Distance from the panel to the load, not there and back — the calculator doubles it for you.
Reference data and planning arithmetic. Does not apply ambient or bundling derating, motor rules, or local amendments. Not a substitute for the NEC edition your jurisdiction has adopted, an inspection, or a licensed electrician.
You should not have to know the resistance
Every voltage-drop formula needs the conductor's resistance per thousand feet, and that number lives in NEC Chapter 9 Table 8. A calculator that asks you for it has not saved you anything — if you have the code book open at Table 8, you can do the arithmetic. So this one holds the table.
Vdrop = K × L × I × R ÷ 1,000
Where R is ohms per 1,000 ft from Table 8, L is the one-way run length, I is the load current, and K is 2 for single phase or DC and √3 (1.732) for three phase.
The three-phase mistake
Three-phase voltage drop uses √3, not 3. This catches people out constantly, and using 3 inflates the answer by 73% — which leads to over-sizing conductors and, occasionally, to concluding a perfectly good design will not work.
The reason is that in a balanced three-phase circuit the currents are 120° apart, so the voltage drops in the phase conductors do not simply add. The vector sum works out to √3 times the drop in one conductor, and there is no neutral current to account for at all.
| Size | 15 A at 120 V | 20 A at 120 V | 30 A at 240 V | 50 A at 240 V |
|---|---|---|---|---|
| 14 AWG | 7.67% ✗ | 10.23% ✗ | 7.67% ✗ | 12.79% ✗ |
| 12 AWG | 4.82% ✗ | 6.43% ✗ | 4.82% ✗ | 8.04% ✗ |
| 10 AWG | 3.02% ✗ | 4.03% ✗ | 3.02% ✗ | 5.04% ✗ |
| 8 AWG | 1.95% | 2.59% | 1.95% | 3.24% ✗ |
| 6 AWG | 1.23% | 1.64% | 1.23% | 2.05% |
| 4 AWG | 0.77% | 1.03% | 0.77% | 1.28% |
What the 3% actually is
The 3% figure is a recommendation, from the informational notes to NEC 210.19(A) and 215.2(A) — 3% on a branch circuit, and no more than 5% across feeder and branch combined. Informational notes are not enforceable code. An inspector will not fail you for 4% drop.
That does not make it unimportant. Voltage drop shows up as motors running hot and starting badly, dimming lights, electronics resetting, and heaters that never reach temperature. The power you lose is dissipated as heat in the wall. It is a performance and efficiency requirement rather than a safety one, which is exactly why it is advisory rather than mandatory.
Where it matters most: long runs to detached garages and workshops, submersible well pumps, anything with a motor, and low-voltage lighting, where the same absolute drop is a much larger percentage.
What this does not account for
- Temperature. Table 8 is DC resistance at 75°C. A conductor running hotter has higher resistance and more drop.
- AC impedance. For large conductors and high currents, reactance matters and NEC Chapter 9 Table 9 is the correct table. Below about 1/0 the difference is small.
- Power factor. Table 9 accounts for it; this resistance-only method does not.
- Starting current. Motor inrush is several times running current, and the drop during starting is correspondingly larger.
Common questions
What is the voltage drop on 100 feet of 12 AWG at 20 amps?
On a 120 V single-phase circuit, 7.72 V, which is 6.43% — well over the 3% recommendation. 12 AWG copper is 1.93 ohms per 1,000 ft, so the maths is 2 x 100 x 20 x 1.93/1000.
How do I calculate three-phase voltage drop?
Use √3 (1.732) as the multiplier, not 3, and not 2. The formula is 1.732 x length x current x resistance / 1,000. Using 3 by mistake inflates the answer by 73%.
Is 3% voltage drop a code requirement?
No. It is a recommendation in the informational notes to NEC 210.19(A) and 215.2(A) — 3% on a branch circuit and 5% total. Informational notes are advisory, so an inspector cannot fail you on voltage drop alone.
How far can I run 12 AWG wire on a 20 amp circuit?
About 47 feet at 120 V before you exceed 3% drop. At 240 V the same conductor reaches about 93 feet, because the allowable drop in volts doubles while the resistance stays the same.