[time-nuts] Line Voltage - USA
cfharris at erols.com
Mon Jan 2 12:17:52 EST 2017
It's not a split phase system in US residential power, it is a
center tapped 240V single phase system. Split phase systems have
historically had a 45-90 degree phase difference between the split
phases. The US system, depending on which wire lead you take as
your reference, has a 0, or a 180 degree, phase difference.
The reason it is done this way, is for safety. The center-tap
of the mains transformer is grounded to earth, as is the neutral,
and service entrance panel grounds. This way, if the power company
installed grounding system is working properly, the highest voltage
that any residential customer could accidentally encounter would
be 120V to ground. (It is not an accident to go mucking around
inside of a 240V range/dryer socket, or the service panel!)
As it has been noted, if a US based system is defective, people can
get hurt. The same is true for the European system.
Back in the dark ages of ~220V electrical distribution systems in
Europe, the reaping due to unintentional grounding of a ~220V wire
was so common and extreme, whole house ground fault interrupters
were mandated for all residential/small business power systems
And, in so far as properly functioning GFI protectors are in use,
and can be maintained, they have been wildly successful!
I will have to leave discussions of which system is better/safer/
cheaper/more reliable, for another time and forum...preferably one
where there is beer and loud music.
Gregory Maxwell wrote:
> On Mon, Jan 2, 2017 at 4:49 AM, Bill Byrom <time at radio.sent.com> wrote:
>> Most US homes and small businesses are powered by what is commonly called a
>> "split-phase" 240 V feed. The final distribution system transformer has a 240 V
>> center-tapped secondary. The center tap is grounded, and three wires are fed to
>> the building (actually it might be up to around 6 houses): (1) Leg L1 or phase A
>> (red wire) -- This wire will measure 120 V to the neutral or 240 V to Leg L2.
>> (2) Neutral (white wire) -- This wire is grounded at the distribution system and
>> at the service entrance to the building. (3) Leg L2 phase B (black wire) -- This
>> wire will measure 120 V to the neutral or 240 V to Leg L1.
> When someone here previously mentioned observing high voltage, one possible cause
> for this in this common "split-phase" configuration is that if the neutral wire
> is overloaded, damaged, poorly connected, or otherwise has high resistance, the
> voltage on the two legs will swing wildly and in opposite directions depending on
> So, e.g. if you put a 1kw load on L1 while L2 is nearly unloaded then perhaps L1s
> voltage drops to 108v while L2 rises to 132v.
> The reason for this is that, e.g. imagine that the neutral were removed completely
> you would effectively be connecting your appliances in a parallel-series circuit
> (all on L1 in parallel, all on L2 in parallel, the both in series) across the 240v
> I've had issues with neutrals several times in the past, and in one instance,
> temporarily dealt with it by moving as much of the load to 240v as I could,
> manually balancing the remaining loads, and then using a digital multi-meter to
> dynamically control some additional load to keep the voltage sane on each side.
> I think the fact that you can end up with a much higher voltages at the outlet if
> the neutral has problems is one of the more unfortunate properties of the
> split-phase approach. _______________________________________________ time-nuts
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