November / December, 2003

By: Truman Surbrook

Electrical power is frequently supplied to farm buildings with either overhead aluminum triplex cable consisting of a bare messenger wire surrounded with two insulated conductors as shown in the figure. For 3-phase there will be three insulated conductors surrounding the messenger conductor. Another popular means of supplying power to a farm building is underground either with direct burial cables or individual conductors in a conduit such as nonmetallic conduit. Underground aluminum direct burial cables are often not recommended because they tend to fail frequently. Minimum depth of burial, whether run in conduit or as direct burial cable, is 2 feet. Overhead conductors are to be installed so that the lowest point of the cable is at least 18 feet above the ground in all areas where farm machinery may be operated. Higher mounting heights are recommended in areas around farm buildings where high equipment such as elevators and grain augers may be used.

The electrical code specifies the minimum size of wire that is permitted to be installed for building services, but for long runs to buildings, these minimum sizes are often too small. The minimum wire size permitted depends upon the rating of the service panel main circuit breaker. But the electrical code minimums do not take voltage drop into consideration. The problem is the resistance of the wire and the load current. The basic electrical rule called Ohm’s law states that current times resistance is voltage drop. The longer the wire the greater the voltage drop. One reason motors are operated at 240 volts rather than 120 volts is to get the same power at less current. If everything possible is done to minimize the load, and the distance to a building cannot be changed, the only thing that can be done to reduce wire resistance is to increase wire size. A larger wire of the same length will have less resistance that a smaller wire.

Voltage drop to electrical current is like a person walking up a hill to do work. You may be too tired after walking up the hill. The result of voltage drop will be reduced performance or improper operation of equipment. Electric motors experiencing excessive voltage drop while starting heavy loads tend to burn out requiring frequent replacement. Excessive voltage drop on the grounded neutral conductor supplying a building is a frequent source of stray voltage that can reach high enough level to bother livestock.

Overhead conductors supplying farm buildings need to be of adequate size to insure proper operation of equipment and to help prevent stray voltage problems. There are a number of variables that need to be considered when determining the minimum size of wires to supply power to a farm building. As a result, the selection process can get so complex that it just simply gets ignored. The following three tables make the process easy. The tables assume the single-phase load on the wires is 80% of the rating of the main circuit breaker in the service panel, and that the voltage drop will be limited to 2%. The electrical code recommends 2% to 3% maximum. Large size wires may not be readily available, so the table recommends two parallel sets of wire as an alternative to a single wire for long runs. The wires can be purchased form an electrical wholesale distributor. For 3-phase services, the wire sizes will be a little smaller than given in the following tables. Table 1 is for copper wires installed underground, Table 2 is for aluminum wires underground, and Table 3 is for overhead aluminum triplex cable.

Table 1 Minimum underground copper wire size for single-phase, 120/240 volt supply to a farm building for not more than 2% voltage drop.

Table 2 Minimum underground aluminum wire size for single-phase, 120/240 volt supply to a farm building for not more than 2% voltage drop.

Table 3 Minimum overhead aluminum triplex cable size for single-phase, 120/240 volt supply to a farm building for not more than 2% voltage drop.

Agricultural Engineering
Michigan State University
A.W. Farrall Hall
East Lansing, MI 48824-1323
(517) 355-4720

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November 24, 2003