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The following formula can be used to check the voltage drop. The resistance/ft can be found in Figures 9-122  and 9-123 for the wire size. 

$_{Calculated voltage drop (VD) = resistance/ft × length ×}$ current 

## Electric Wire Chart Instructions 

To select the correct size of electrical wire, two major requirements must be met: 

1. The wire size should be sufficient to prevent an excessive voltage drop while carrying the required current over the required distance. [Figure 9-121] 

2. The size should be sufficient to prevent overheating of the wire carrying the required current. (See Maximum Operating Temperature earlier in this chapter for computing current carrying capacity methods.) 

To meet the two requirements for selecting the correct wire size using Figure 9-116 , the following must be known: 

1. The wire length in feet. 

2. The number of amperes of current to be carried. 

3. The allowable voltage drop permitted. 

4. The required continuous or intermittent current. 

5. The estimated or measured conductor temperature. 

6. Is the wire to be installed in conduit and/or bundle? 

7. Is the wire to be installed as a single wire in free air? 

## Example A. 

Find the wire size in Figure 9-116  using the following known information: 

1. The wire run is 50 feet long, including the ground wire. 

2. Current load is 20 amps. 

3. The voltage source is 28 volts from bus to equipment. 

4. The circuit has continuous operation. 

5. Estimated conductor temperature is 20 °C or less. The scale on the left of the chart represents maximum wire length in feet to prevent an excessive voltage drop for a specified voltage source system (e.g., 14V, 28V, 115V, 200V). This voltage is identified at the top of scale and the corresponding voltage drop limit for continuous operation at the bottom. The scale (slant lines) on top of the chart represents amperes. The scale at the bottom of the chart represents wire gauge. 

Step 1—From the left scale, find the wire length 50 feet under the 28V source column. 

Step 2—Follow the corresponding horizontal line to the right until it intersects the slanted line for the 20-amp load. 

Step 3—At this point, drop vertically to the bottom of the chart. The value falls between No. 8 and No. 10. Select the next larger size wire to the right, in this case No. 8. This is the smallest size wire that can be used without exceeding the voltage drop limit expressed at the bottom of the left scale. This example is plotted on the wire chart in Figure 9-116 . Use Figure 9-116 (top)  for continuous flow and Figure 9-116 (bottom)  for intermittent flow. 

## Example B. 

Find the wire size in Figure 9-116  using the following known information: 

1. The wire run is 200 feet long, including the ground wire. 

