**Voltage Drop**

Series Circuit

In a series circuit, voltage (electromotive force) pushes current (electrons) through a single path (conductor) around a circuit and back to its power source. A portion of this voltage is dropped off at each resistor and, by the time all of the voltage passes through all of the resistors, the total voltage is used up. This is consistent with Kirchoff’s Law, which states that “the sum of the voltages dropped at each resistor is equal to the source voltage.” Notice the series circuit below, all of the voltages used at all of the resistors add up to 24 volts. Also, all of the resistances add up to the total resistance of the circuit, This is because all of the resistance in a series circuit is accumulated along a single path. In this case 12 ohms.

In a parallel circuit, all of the voltage at the source is available to each resistor, even though there are several paths for current to travel. Notice in the parallel circuit below; the entire 12 volts at the source is available at each resistor. Resistance in a parallel circuit is not accumulated, or added together along one path, like a series circuit. The reason why is that the voltage, or force, is the same everywhere in the circuit.

Resistance (Good And Bad)

Up to now we have been talking about resistors as symbols in a circuit diagram. Most resistors, like light bulbs, toasters and electric heaters, in reality, are very useful devices. Even though these gizmos serve useful purposes by using electricity to do actual work, they aren’t perfect, and to some extent waste energy. A light bulb for instance gives off light but, at the same time gives off wasted heat.

Copper and aluminum conductors also are a little imperfect and contain some resistance. Chapter 9, Table 8 in the National Electrical Code gives us the actual resistance of conductors (per 1000 feet).

Copper and aluminum conductors also are a little imperfect and contain some resistance. Chapter 9, Table 8 in the National Electrical Code gives us the actual resistance of conductors (per 1000 feet).

Notice the three columns on the right side of the Table. Copper Uncoated, Copper Coated and Aluminum. Copper Uncoated includes most conductors, like thermo-plastic insulated and others. Copper Coated conductors includes rubber insulated (R-series) conductors which require an outer coating to keep the insulation from stretching. Aluminum conductors are listed on the last column.

Remember, the resistances given in Chapter 9, Table 8 are per 1000 feet. The resistance per 1000 feet of #14 stranded copper wire (quantity 7, i.e. strands) for instance is 3.14 ohms, while the resistance of #14 solid copper wire (quantity 1) is 3.07 ohms. As you can see, there is less resistance in a solid wire than in a stranded. As you know, most conductors come in rolls of 500 feet, so the values in the Table will have to be adjusted for different lengths. Here’s a formula for finding resistance for conductors of varying lengths:

Remember, the resistances given in Chapter 9, Table 8 are per 1000 feet. The resistance per 1000 feet of #14 stranded copper wire (quantity 7, i.e. strands) for instance is 3.14 ohms, while the resistance of #14 solid copper wire (quantity 1) is 3.07 ohms. As you can see, there is less resistance in a solid wire than in a stranded. As you know, most conductors come in rolls of 500 feet, so the values in the Table will have to be adjusted for different lengths. Here’s a formula for finding resistance for conductors of varying lengths:

Keep in mind that this formula calculates the resistance of one conductor only. To find the resistance of two conductors you’ll have to double the resistance, etc. For conductors in parallel, you must halve the resistance, (or divide by 2). This is because two conductors in parallel are taking the place of one conductor.

Calculating Voltage Drop

The simplest way to calculate the voltage drop on conductors in a circuit you can use the formula...

V.D. = I x R

Check the following circuit...

In the above circuit the amps (I) equals 3. The resistance (R) for 350’ ( 175’ times two conductors) of #14 copper wire

(3.07 x 350’)/1000 = 1.07 ohms. The V.D. = I x R formula should look like this...

(3.07 x 350’)/1000 = 1.07 ohms. The V.D. = I x R formula should look like this...

V.D. = 20 x 0.99 or 19.8 volts dropped

Voltage Drop Allowed

The National Electrical Code has some recommendations as to how much voltage drop. or loss, can be tolerated on conductors. Here they are....

For Branch Circuits...

For Branch Circuits...

*210.19 Informational Note. 4: Conductors for Branch Circuits (as defined in Article 100), sized to prevent a voltage drop exceeding 3 percent (at the farthest outlet of power, heating, and lighting loads, or combinations of such loads) and a maximum total voltage drop on both feeders and branch circuits (to the farthest outlet) not to exceed 5 percent, will provide reasonable efficiency of operation.*As far as Feeders are concerned...

215.2 Informational Note 2: Conductors for Feeders (as defined in Article 100), sized to prevent a voltage drop exceeding 3 percent (at the farthest outlet of power, heating, and lighting loads, or combinations of such loads) and a maximum total voltage drop on both feeders and branch circuits (to the farthest outlet) not to exceed 5 percent, will provide reasonable efficiency of operation.215.2 Informational Note 2: Conductors for Feeders (as defined in Article 100), sized to prevent a voltage drop exceeding 3 percent (at the farthest outlet of power, heating, and lighting loads, or combinations of such loads) and a maximum total voltage drop on both feeders and branch circuits (to the farthest outlet) not to exceed 5 percent, will provide reasonable efficiency of operation.

Voltage Drop Formulas

To calculate how much voltage is dropped on the conductors of a circuit we use the following formula...

“VD” of course stands for voltage drop

“2” represents the two conductors in a circuit

“K” is the constant or resistance factor (copper = 12.9, aluminum = 21.2)

“L” is the length of the conductor in one direction (2 is built into the formula)

“I” stands for amps of the load.

“CM” is the circular mill (thickness) of the conductor from Chapter 9, Table 8.

“2” represents the two conductors in a circuit

“K” is the constant or resistance factor (copper = 12.9, aluminum = 21.2)

“L” is the length of the conductor in one direction (2 is built into the formula)

“I” stands for amps of the load.

“CM” is the circular mill (thickness) of the conductor from Chapter 9, Table 8.

Let’s put this formula to work in a problem...

Remember; we are trying to find the voltage drop of the circuit not the load. This is the wasted energy of the circuit not the energy used by the heater to do useful work, (like keeping us warm). Also, assume the conductor in the problem is copper, unless otherwise stated, (Article 110.5).

Example: A 240 volt, 25 amp heater is located 135 feet from a panel fed with two #10 THW conductors. Let’s find the voltage drop of the circuit ?

O.K., the formula for voltage drop is...

Remember; we are trying to find the voltage drop of the circuit not the load. This is the wasted energy of the circuit not the energy used by the heater to do useful work, (like keeping us warm). Also, assume the conductor in the problem is copper, unless otherwise stated, (Article 110.5).

Example: A 240 volt, 25 amp heater is located 135 feet from a panel fed with two #10 THW conductors. Let’s find the voltage drop of the circuit ?

O.K., the formula for voltage drop is...

The variables are...

K = 12.9 (copper)

L = 135’

I = 25 amps

CM = 10,380 Chapter 9, Table 8

Plugging these values into the formula...

L = 135’

I = 25 amps

CM = 10,380 Chapter 9, Table 8

Plugging these values into the formula...