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In general, except for Motion Picture and Television Studios and Fire Pumps the National Electrical Code does not have mandatory voltage drop requirements. There are voltage drop recommendations in the form of fine print notes for feeders and branch circuits. However, if all requirements of the NEC are met, and the design did not take into consideration the voltage drop, there might not be enough volts at the end of a circuit to perform the designated function whether powering a relay or a small motor. Voltage drop is the volts which are lost in the conductors due to the resistance of the wire. Voltage drop can be calculated by several different methods. In general voltage drop should not exceed 2 to 5 percent.

The easiest method is to use the resistance of the conductors given in Table 8 of chapter 9.

Voltage drop = (2) x (the circuit length in ft.) x (resistance per 1 ft.) x (current)
Example: Circuit is a two wire circuit, 100 feet long, the wire is copper No. 16. What is the voltage drop for 4 amperes? By Table 8 Chapter 9 the resistance of 1 000 ft. of No. 16 copper conductor is 4.99 ohms (uncoated and stranded).
Voltage drop = 2 x 100 x 4.99/1000 x 4 amperes
= 4.0 volts

For a voltage of 24 volts this converts to 4.0/24 x 1 00 percent or 16.7 percent which is probably too high for most applications therefore a larger wire size would be required.

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