Power line voltage

Power line voltage

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“How large should my power line wiring be?”, “What outlet size do I need?”,
and “Do I need 240 volts?” are commonly asked questions. There seem like simple
questions, but the simple answers are usually wrong. Even technical editors at
the ARRL have had problems understanding power line systems and how to evaluate
them! The early AL1200 review was flawed because the ARRL Lab used a defective
or inadequate power line source in the review. They were tripped-up by the very
same thing that commonly fools other people.

Single family residential power lines in USA homes and most apartments are
120/240 volt single-phase 60 Hz systems. These lines have a center-tapped 240
volt winding. They use a common neutral and ground at the distribution
transformer and at the dwelling entrance, with two 120-volt lines of opposite
polarity. One could also say the “hot” conductors of 240 volt systems are 180
degrees out-of-phase with reference to ground, or to neutral, or to the supply
center tap. Note that this does NOT make it a two-phase system! It is a
single-phase 240 volt system with a center tap. The “hot” lines are opposite
polarity and (essentially) equal voltages, but they are not a different “phase”.
It is a simple grounded center tap system. 

The neutral is bonded or connected at the breaker panel to a safety ground. The safety
ground is the round “third pin” of the outlet. The breaker or distribution panel
is the only point where the safety ground should be grounded in house wiring,
although a second ground point to the safety ground is generally unavoidable in
amateur radio installations. The second ground point in amateur radio installations will render any GFI
outlets or breakers for that outlet string unusable or unreliable. This ground loop is caused
by the third “safety ground” wire that connects to equipment cabinets or chassis. 


Safety First! USA

Code requirements mandate the radio room safety and cable
entrance ground rod and the power mains ground be electrically bonded.
The NFPA Code is a minimal requirement, we really should do better if we
want to minimize danger of lighting damage.

All coaxial cables should pass through a plate that is grounded and
bonded  to the power mains ground system.


Most modern wiring provides 15 amperes at any normal 120 volt outlet. If the
wire to the breaker or distribution panel is not too long, a normal 120 Vac
outlet will often be adequate for amplifiers up to 1200 watts PEP voice SSB output, or
600 watts CW output. The key here is that the power mains wiring to the breaker
or distribution panel must be reasonably short, and must not be a shared outlet
with critical loads. Typically for standard 14 AWG copper wiring:

CW  Carrier/SSB PEP Output max avg amperes 120v peak current


distance 120v

2% reg

max avg amperes 240v peak current


distance 240v

2% reg

600/1200 capacitor input filter 12 25 27 ft 6 12.5 108
1500/3000 capacitor input filter 26 54 12.5 ft 13 27 50
600/1200 choke input filter 12 12 56.25 ft 6 6 225
1500/3000 choke input filter 26 26 26 ft 13 13 104


History of USA Power Mains Voltages


Power line voltage is always specified in RMS (root mean
square) voltage.  RMS is the quadratic mean, or square root of the mean of the
squares of waveform values.  RMS relates to perfectly shaped sine wave voltage
or current waveforms by the special case of 2, where it is the square root of 2
(1.414) or inverse of 1.414 (.707).  With a square wave, RMS voltage is equal to
peak voltage, and RMS current is equal to peak current.

RMS quantifies voltage or current in a way useful for
determining the work that can be done, such as when heating something. At the
same time, there is no such thing as RMS power, although audio people often use
the nonsensical term “RMS power” to describe sine-wave power.  The consumer
audio confusion probably comes from use of a sine wave’s RMS current and voltage
to calculate power.

With a perfect sine wave, peak voltage is 1.414 times RMS
voltage. In other words, sine wave RMS voltage is 1/1.414, or .707, times peak
voltage under perfect undistorted sine wave conditions.


USA Standard Residential Voltage

Although voltage has changed over the years, residential USA power mains have
maintained a single phase 60 Hz frequency.  Standardized power mains voltages
started as 110 and 220 VAC systems. Around the end of WWII, standardized mains
voltage increased from 110/220 to 117/234 VAC.

117/234 VAC remained the residential voltage standard for a few decades,
changing to 120/240 VAC in the 1960’s.

In the 1970’s, the American National Standards Institute (ANSI) set current
120/240 specifications through ANSI Standard C84.1-1970. This standard specifies
two voltage ranges, Range A and Range B, which included both service voltage and
utilization voltage. Service voltage for this situation was usually interpreted
to be at the meter, and utilization voltage was at the terminals of the
utilization equipment. Electric supply systems were to be so designed and
operated that most service voltages would be within the limits specified in
Range A. The occurrence of service voltages outside Range A was to be
infrequent. Range A service voltage was to be 120 V (+/-5 percent), which would
be 114 to 126 V. Range A utilization voltage was specified to be between 110 and
126 V.

The current line voltage, since the late 1960’s in most locations and
since the 1970’s by written uniform standard ANSI Standard C84.1, is 120/240 + –
5%. After 40 years or more of being 120/240, it is probably time we stopped
calling it “220”, “117”, “115”, or “110”. In the USA, it is 240 volts or 120


Entrance Wiring

USA residential lines employ a common-ground center tap at
the transformer, where primary feeder neutral connects to the residential
secondary center tap. A small generally-poor earth ground connects to each
supply pole, and occasionally along long primary runs without transformers.

At the dwelling entrance, as required by national safety
codes, all cables entering the building must share a common ground point. This
is also the common bonding point for the safety ground wire to the neutral. This
common ground point prevents significant voltage differences between grounds on
cables or wiring inside the dwelling. There is a small earth ground rod or
earthing system required. Generally this system has an earthing resistance well
over 30 ohms, so it isn’t really much of a ground. It is, however, better than
no ground at all.

By legal requirement, all cables entering the Hamshack,
including the Hamshack ground rod or ground system, must be bonded to the power
mains entrance ground. Again, as with the power, CATV, and Telco grounds, this
is to prevent ground potential differences inside the dwelling.

The pole ground and the house ground help protect against
voltage rise in the event of lightning strikes, power line ground faults, or
open neutrals. While there should not be much potential difference, there is
always some current flowing into these grounds to earth. This current flows
because there is always some voltage drop along neutrals. That voltage drop
excites the grounds rods with respect to earth and other grounds distributed
along the power grid. As a matter of fact, if we drive two ground rods into the
earth some distance apart, even somewhat far from power mains, 60 Hz voltage can
detected! This voltage is excited by the earthing currents in our mains system.

Amplifier Circuitry

As described above, power line voltage is specified in
voltage based on a pure, ideal, sine waveform.

Most amplifiers and power supplies, including switching
supplies, use capacitor input filters. While most meters respond something
around average or RMS voltage, capacitor input supplies operate from peak
voltage. Peak voltage of a perfect sine wave is 1.414 times RMS, so our 120 VAC
mains (without harmonics or clipping) crest at 169.68 volts peak.  If we
rectified the power line and filtered the DC with a perfect capacitor input
supply, like most conventional and switch mode supplies do, we would have about
170 volts DC. This happens because the capacitor charges to the power line’s
peak voltage at the crest of the sine wave.

When near full voltage, the power supply draws current only
at peaks. If the supply is delivering 1 ampere DC near 170 volts, all of the
energy would be supplied during a very short period at sine wave crest.  Current
from the power line would be many amperes, but for extremely brief periods of

Since load power is drawn only at sine wave voltage peaks,
there is a high ratio of peak current to average or “heating” current. In the
example above, while average current might be near 1.4 amperes, peak current
would be several amperes. This ratio of peak to average current gives rise to
something called apparent power factor (APF). APF is based on peak to average
current, and is not same as standard power line phase-shift power factor caused
by inductive loads, such as motors.

Nearly all radio and amplifier power supplies, since they
almost always employ capacitor input filters, have a very high apparent power
. The more robust we make power supply components, and the stiffer we
design a supply in an attempt to stay near 1.414 times AC RMS voltage, the
greater APF becomes.  The stiffest, largest, most oversized supplies have the
highest APF’s, demanding greatest attention to power mains’ equivalent series
resistance (ESR) if we want to maintain that regulation.

Because the typical supply mainly works from peaks, average
or RMS voltage has little practical use (aside from calculating heat). While
this may seem complex, power supply regulation has to be calculated using peak
current and/or peak voltage. It is common to see 5% voltage drop on an average
or RMS meter, while power line peak voltages are dropping 15% or more. This can
mislead us into thinking the power mains regulation is good and a power supply
bad, even when the bulk of the problem is actually in the mains.

Amplifier Example

In an AL1200 amplifier operating from very stiff power
lines, peak-to-average current ratio is about 4:1.

With 12 amperes RMS (heating) current, peak current will be
about 48 amperes. While power line heating is calculated at 12 amperes, voltage
drop is calculated at 48 amperes.  A power line resistance of one ohm, for
example, would produce only 12 watts of heat, while the same one-ohm resistance
would reduce peak line voltage by 48 volts if the power supply system maintained
the same APF! With a nominal 240 V RMS line, peak line voltage would drop from
339 volts down to 291 volts.

This is a 14% reduction in high voltage, while meter
measured RMS or average line voltage would typically only change about 5%. (The
exact amount would depend on waveform distortion and the meter.)   

Example of a Mains Problem

Power mains problems can be tricky, escaping even the most
experienced amateurs. Even someone as experienced as the ARRL Lab can miss
problems like this. The ARRL lab, reviewing an AL1200 amplifier, measured
operating DC plate voltage of an AL1200 amplifier as 2900 volts under load,
while the average-based RMS line voltage was fairly steady near 240 VAC.

Despite emphatic warnings the ARRL Lab had something wrong
in the lab power mains system, the ARRL failed to investigate properly before
releasing the review.  The ARRL finally realized the error when, after
installing the amplifier at W1AW, loaded voltage was suddenly a normal 3300-3400

The ARRL Lab’s problem was an expensive voltage regulator
that held power mains’ average or RMS voltage steady, while allowing line peaks
to sag over 15%. This caused the normal 3400 volts full load voltage of an
AL1200 to drop to 2900 volts, while the metered voltage at the socket barely
changed. The Lab measured good, stable, line voltage on a typical meter, but
peak regulation was terrible because their expensive voltage regulator could not
handle the amplifier’s APF.

While the realization they had a problem came too late to
prevent false review data, at least it is a good learning tool for others. Good
stable voltage on a traditional meter does not mean the power line system is
problem free. The Lab missed a simple, easy, observation. It is electrically
impossible to significantly decreased dynamic regulation inside a supply with
accompanying heating or ripple. 

Determining Power Line Sag

Apparent power factor (APF) is high in capacitor input
supplies, with peak currents 2-5 times average currents. The better the power
transformer, the more disproportionate peak current becomes when referenced to
average current. Because of high APF, voltage regulation in capacitor input
supplies is largely a function of series impedances from the filter capacitors
back to the power source. This undesired series impedance is normally dominated
by resistance in the wiring back to the pole transformer, and by the amplifier’s
power transformer.

Evaluating regulation with voltage measurements requires
thought and care. A capacitor input supply works from the peak line voltage.
Peak voltage does not change in proportion to average or RMS voltage. As a
matter of fact, average voltage often barely changes when peak voltages drop a
very noticeable amount.

Virtually all multimeters do not detect true peak voltage,
and they also do not read RMS or average voltage. Most multimeters detect
something around average AC voltage, ranging up toward peak voltage. Whatever
they happen to read is corrected or adjusted to provide a pseudo-RMS voltage on
the display.  Unfortunately, this only works well with a sine wave. Since the
supply only loads the peaks, the waveform squares. The average voltage hardly
changes even with significant fractional-cycle peak clipping, which means
significant DC voltage loss without a similar change on the power line meter.

To actually determine power line regulation when feeding a
capacitor input supply, the multimeter should be a true peak reading meter. 

In almost every tube- type amplifier, the high voltage
meter provides a good way to determine power line quality. If plate voltage runs
normal at idle, but falls well below manufacturer’s rated specifications under
full load without undue carrier hum or power supply components heating, chances
are good power line equivalent series resistance (ESR) is too high.  It is
electrically impossible to significantly decreased dynamic regulation inside a
supply with accompanying heating or ripple.

or 240 Volt Operation

Normally, losses inside an amplifier do not change much
with power line voltage changes. Changing from 120 volts to 240 volts might
increase or decrease life of some components, such as switches and relays, but
overall dynamic regulation is generally not changed much. Operating voltage is
not changed at all, provided the primary system is wired to exactly double
voltage. This happens because most systems employ identical dual primaries,
which paralleled for 120 volts and in series on 240 volts. With dual primaries,
current in each primary and voltage across each primary remains the same
regardless of 120 or 240 volt wiring, causing transformer losses and ESR to
remain exactly the same. 

Because of high APF, ESR which causes noticeable regulation
issues can be surprisingly low. Wiring that normally handles a 1500-watt
resistive load with minimal drop can have much worse regulation with a 1500-watt
power supply load. Worse, a conventional multimeter might not show the line
voltage loss.

This is because APF causes peak current demand to be high,
which in turn clips the sine wave into a flat-topped waveform.

Normally, changes in performance come from changes in power
line loading outside the amplifier. Performance changes do not come from
efficiency changes inside the amplifier.  By doubling voltage from 120V to 240V,
we halve current. All things equal, the system has half the voltage drop at
twice the line voltage. This results in four times better regulation, when
expressed as a percentage, with no change in wire size.

Keep in mind this is four times improvement.  A 0.1 ohm ESR
line with 40 amps peak current would drop 4 volts out of 170 volts peak. This
translates to a 2.4% loss in voltage. A system change to 240-volts results in
2-volts drop out of 340 peak volts. This is about 0.6% regulation loss.  With a
3000 volt supply, we can expect about 50 volts more high voltage under load from
power line changes.

This is, of course, a fictional case with 0.1 ohms loop
resistance. This would be typical for a 25-foot run of #12 AWG (.05 ohms) to a
good 200-ampere breaker box system with nearby pole transformer (typically
around .05 ohms). My shop workbench feed measures ~0.1 ohms ESR, including the
line transformer.  If the power line has significant ESR, a change from 120 to
240 volts can greatly improve dynamic regulation.  Whatever portion of
total sag
is caused by a 120V power line, that sag will be ¼ the
amount.  Voltage sag inside the amplifier will not change very

vs. CW and Carrier Modes

APF effect on dynamic regulation is less problematic on
voice SSB. The power supply filter capacitors supply energy for voice peaks; the
power line never seeing the full peak power input demand used on SSB voice.


Meters on Power Lines for Amplifiers

If your amplifier uses a capacitor input power supply (including most
domestic high power switching supplies), do not rely on normal ac voltmeters for
power line stability measurements. Normal ac meters are usually fine for choke
input supplies, or waveform distortion corrected power supplies.

Most amateur power supplies are capacitor input filter systems. When measuring power line voltage or current for high power amplifiers,
almost every meter measures the wrong thing! The power supply runs of a very
small portion of the sine wave near the top of each half, especially on the
rising edge of the waveform. Virtually all meters measure average or
pseudo-average voltage and current, so they do not measure the voltage the
amplifier power supply requires. Meters are often calibrated in peak or RMS,
but they often just apply a correction factor to the average voltage that is
actually measured.

We might do the same thing manually, assuming peak voltage is 1.414 times
indicated RMS
voltage. This method is correct, and meters are often very close, when the
measured waveform is a perfect
sine wave. This is not the case with capacitor input power
supplies. Conventional meters cannot be trusted to reliably evaluate power line
health when the powerline is loaded by a capacitor input power supply. With a
non-peak meter, a
power line voltage reading can show no significant measured voltage drop, yet
the powerline can be causing terrible
voltage regulation and performance in a capacitor input supply. The ARRL case
was an almost perfect example of a meter indicating a stable mains source, while
the mains source was almost useless. 

Harmonic Distortion and Apparent Power Factor

A capacitor input supply amplifier develops high voltage on sine wave crests
or peaks. Because of that, the power line current to operate capacitor input
supplies comes on peaks.

Here is an example of 811H secondary current and
voltage for a plate current of 750 mA:

Secondary current is just over 4 amperes, while secondary voltage is just
over 1500 volts peak. Transformer and power line loading is on the rising
waveform edge, with a load duration of about 2.5 mS during each 8.3 mS power
line half-cycle.  

Measured transformer current roughly agrees with secondary current. At
120 volts, measured primary peak current is 32-amperes. One-cycle average current
with a steady 750 watt carrier is 11-amperes.

This is why voltage drop needs to be measured with a true peak reading AC
power line meter, or roughly estimated by watching the HV meter inside the
amplifier. Abnormal voltage sag under load is almost always caused by
inadequate power line regulation.

Power lines and circuit breakers should
be minimally sized for heating currents, which are average
currents. When using capacitor input supplies, voltage supply
calculations should use about three times average current
for maximum power amplifier dc input power.