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ELECTRIC POWER SUPPLY
All wiring should be in accordance with National Electric Code and local standards.
Before making an installation, check the electric power supply. The power supply must be of the same characteristics as
that on the nameplate.
Normally the wire size used establishes the circuits maximum permissible load. For example a number 12 wire circuit is
considered to be a 20 ampere circuit and is protected by a 20 ampere fuse. The unit amperage nameplate rating should
not exceed 80% of the circuit rating, 16 amperes.
When identifying electrical trouble in any unit, check the power supply at the main disconnect switch and the unit. A high
or low voltage condition may prevent the unit from starting, or if the unit is running, compressor can cut-off automatically
on the motor overload. On some installations, the voltage may be entirely adequate when tested, but may drop or surge
some time later due to increase or decrease of load on the power line. This condition may be repeated daily at the same
hour. When this condition is suspected, make arrangements with the local power supplier to place a recording voltmeter
on the line to check and correct the erratic voltage conditions.
The voltage drop should not be more or less than 10% of the units rated voltage. If the voltage at the meter is correct, but
the voltage at the unit fluctuates 10% or more, the branch circuit is not adequate.
UNBALANCED VOLTAGE
Unbalanced voltage on a three phase circuit may be the source of electrical trouble. If balanced supply voltages are not
always maintained, so with the three line currents will not be equal. Unbalanced voltage on a three phase compressor
motor may overheat and trip the motor on external or internal current sensing motor protector, whichever one is provided.
To determine that an unbalanced voltage condition exists, the supply voltage at the main disconnect switch and at the
unit disconnect switch should be measured. Voltage unbalance is calculated as follows.
The percentage of voltage unbalance equals one hundred times the ratio of the Maximum Voltage Deviation from the
Average Voltage, and the Average Voltage.
The percentage increase in temperature rise in a phase winding resulting from voltage unbalance will be approximately
two times the square of the voltage unbalance.
Because of the unbalanced voltage condition in a three phase circuit, it is possible that one phase winding in the com-
pressor motor may be over-heated while the other have a temperature within normal limits. Where unbalanced voltage on
a three phase circuit exists, the power company should be notified of such unbalance to determine, if the situation can be
corrected.
A common source of unbalanced voltage on a three phase circuit is a single phase load between two of the three phases.
A large lighting or heating circuit connection to one phase of a three phase circuit can easily cause sufficient variations in
motor currents to endanger the motor. Interchange the single phase loads to equally divide the entire loads among the
three phases. Balance the supply voltage as evenly as possible.
Unless the unbalanced voltage condition can be corrected, the only way to insure motor safety is to be sure that the
protectors are in the high current phase when using two leg protection, or to use protectors in all three legs.
ELECTRICAL
Voltage unbalance, % = 100 x
Average voltage
Maximum voltage deviation from average voltage
