Voltage drop practice problems

The voltage drop formula

Most exam questions use the circular-mil formula. It needs four things: a multiplier for the type of circuit, the K value of the conductor metal, the load current, and the one-way length of the run.

The 2 in the single-phase formula accounts for current flowing out on one conductor and back on the other, so you always use the one-way distance. In a balanced three-phase circuit the return current is shared, and the multiplier becomes the square root of 3, or 1.732.

What K means

K is the resistance of one circular mil of conductor that is one foot long. Exam prep books and most test questions use 12.9 for copper and 21.2 for aluminum. Some questions give you a different K, so always use the value in the question when there is one.

Where circular mils come from

The area of each conductor size is listed in Chapter 9, Table 8 of the NEC. A #12 conductor is 6,530 circular mils and a #6 is 26,240. These values never change, which is why the first line of the worksheet asks you to look one up. On the real exam you'll be flipping to that table constantly, so knowing where it is saves minutes.

Worked example: find the voltage drop

A 240 V single-phase load draws 24 amperes and is 120 feet from the panel. The circuit uses #10 copper. Is the voltage drop within 3%?

Notice how close that is. Questions like this are built to catch people who round too early or forget to use the one-way length.

Worked example: size the conductor

A 208 V three-phase load draws 60 amperes and is 200 feet from the source. What is the smallest copper conductor that keeps the drop at 3% or less?

When you size for voltage drop, you rearrange the same formula and solve for circular mils. Then you go back to Chapter 9, Table 8 and take the first size that is equal to or larger than your answer. Never round down, even when the number is close.

The 3% and 5% recommendations

The NEC doesn't require a voltage drop limit for ordinary branch circuits and feeders. Informational notes in 210.19 and 215.2 recommend 3% for a branch circuit or feeder and 5% for the feeder and branch circuit combined. Because these are recommendations, exam questions spell out the limit they want you to use. Read the question for it.

Common voltage drop mistakes on the exam

• Doubling the length. The 2 in the formula already covers the return path. If you also double the distance, your answer is twice as large as it should be.
• Using 1.732 on a single-phase circuit. A 240 V circuit is still single-phase. Only use 1.732 when the question says three-phase.
• Forgetting the multiplier. Leaving out the 2 or the 1.732 gives an answer that is half, or about 58%, of the right one. Exam writers put that number in the choices.
• Mixing up copper and aluminum. Aluminum's K of 21.2 is much higher, so the drop is about 64% larger on the same size wire.
• Calculating percent from the wrong voltage. Divide by the system voltage the load is served at, not by 120 when the circuit is 240.
• Rounding down when sizing. If you need 42,967 circular mils, a 41,740 cmil conductor doesn't meet it.

Another way: the resistance method

Some questions give you conductor resistance from Chapter 9, Table 8 or Table 9 instead of K. Then voltage drop is current times resistance: VD = I × R, where R is the resistance of both conductors over the run for single-phase. The circular-mil formula and the resistance method give close answers, and the question usually tells you which one to use.

How voltage drop appears on the Texas Calculations exam

In March 2025 Texas split the journeyman exam into a code knowledge part and a separately scored calculations part with 26 questions in 110 minutes. That works out to about 4 minutes and 14 seconds per question. A voltage drop question should take you two minutes or less, which buys time for longer problems like dwelling unit load calculations. The timer on each practice problem shows how you're tracking against that pace.

How to practice voltage drop effectively

1. Work ten problems in the worksheet until each line is automatic.
2. Switch to the practice exam and solve them as multiple choice, without the steps.
3. When you miss one, look at which wrong answer you picked. It tells you which mistake you made.