| In our office - we have an electrical outlet (large 3 prong
| electricians just installed). It is supposed to be 208 Volts...
| we were having some problems with our equipment, so I ran a multimeter
| on it - and when I hook up the hot and neutral to the meter (analog
| type), I get the correct reading of 210volts +/-.
| when I connect to the hot and the GROUND I get 120 volts... is that
| correct, or does the electrician need to come back out? I thought hot
| and ground should also produce the 210 volt reading...
In the USA, the standard voltage for common appliances is 120 volts. That
voltage is between the hot line wire and the grounded neutral wire. Most
other places in the world have a higher voltage in the 220 to 240 volt
range, also wired the same way.
Heavy duty appliances would draw quite a lot of current if wired to 120
volts. Even the sum of all the small usages on 120 volts would add up to
a large current. The system in North America and a few other places in
the world uses TWO hot line wires to achieve a higher voltage. Voltage
is figured _between_
two wires. Normally in a single phase system there
are two hot line wires with the AC sine wave phased exactly opposite at
180 degree phase difference. That results in the voltage BETWEEN THE TWO
HOT WIRES to be 240 volts.
This method of wiring originated with Thomas Edison's DC power system.
His system had a positive wire at 110 volts and a negative wire at 110
volts. Even though the light bulbs operated only at 110 volts (not 220)
this scheme reduced the costs of wiring and allowed electric usage at a
greater distance from the power plant. The DC system generated power at
the same voltage it was used, so he got none of the advantages of high
voltage distribution we have today. But his "split" system lives on in
the AC power system as "split phase".
In many places, usually commercial buildings, three phase power is provided
instead of single phase power. Three phase power comes with three hot line
wires instead of just two. In the majority of three phase systems, all 3
of these wires are 120 volts relative to the 4th grounded neutral wire, but
each with a sine wave phase shifted at 120 degree intervals. The voltage
BETWEEN ANY TWO of these wires is 208 volts, not 240 volts. This is because
the sine wave phase angles are somewhat closer at 120 degrees instead of 180
degrees. Labeling these wires A, B, and C for the phase hot wires, and N
for the neutral, these connection pairings have these nominal voltages:
A-N, or B-N, or C-N : 120 volts
A-B, or B-C, or C-A : 208 volts
If the appliance to be served only needs single phase power at 208 volts,
it can be wired with only 2 of the 3 phase wires. Just omit C above and
label the two hot wires with A and B.
This method of three phase power is used where most of the electric usage
is single phase 120 volts, and there is a lot of usage. This allows the
power loading to be balanced over all three phases.
An older three phase power system was wired somewhat differently, where
the voltages would be:
A-N, or B-N : 120 volts
C-N : 208 volts (referred to as "high leg" or "stinger")
A-B, or B-C, or C-A : 240 volts
This older system has several disadvantages. Where there is a lot of 120
volt usage, this can cause the power loading coming in to be out of balance.
It also can result in blown transformer fuses more often due to its "delta"
style wiring (of the "closed delta" type).
The nominal voltage standard is 120, 208, or 240 volts, depending on what
the wiring connection is. Your actual voltage could vary above and below
that amount a few percent. In some countries the standard is different.
A less common industrial three phase system involves 277 volts from any
hot wire and the neutral, and 480 volts between two hot wires. If 120
volts is also needed, it requires an additional transformer to get it.
In Canada, these voltages may be 346 and 600 volts.
Fluorescent and high intensity discharge lighting can be run directly on
voltages above 120 volts with a special ballast, and this is often done
to reduce costs.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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