[time-nuts] Line Voltage - USA

Bill Byrom time at radio.sent.com
Sun Jan 1 23:49:03 EST 2017

There are a couple of recent threads concerning the power line mains
voltage standards. After a bit of research and thinking, I have found
that this is a complex topic. The simple answer is:

* The standard in the US for the past 50 years has been 120/240 V +/- 5%
  RMS at the service entrance to the building. This is a range of
  114/228 V to 126/252 V.
* The load voltage could be as low as 110/220 V and as high as 125/250 V
  and be within specifications.

There are two voltage measurement points to consider:

(1) Service voltage: This is the RMS voltage measured at the service
    entrance to the building (at the metering point).
(2) Utilization voltage: This is the RMS voltage measured at the load.
    It might be measured at an unused socket in a power strip feeding
    several pieces of electronic equipment, for example. There are of
    course many different utilization voltages present in a home or
    business, depending on where you make the measurement.

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.

Large appliances and HVAC systems are usually connected across L1-L2
(240 V), while most sockets are on circuits either connected across L1-
neutral (120 V) or L2-neutral (120 V).

The voltages I have described are the current standardized values for
the service voltage which have been in general use for about 50 years
(120/240 V +/- 5%). I believe that the original systems installed before
1940 were designed for a 110/220 V nominal service voltage, but after a
report in 1949 the nominal service voltage was increased to 117/234 V,
as specified in ANSI C84.1-1954. After research in actual buildings, in
the 1960's the nominal service voltage was increased again, to 120/240 V
in the ANSI C84.1-1970 standard. The purpose is to keep the utilization
voltage at the load above 110/220 V.

The voltage at the service entrance should in most cases be in Range A
(120/240V +/-5%). On each 120V leg the service voltage should therefore
be between 114 and 126 V. The utilization voltage at the load should be
between 110 and 125 V due to losses in building wiring.

See details of the current specifications at:


These voltage specifications were designed for resistive loads and
measurement of the true RMS voltage. In most electronic equipment built
over the past 50 years, the power supply input circuitry is basically a
rectifier connected to a smoothing capacitor. This leads to high input
current surges during the peaks of the waveform, so that the peak
voltage is reduced much more by the building wiring resistance than if
the load was resistive for the same power consumption.

So the waveform shape at different utilization locations in a building
(with active equipment loads) may be different, so the voltage measured
by different AC measuring instruments can differ. Many meters are full
wave average measuring but calibrated so they only read RMS voltage
correctly on pure sinewaves. Other meters are true RMS measuring and
will read very close the correct RMS voltage even if the waveform is

Bill Byrom N5BB

On Sun, Jan 1, 2017, at 12:16 PM, CIW308 VE6OH wrote:

> Mark,


> CSA have standards for over and under voltage, Typical no more that 3%
> over and 5% under if memory serves me.


> This might help (

> http://www.safetyauthority.ca/sites/default/files/csa-fia3660-voltagedropcalc.pdf
> )

> The power companies here in Alberta are generally good about fixing

> problems with line regulation.

> There can be problems with industrial areas and big welders or motors
> staring as I am sure you know.

> I am sure they do not want the bill for replacing equipment that was

> subjected to over voltage.


> On UPSs: I am sure you are aware that may of them are not TRUE
> sine wave
> so the DMM may not read correctly.


> Mitch

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