hot wire, which is more
of a loop than a straight line.  

Actually (!), since its AC, the electricity just jiggles back and forth in
both wires. 

Re #1: It would only qualify as a ring if there were an
  additional pair of conductors from the power supply (fuse
  box) to the last outlet in the chain.  That is the norm in
  Britain.

Re #4: I love that description :-)

They must use lighter wire in home wiring in Britain. There is no need for
completing a circle otherwise. Or perhaps they don't install as many separate
circuits? There are something like 6 separate outlet cicuits in my house,
plus additional applience outlet circuits to the kitchen and separate circuits
for the stove, dishwasher, washer and drier, furnace, and AC. 

A ring configuration *would* reduce the problem of voltage drop at the 
later outlets in the string.  I had a big problem with that in my old 
apartment.  (In the one I'm in now, the voltage is just plain low 
everywhere.  And some of the outlets aren't grounded.  The trials of 
living in an old complex.)

The problem of voltage drop is caused by using an improper wire gauge (or
overloading the circuit). Britain uses 220 volt circuits so current demand
is halved for the same power, so they can use finer wire - but perhaps they
overdo that. 

240V AC, 50 Hz, usual ring main is 30A and (I'm told) uses
2.5mm^2 conductors.

That *is* thinner than U.S. practice.  In the U.S. we'd use 10 gauge 
for that current level, which is 5.26 mm^2.  But I suppose using a ring 
structure means you're effectively splitting the load across two 
conductors, allowing each individual conductor to be thinner.  Very 
interesting.

It's worth noting that in most U.S. homes there are two types of 
circuits.  Most general-purpose outlets are 120V, 15A.  High-power 
appliances such as electric clothes dryers, stoves, and water heaters 
are powered by 240V, 30A circuits.  Power is delivered to the house on 
three wires -- two "hot" and one neutral.  Going from either hot wire 
to neutral gives 120V, going across the two hot wires gives 240V.  This 
is called a "3-wire split-phase" system.

Sounds like a centre-tapped winding on the transformer. Some
notes that I found suggest that you can use 2.5mm^2 on a
ring main where you would have to use 4mm^2 for a point-to-
point or bus arrangement. If I bought a new house (unlikely)
should I expect NEMA 14-30 sockets for things like stove and
dryer hookups?

Re resp:11: Yes, that's exactly what it is.  The secondary on the 
transformer is center-tapped, and the center tap is the neutral.  It's 
also grounded (or "earthed," for you British types.)  This gives a 240V 
system where no conductor is more than 120V above ground potential.

If it's a very new house, you might find NEMA 14-30 sockets.  Until 
relatively recently NEMA 10-30 and 10-50 sockets were more common.  
They're a bit of a throwback as they don't have a separate safety 
ground conductor.  10-30 was usually used for dryers and 10-50 for 
stoves.

I just had a look at an electric tumble dryer and sure
enough, it has what looks like a NEMA 10-30 plug.

Let's see if anyone is still around here.....

This is only somewhat "Electrical Wiring" related, but there is no other more
relevant item, so I'll try it here.

I have a household X-10 light control system that sends activation codes
to switch and outlet units at programmmed times to turn lights on and off.
Over the past year the system became more and more erratic, until hardly
any control worked. The control xmttr has a LED pilot light that also, over
the past year, began to flicker. This is attributed on one discussion group
to line noise produced by CFLs. The situation became so bad I have replaced
several X-10 switch units with plug-in timers. However these can't be used
for outside lights, which stopped being controller.

Two days ago we arrived home after dark and discovered *half* of our 
house power had failed. We have a three wire service, with two 120v hot 
wire branches 180 degrees out of phase. This produces 240 volts for the 
electric stove. All the 120 volt outlets and switches are distributed 
about equally between the two branches. The power had only gone out on 
one of the 120 volt branches. We've had many power failures in the past, 
but they had always been total, not just on one branch.

The full power came back on after about 30 minutes, so no harm done. But 
we subsequently noticed that the X-10 controlled lights were working 
again: in particular, the outside lights went on in the evening and out 
at morning. I finally noticed also that the X-10 Xmttr LED pilot light 
no longer flickered!

Apparently power line noise much have been developing somwewhere outside 
the house and got fixed during the 30 minute single-branch blackout. It 
also means that CFL noise was not the culprit for the problem with X-10 
control.

Has anyone else ever observed single-branch power noise and eventual 
failure on a three-wire service, and know what in the power distrikbtion 
system might cause this?

I've also submitted an inquiry to DTE.

Jim suspects that DTE was working somewhere down the line and they replaced
something substandard and now it works.

I suspect that also - but WHAT "something"? 

While the problem persisted, there was no dimming or fluctuations 
observable from the lights on the separate branches. Also, I have 
couplers (bridges) between the two phases as the X-10 control signal is 
injected into only one of the branches. So "noise" at the X-10 carrier 
frequency (120kH) would be found on both branches. (Of course I COULD 
have isolated the two branches and put a scope on each to find out which 
was the sources, but I mostly felt because of the online conference 
comments that the cause was CFLs. It turns out, they were wrong.).

I had electical problems at one home that left something over 140 volts
on one side of the three line.  The other side was fairly normal.

In talking informally to a local electrician from a good contracting
house I was given two possibilities for my problem.  First a bad
connection somewhere on my neutral leg might allow one side to pull the
neutral away from center leaving too much voltage on the other leg.  The
other, a bad transformer nearby.

Turned out to be the latter although I moved out before it was
corrected.

I suspect the same for you.

When the power failed it may well have been the transformer for your
block that died, or the outage took it past its last straw and either
way DTE put in a new one.

Quite likely your neighbors wouldn't have noticed problems unless they
also have some X10.

You're probably right. I could not get anyone at DTE to tell me what had 
happened. I dind't ask the neighbors if they had a power outage as I 
just assumed they did. Now you mention it, I wonder if any noticed the 
one-sidedness of the outage.

I'm in the area of Prescott, AZ.  We have a lot of really crazy thunder
storms this time of year.  I just remembered that friends from work had
one of these one sided outages recently.

The power company where ever you are is in the business of telling
customers nothing no matter what.  At least in the US.  perhaps
elsewhere?...

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