107 90 88 100 20 20 20 20 No. 6 No. 4 No. 12 No. 14 VD = (0.00044 ohms/feet) (107 x 20) = 0.942 VD = (0.00028 ohms/feet) (90 x 20) = 0.504 VD = (0.00202 ohms/feet) (88 x 20) = 3.60 VD = (0.00306 ohms/feet) (100 x 20) = 6.12 1 0.5 4 7  Voltage drop  Run lengths (feet) Circuit current (amps)  Wire size from chart Check calculated voltage drop (VD) = (resistance/feet) (length) (current) No. 10 --- --- --- 20  39  19.5 156 273 VD = (0.00126 ohms/feet) (39 x 20) = 0.98 VD = (0.00126 ohms/feet) (19.5 x 20) = 0.366 VD = (0.00126 ohms/feet) (156 x 20) = 3.93 VD = (0.00126 ohms/feet) (273 x 20) = 6.88 1 0.5 4 7  Maximum Voltage drop Wire size Circuit current (amps)  Maximum wire run length (feet) Check calculated voltage drop (VD) = (resistance/feet) (length) (current) Figure 9-122.  Determining required tin-plated copper wire size and  checking voltage drop.  Figure 9-123.  Determining maximum tin-plated copper wire length  and checking voltage drop. To meet the two requirements for selecting the correct wire  size using  Figure 9-116 , the following must be known: 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. Find the wire size in  Figure 9-116  using the following known  information: To select the correct size of electrical wire, two major  requirements must be met: $_{Calculated voltage drop (VD) = resistance/ft × length ×}$  current 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. Electric Wire Chart Instructions Example B.  Example A.  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.) 1.  The wire run is 200 feet long, including the ground  wire. 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. 4.  The circuit has continuous operation. 3.  The voltage source is 28 volts from bus to equipment. 2.  Current load is 20 amps. 1.  The wire run is 50 feet long, including the ground  wire. 7.  Is the wire to be installed as a single wire in free air? 6.  Is the wire to be installed in conduit and/or bundle? 5.  The estimated or measured conductor temperature. 4.  The required continuous or intermittent current. 3.  The allowable voltage drop permitted. 2.  The number of amperes of current to be carried. 1.  The wire length in feet. Find the wire size in  Figure 9-116  using the following known  information: 9-76