Field Wire Length
Original instructions
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Introduction
This section describes some considerations in calculating field wire length.
Effect of IR Drop
The IR drop is the product of the resistance of the wire (depends on wire gauge size) and the current drawn by the load. (IR = volts) After calculating the IR drop of the field wire, what you have left is available at the module input.
Example
The following example shows how to calculate the IR drop to see if enough is left over to turn on an I/O base’s input point.
Step
Action
1
Assume an I/O base needs 80 VAC minimum to turn on voltage and assume a field source of 120 VAC.
2
Assume the current drawn by the I/O base is 6 mA.
3
Consult the vendor of the wire to get the resistance of the wire (usually given in Ohms per 1000 feet, this depends on the gauge and length of the wire). For this example, assume the total resistance of the wire length is 1000 Ohms.
4
Calculate .006 A x 1000 Ohms = 6 VAC. This is the IR drop.
5
Calculate 120 VAC - 6 VAC = 114 VAC. This is plenty to turn on the inputs, as the minimum required is 80 VAC.
Empirical Testing Required
The IR drop calculation can only be a rough estimate. Empirical testing is required to fine-tune the wiring length. The result will depend on the following variables: