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How to calculate voltage drop the easy way
Voltage drop shows up on exams and in real life on long runs. Here's the formula, a worked example, and what's considered acceptable.
The breakdown, in plain English
- Single-phase formula: VD = (2 × K × I × L) ÷ CM.
- K is the resistance constant: about 12.9 for copper, 21.2 for aluminum.
- I is the load in amps, L is the one-way length in feet, CM is the wire's circular mil area (from Chapter 9, Table 8).
- For three-phase, swap the 2 for 1.732.
- The code recommends no more than 3% drop on a branch circuit and 5% total. That's a recommendation in the notes, not a hard rule, but exams ask about it.
Worked example
Example: 16 A at 120 V, 100 ft one way, #12 copper
- #12 copper = 6,530 circular mils.
- 2 × 12.9 × 16 × 100 = 41,280.
- 41,280 ÷ 6,530 = 6.32 volts dropped.
- 6.32 ÷ 120 = 5.3%. That's over 3%, so you'd upsize to #10.
Codes to look up
NEC 210.19(A) Informational Note
Sizing branch circuits for about 3% drop, and 5% total with the feeder, gives reasonable efficiency.
NEC Chapter 9, Table 8
Lists the circular mil area and resistance of each wire size.
These are plain-English summaries, not the code's exact wording. Always check the full section in the NEC edition your area has adopted, plus local amendments.
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