2.5 Voltage Drop and Design Judgment

Voltage drop is not ampacity

Voltage drop is the reduction in voltage between the source and the load caused by conductor impedance and load current. It affects equipment performance, motor starting, lighting quality, heat, nuisance trips, and customer complaints. It is not the same as ampacity. A conductor may be large enough to carry current safely under the ampacity rules and still be too small for good voltage performance on a long run.

A master electrician should treat voltage drop as a design check. The NEC includes voltage drop recommendations in informational material for many general situations, but the enforceable requirement depends on the specific installation, listed equipment instructions, contract documents, engineering design, local amendments, or special equipment rules. On an exam, read whether the question asks for voltage drop, conductor ampacity, or code minimum. Do not answer a voltage-drop design question with only an ampacity table.

Formula setup

For a simple single-phase two-wire circuit, a common approximation is voltage drop = 2 x K x I x D / circular mil area, where K is conductor resistivity constant, I is current, and D is one-way distance in feet. The factor 2 accounts for the outgoing and return path. For a balanced three-phase circuit, a common approximation is voltage drop = 1.732 x K x I x D / circular mil area. Percent voltage drop is voltage drop divided by system voltage, multiplied by 100.

Some tables and software use conductor resistance in ohms per thousand feet. With that method, single-phase voltage drop is approximately 2 x I x R x D / 1000, and three-phase voltage drop is approximately 1.732 x I x R x D / 1000. Be clear whether D is one-way length and whether R is for one conductor per 1000 feet. Most exam errors come from doubling the length twice, failing to double it at all on single-phase circuits, using the wrong voltage, or using amperes after demand when the stem asks for load current.

Example

A 120 volt, single-phase branch circuit supplies 16 amps at a one-way distance of 150 feet using copper conductors with approximate K = 12.9 and conductor area of 6,530 circular mils for 12 AWG. Voltage drop is 2 x 12.9 x 16 x 150 / 6,530 = about 9.5 volts. Percent drop is 9.5 / 120 x 100, or about 7.9 percent. That is likely a poor design even if the conductor ampacity could be acceptable for a particular branch-circuit rule.

If the circuit were changed to 10 AWG copper at about 10,380 circular mils, the drop would be 2 x 12.9 x 16 x 150 / 10,380 = about 6.0 volts, or 5.0 percent. If the load could be served at 240 volts instead of 120 volts for the same power, current would be lower and percent drop may improve. If the panel could be located closer, the drop would improve without increasing conductor size. That is why voltage drop is a design problem, not just a table lookup.

Design options table

Motors and starting

Motor loads deserve special attention because starting current can be many times running current. A feeder that looks acceptable at full-load current may allow a deep voltage sag during starting. That sag can affect the motor itself and other loads on the same system. Long well-pump circuits, rooftop HVAC equipment, elevators, compressors, and large exhaust fans are common field examples.

The master electrician should review nameplate data, controller requirements, conductor length, starting method, transformer capacity, generator capacity, and utility service stiffness. Reduced-voltage starters, variable frequency drives, larger conductors, local transformers, or different distribution architecture can all be part of the solution. The NEC calculation may establish a minimum, but the system still has to work.

Exam traps

A typical exam problem gives voltage, current, distance, conductor material, and circular mil area. Underline whether the circuit is single-phase or three-phase. Underline whether the length is one-way. If the answer options are percentages, calculate voltage first and then divide by the correct system voltage. Do not use line-to-neutral voltage for a three-phase line-to-line load unless the problem says the load is connected line-to-neutral.

Another trap is assuming voltage drop automatically permits a smaller overcurrent device or changes the load calculation. It does not. Voltage drop is checked after the load and minimum conductor basis are known. If a larger conductor is chosen for voltage drop, the overcurrent device is still selected under the applicable branch-circuit, feeder, or equipment rule. The larger conductor may also require termination and equipment compatibility checks.