Using Wire Tables / Determining Conductor Sizes--part 3: Voltage Drop, Parallel Conductors, Testing Installations

Calculating Voltage Drop

Sometimes it’s necessary to calculate the voltage drop of an installation when the length, size of wire, and current are known. The following formula can be used to find the voltage drop of conductors used on a single-phase system:

E_D = 2 KIL / CM

where:

E_D = voltage drop

K = ohms per mil ft

I = current

L = length of conductor in ft

CM = circular mil area of the conductor

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EXAMPLE:

Determine the ohms-per-mil foot at 75°C for a copper conductor? Solution Use the formula R = R_ref

[1 + a(T x Tref )] where

R = Conductor resistance at temperature "T"

R_ref= Conductor resistance at reference temperature (20°C in this example)

‘a’ = Coefficient of resistance for the conductor material

T = Conductor temperature in °C

T_ref = Reference temperature that ‘a’ is specified at for the conductor material.

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Coefficient of Temperature:

The temperature of a conductor can greatly affect its resistance. === lists the ohms-per-mil foot (K) at 20°C for various materials. The resistance of a material is generally given at 20°C because it’s the standard used in the American Engineers Hand book and is considered a standard throughout the United States. The temperature coefficient can be used to determine the resistance of a material at different temperatures. Most conductors will increase their resistance with an increase of temperature. Semiconductor materials such as silicon, germanium, and carbon will exhibit a decrease of resistance with an increase of temperature. These materials have a negative coefficient of temperature.

+++++ A single-phase conductor causes heat to be produced in the conduit. Conduit Conductor of a single phase.

+++++ The magnetic field expands and contracts. Conduit Conductor Magnetic field.

+++++ Eddy currents are currents induced in metals.

Parallel Conductors

Under certain conditions, it may become necessary or advantageous to connect conductors in parallel. One such condition for parallel conductors is when the conductor is very large as in the earlier example, where it was calculated that the conductors supplying a motor 2500 feet from its source would have to be 500 kcmil. A 500-kcmil conductor is very large and difficult to handle. Therefore, it may be preferable to use parallel conductors for this installation. The NEC lists five conditions that must be met when conductors are connected in parallel (310.10 (H)). These conditions are listed here:

1. The conductors must be the same length.

2. The conductors must be made of the same material. So, all parallel conductors must be either copper or aluminum. It’s not permissible to use copper for one of the conductors and aluminum for the other.

3. The conductors must have the same circular mil area.

4. The conductors must use the same type of insulation.

5. The conductors must be terminated or connected in the same manner.

In the example, the actual conductor size needed was calculated to be 440,778.443 CM. This circular mil area could be obtained by connecting two 250-kcmil conductors in parallel for each phase, or three 000 (3/0) conductors in parallel for each phase. [Note: Each 000 (3/0) conductor has an area of 167,800 CM. This is a total of 503,400 CM.] Another example of when it may be necessary to connect wires in parallel is when conductors of a large size must be run in a conduit. Conductors of a single phase are not permitted to be run in metallic conduits [NEC 300.5(I), NEC 300.20(A), and NEC 300.20(B)], because when current flows through a conductor, a magnetic field is produced around the conductor. In an AC circuit, the current continuously changes direction and magnitude, which causes the magnetic field to cut through the wall of the metal conduit. This cutting action of the magnetic field induces a current, called an eddy current, into the metal of the conduit. Eddy currents are currents that are induced into metals. They tend to move in a circular fashion similar to the eddies of a river, hence the name eddy currents (). Eddy currents can produce enough heat in high-current circuits to melt the insulation surrounding the conductors. All metal conduits can have eddy current induction, but conduits made of magnetic materials such as steel have an added problem with hysteresis loss. Hysteresis loss is caused by molecular friction. As the direction of the magnetic field reverses, the molecules of the metal are magnetized with the opposite polarity and swing to realign themselves. This continuous aligning and realigning of the molecules produces heat caused by friction. Hysteresis losses become greater with an increase in frequency.

+++++ The molecules reverse direction each time the magnetic field changes direction.

+++++ Each conduit contains a conductor from each phase. This permits the magnetic fields to cancel each other.

To correct this problem, a conductor of each phase must be run in each conduit. When all three phases are contained in a single conduit, the magnetic fields of the separate conductors cancel each other resulting in no current being induced in the walls of the conduit.

+++++ Battery-operated MEGGER.

+++++ Testing for shorts with a MEGGER.

+++++ Testing for grounds with a MEGGER

Testing Wire Installations

After the conductors have been installed in conduits or raceways, it’s accepted practice to test the installation for grounds and shorts. This test requires an ohmmeter, which not only measures resistance in millions of ohms but also provides a high enough voltage to ensure that the insulation won’t break down when rated line voltage is applied to the conductors. Most ohmmeters operate with a maximum voltage that ranges from 1.5 volts to about 9 volts depending on the type of ohmmeter and the setting of the range scale. To test wire insulation, a special type of ohmmeter, called a MEGGER, is used. The MEGGER is a megohmmeter that can produce voltages that range from about 250 to 5000 volts depending on the model of the meter and the range setting.

One model of a MEGGER is shown. This instrument contains a hand crank that is connected to the rotor of a brushless AC generator. The advantage of this particular instrument is that it does not require the use of batteries. A range-selector switch permits the meter to be used as a standard ohmmeter or as a megohmmeter. When it’s used as a megohmmeter, the se lector switch permits the test voltage to be selected. Test voltages of 100 volts, 250 volts, 500 volts, and 1000 volts can be obtained.

MEGGER can also be obtained in battery-operated models. These models are small, lightweight, and particularly useful when it becomes necessary to test the dielectric of a capacitor.

Wire installations are generally tested for two conditions, shorts and grounds. Shorts are current paths that exist between conductors. To test an installation for shorts, the MEGGER is connected across two conductors at a time. The circuit is tested at rated voltage or slightly higher. The MEGGER indicates the resistance between the two conductors.

Because both conductors are insulated, the resistance between them should be extremely high. Each conductor should be tested against every other conductor in the installation.

To test the installation for grounds, one lead of the MEGGER is connected to the conduit or raceway. The other meter lead is connected to one of the conductors. The conductor should be tested at rated voltage or slightly higher. Each conductor should be tested.