NEC voltage drop formulas and recommended limits

Where to turn

• Chapter 9, Table 8 gives conductor circular mil area and direct current resistance by size. Use it for the area in the formula.
• Informational notes under 210.19(A) and 215.2(A) carry the 3% branch circuit, 3% feeder and 5% combined recommendations (2023 numbering). Confirm the note numbers in the 2026 book.

An informational note explains and recommends. It does not make a rule. If a question asks what the NEC requires, the answer is that it sets no mandatory voltage drop limit for ordinary branch circuits and feeders. If it asks what the NEC suggests, the answer is the percentage in the note.

The formulas

• Single-phase, two-wire: drop equals 2 × K × I × L, divided by circular mils.
• Three-phase: drop equals 1.732 × K × I × L, divided by circular mils.
• K is about 12.9 for copper and 21.2 for aluminum, in ohm-circular mils per foot. L is the one-way length in feet. I is the load current.
• Percent drop is the calculated drop divided by the source voltage, times 100.

The drop itself is Ohm's law applied to the wire (V = I × R), and the Ohm's law calculator lets you practice that step and three-phase power.

Worked example

A 208-volt, three-phase feeder carries 45 amperes to a panel 180 feet away. The conductors are copper. Is 6 AWG enough to stay within 3%?

• 6 AWG has 26,240 circular mils in Table 8.
• Drop = 1.732 × 12.9 × 45 × 180 ÷ 26,240 = 6.90 V.
• Percent = 6.90 ÷ 208 = 3.3%. That is above the 3% recommendation.
• 4 AWG has 41,740 circular mils. Drop = 4.34 V, or 2.1%. That meets it.

The 6 AWG conductor is legal if the ampacity works. Choosing 4 AWG is a design choice to follow the recommendation, and then the equipment grounding conductor must grow in proportion, under 250.122(B).

Common traps

• Treating 3% as a rule. It is an informational note. Look at the wording of the question: required or recommended.
• Using round-trip length for L. The 2 in the single-phase formula already covers the return path, so L is one way.
• Using 2 for three-phase. Use 1.732.
• Dividing by the wrong voltage. Percent drop is based on the source voltage the problem gives.
• Using the breaker rating for I. Use the actual load current unless the question says otherwise.
• Mixing up K. Copper and aluminum differ, and a question may give a different K.
• Forgetting the 5% total. Feeder and branch drops add, and the combined recommendation is the larger figure.

Working it with the book open

Voltage drop needs only Chapter 9 Table 8 from the book, so most of the work is on your calculator. Write the formula first, with K, the current, the one-way length and the circular mils lined up, and compute the numerator before you divide. If the problem asks for the smallest conductor that meets a percentage, rearrange for circular mils, divide, then pick the next larger size in Table 8.

How to practice

Invent eight runs with different voltages, lengths and sizes. Work the drop in volts, then the percent, then pick the next size up if it misses. Work the same run in single-phase and three-phase to feel the 2 versus 1.732 difference. Aim for two minutes a problem.

Check a run in seconds with the voltage drop calculator, which shows every step.

Related guides: conductor ampacity and grounding electrode conductor sizing.

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Related exam prep

• Fire alarm voltage drop formula on Alarm Ladder: the same calculation for notification appliance circuits.

Sources

Show the list

• Candidate Information Bulletin, Texas electricians (updated 9/15/2026), PSI for the Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• Electrician exam information, Texas Department of Licensing and Regulation. Read 24 Oct 2026.
• NFPA 70, National Electrical Code (2026 edition), NFPA. Free read-only access with an account.