Voltage Drop Calculator
Estimate voltage drop and percentage drop for copper or aluminum conductors from 14 AWG to 1000 kcmil — single-phase or three-phase. Results update as you type. Built by a Phoenix electrical contractor for long, hot Valley site runs.
How it’s calculated. NEC approximate (circular-mil) method:
VD = (phase × K × I × L) ÷ (CM × sets). The phase factor is
2 for single-phase 2-wire and 1.732 (√3) for three-phase. L is
one-way feet — do not double it, because the phase factor already accounts for the return path.
CM is circular mils. The third option, per-conductor only, applies a factor of 1 and reports the
drop in a single conductor; it is an advanced check, not a system type. Use 2-wire or three-phase for a real circuit.
The insulation / jacket sets the conductor’s temperature rating, which changes its resistance.
We adjust K from the 75°C base (12.9 copper / 21.2 aluminum, Ω·cmil/ft) to the selected
rating with K·(T∞ + T) ÷ (T∞ + 75) (T∞ = 234.5 copper, 228.1 aluminum) —
e.g., copper at 90°C ≈ 13.5, at 60°C ≈ 12.3. Using the rated temperature is the conservative choice.
NEC recommends (informational) ≤3% branch and ≤5% total. Resistance-only — ignores power factor and reactance. For design/permit work rely on a stamped calculation; our engineering team performs full studies.
Worked example. A 20 A, 120 V circuit feeding a receptacle 150 ft from the panel on 12 AWG copper THWN (75°C, K = 12.9, 6,530 circular mils): VD = 2 × 12.9 × 20 × 150 ÷ 6,530 = 11.85 V, or 9.9% — more than three times the 3% branch-circuit guideline, and the receptacle would see about 108 V under full load. Moving to 10 AWG (10,380 cmil) brings it to 7.46 V (6.2%); 8 AWG (16,510 cmil) to 4.69 V (3.9%); and 6 AWG (26,240 cmil) to 2.95 V, or 2.5% — the first size inside 3%, and the calculator’s “min size” answer. That is why a long 120 V run to a parking-lot pole or a remote outbuilding so often lands several sizes above what its ampacity alone requires. Higher voltage is the cheaper fix: 100 A three-phase at 480 V on 1/0 copper (105,600 cmil) over 250 ft drops 1.732 × 12.9 × 100 × 250 ÷ 105,600 = 5.29 V, only 1.1%.
Section numbers follow the 2023 NEC; the edition adopted by your jurisdiction governs.
Need it engineered and stamped?
Voltage-drop and feeder sizing on a real project? Our commercial electricians in Phoenix size the conductors, run the studies, and self-perform the install.
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Read the field guide
The calculator gives you the number; the guides show where it comes from — sizing a motor branch circuit, feeding a light pole across a parking lot, and running conductors to an EV charger, the three places a long run most often fails the 3% check.
Voltage drop on a Phoenix commercial site
We built this for our own estimators pricing metro Phoenix work, where the two things that push a feeder past the 3 percent branch and 5 percent total recommendations are both common: long runs across large flat commercial sites — parking lot lighting, remote equipment pads, underground feeders to outbuildings — and conductors that spend the summer well above the 75°C-column assumptions.
Heat matters here in a way the code book does not spell out for you. Copper resistance rises with temperature, so a conductor running hot in a rooftop or slab-embedded raceway in July drops more volts over the same length than the table value suggests. The calculator gives you the geometry answer; on a Valley site, treat it as the floor and confirm the ambient case with the wire ampacity calculator before you size the feeder.
Which code edition applies
Arizona has no statewide electrical code, so the edition depends on the jurisdiction. Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC; Scottsdale is on the 2020; unincorporated Maricopa County is still on the 2017. For Gilbert, Peoria and Tempe we confirm the adopted edition with the jurisdiction before design starts. The 3 and 5 percent figures are informational-note recommendations rather than mandatory limits in every edition, but reviewers across the Valley treat them as the expectation on commercial sets, and energy-code compliance can make them binding.
Sizing a feeder for a real project in Maricopa County? Service and distribution covers how we design and run it, and the engineering page covers what goes into the permit set. Or use the form at the bottom of any service page.
Frequently asked questions
Does the calculator account for Phoenix heat?
Not directly. It calculates drop from conductor size, material, length, load and phase. Conductor resistance rises with temperature, so a run that sits hot in a rooftop or slab raceway in a Phoenix summer will drop more than the table figure. Use it for the geometry, then check the ambient case with the wire ampacity calculator.
Is 3 percent voltage drop a code requirement in Arizona?
In the NEC it is an informational-note recommendation for branch circuits, with 5 percent total to the farthest outlet, rather than a hard limit in the general articles. Valley plan reviewers expect commercial sets to respect it, and energy-code provisions can make it binding for certain circuits. Which NEC edition applies depends on the city, because Arizona has no statewide adoption.
Why does my long site run keep failing the 5 percent check?
Usually length. Large flat commercial lots in the Valley put lighting poles, gates and remote pads a long way from the service. The fixes are the ones the calculator lets you test: upsize the conductor, raise the voltage for the run, or move the distribution point closer to the load.
Ready to price your Phoenix project?
Send us the plans, the scope, or just the problem. We will tell you what it takes. Call (602) 396-5216 or request a bid online.