Voltage Drop

What is the meaning of voltage drop?

Voltage drop is defined as the amount of voltage loss that occurs through all or part of a circuit due to impedance. A common analogy used to explain voltage, current and voltage drop is a garden hose. Voltage is analogous to the water pressure supplied to the hose.

Allowable Voltage Drop
The voltage drop in the main power wires from the generation source or the battery to the bus should not exceed 2 percent of the regulated voltage when the generator is carrying rated current or the battery is being discharged at the 5-minute rate.
It’s caused by wire resistance leading up to a load. The voltage dropped at DC is the Ohm’s law V = IR. In fact it is sometimes called “IR drop.” … The device feels like it’s connected to a source that has no internal resistance.

Why is there a voltage drop across a resistor?
The larger the resistor, the more energy used by that resistor, and the bigger the voltage drop across that resistor. Ohm’s Law can be used to verify voltage drop. In a DC circuit, voltage equals current multiplied by resistance. .

Is potential difference and voltage drop the same thing?
Voltage is the potential difference between the source & any point in the circuit. Voltage drop means, amount of voltage by which voltage across load resistor is less then the source voltage. there is voltage drop in wire connected to the positive terminal of the source to load resistor.

How do you figure out voltage drop?
Voltage drop of the circuit conductors can be determined by multiplying the current of the circuit by the total resistance of the circuit conductors: VD = I x R. “I” is equal to the load in amperes and ”R” is equal to the resistance of the conductor as listed in Chapter 9, Table 8 for direct current circuit, or in …

How do you know if your wiring will provide a reasonable efficiency of operation?

The National Electrical Code, 210-19(a) (FPN 4) and 215-2 (b) (FPN 3), recommends 5% voltage drop for feeder circuits and 3% for branch circuits. Let’s work some examples, using the equations in the sidebar (right). Our examples use uncoated copper wire in steel conduit, for 480V branch circuits; we’ll use NEC Table 9’s power factor column.

Example 1: Determine voltage drop Run a No. 10 stranded wire 200 ft at 20A. Per Table 9, our “ohms to neutral per 1,000 ft” is 1.1 ohms. To complete the numerator, multiply as follows: (2 x 0.866) x 200 ft x 1.1 ohms x 20A = 7620.8 Dividing 7621 by 1000 ft gives a voltage drop of 7.7V. This drop is acceptable for our 480V circuit. A No. 12 would drop 11.8V. Boost the length to 500 ft, and that No. 10 drops 18V; the No. 12 drops 29V.

Example 2: Determine wire size Run a stranded copper wire 200 ft at 20A. You can find the wire size by algebraically altering the first equation, or you can use the following method. To complete the numerator, multiply as follows: 1.73 x 212.9 ohms x 200 ft x 20A = 89371.2 Dividing the 89371.2 by the acceptable voltage drop of 14.4V gives you 6207 circular mils. NEC Table 8 shows that a No. 12 wire satisfies the voltage drop recommendation.

Example 3: Determine wire length Run a stranded copper No. 10 wire for a 20A circuit. To complete the numerator, multiply as follows: 1000 x 14.4V = 14400 To complete the denominator, multiply as follows: (2 x 0.866) x 1.1ohms x 20A = 38.104 Finally, divide the numerator by the denominator, as follows: 14400 / 38.1044377 ft If you ran the No. 12 wire for the same circuit, you could run it 244 ft.

Example 4: Determine maximum load Run a stranded copper No. 10 wire for a 200 ft circuit. To complete the numerator, multiply as follows: 1000 x 14.4V = 14400 To complete the denominator, multiply as follows: (2 x 0.866) x 1.1 ohms x 200 ft = 381.04 Finally, divide the numerator by the denominator, as follows: 14400 / 381.04437A This circuit could handle 37A on each phase conductor. A 200 ft No. 2 could handle 24A.

* The number “0.866” is for 3-phase only. It converts the number “2” to “1.732” (the square root of 3). For single-phase circuits, don’t use the “0.866” in the calculations. * “CM”denotes wire size in circular mils, as shown in Table 8. * To calculate wire size, use 12.9 as your K for copper and 21.2 as your K for aluminum. * “L” is the one-way wire length in ft. * “R” is the resistance per 1,000 ft. Use NEC Table 9 for AC wiring. If you have non-linear loads, use the column that helps account for power factor.

Equation 1: Calculating the actual Voltage Drop in volts Volts Dropped = (2 x 0.866) x L x R x Amps/1000

Equation 2: Calculating the Wire Size in circular mils CM = 2 x K x L x Amps/Acceptable Voltage Drop Alternatively, you can algebraically manipulate Equation 1 to: R410002Acceptable Voltage Drop/1.732 x L x Amps and then look up the wire size according to its AC resistance.

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