NEC 2026

Voltage Drop Calculator

Find the voltage drop of a copper or aluminum run in volts and percent, single-phase or three-phase, and the smallest size that keeps it within the suggested limit. The result updates as you type.

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Enter the voltage, load in amps and one-way length in feet to see the drop.

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For reference only. Always verify with the NEC edition adopted by your AHJ.

How voltage drop is calculated

Two formulas cover most runs. Single-phase and DC: drop = 2 x one-way length x resistance per 1000 ft x amps / 1000. Three-phase: drop = square root of 3 x one-way length x resistance per 1000 ft x amps / 1000. The percentage is the drop divided by the supply voltage. The result also shows the voltage left at the load.

The resistance per 1000 ft comes from Chapter 9 of the NEC. The DC method reads Table 8, the direct-current resistance of a stranded conductor. The AC method reads the Effective Z column of Table 9, the impedance for alternating current, which includes reactance and changes with the raceway material (PVC, aluminum or steel). Table 9 Note 2 says Effective Z is a line-to-neutral value, so both methods use the same length factors above. On small conductors the two methods are close; on large conductors the AC figure is the more realistic one.

The 3% and 5% figures are recommendations in Informational Notes (210.19 Informational Note and 215.4(A)(2) Informational Note No. 2), not mandatory rules. When the drop is higher, the calculator says it is above the figure suggested in the Informational Note and gives the smallest size that stays within 3%. A size chosen for voltage drop must still meet ampacity, termination and overcurrent rules: use the wire size calculator for those.

Voltage drop questions

Is a 3% voltage drop required by the NEC?
No. The 3% figure for branch circuits and the 5% figure for feeders plus branch circuit combined appear only in Informational Notes (210.19 Informational Note and 215.4(A)(2) Informational Note No. 2). Informational Notes are recommendations and are not mandatory rules, though other codes, a specification or the equipment maker may set a limit, so check what applies to the job.
What is the voltage drop formula?
For single-phase, drop in volts = 2 x one-way length x resistance per 1000 ft x amps / 1000. For three-phase, replace the 2 with the square root of 3. Percent drop = drop in volts / supply voltage x 100. The resistance per 1000 ft comes from Chapter 9 Table 8 or Table 9, whichever method you choose.
Which method should I use, Table 8 or Table 9?
Table 8 gives the direct-current resistance of the conductor, which is simple and slightly conservative for small sizes on resistive loads. Table 9 gives the effective impedance for alternating current at a fixed power factor and depends on the raceway material (PVC, aluminum or steel), which matters most on larger conductors where reactance is a real part of the impedance. For motors and other loads with a different power factor, use the actual power factor in a full impedance calculation.
Why does the length count twice for single-phase?
The current travels out on one conductor and back on the other, so the resistance in the circuit is that of twice the one-way length. The calculator takes the one-way length of the run. Three-phase balanced circuits use the square root of 3 instead, because the line-to-line drop depends on the phase relationship between the conductors.
Does this calculator pick a wire size?
It shows the drop for the size you select and, as a second result, the smallest size in the tables that keeps the drop within the suggested Informational Note figure for your inputs. That size is for voltage drop only; the conductor must also be sized for ampacity, termination temperature and overcurrent protection, so check the wire size calculator as well.

You can read the NEC 2026 for free at NFPA free access.