What the result means
Every conductor has resistance, so some voltage is lost between the panel and the load. Long runs, high current and small conductors all increase that loss. The result shows how many volts are lost and what that is as a percent of the source voltage.
If the percent is above 3% on a branch circuit, motors run hotter, lights dim at start-up and electronics may misbehave. Switch to the "smallest wire" mode to find the size that meets your target, then confirm that size also meets ampacity, terminal temperature and derating rules.
How it's calculated
- Single-phase: VD = (2 × K × I × L) ÷ CM
- Three-phase: VD = (1.732 × K × I × L) ÷ CM
- %VD = VD ÷ source voltage × 100
- Required CM = (2 or 1.732) × K × I × L ÷ (max %VD ÷ 100 × V)
K = 12.9 ohm-cmil/ft for copper, 21.2 for aluminum.
L is the one-way length in feet. CM is the conductor area in circular mils (NEC Chapter 9, Table 8).
Worked example
A 20 A load on 12 AWG copper, 100 ft from a 120 V single-phase panel.
- 12 AWG = 6,530 cmil
- VD = 2 × 12.9 × 20 × 100 ÷ 6,530 = 7.90 V
- %VD = (2 × 12.9 × 20 × 100 ÷ 6,530) ÷ 120 × 100 = 6.58%
6.58% is above the 3% suggestion. Check both voltage drop and conductor ampacity before selecting a conductor.
Reference table
The values below are computed from the same calculator functions used above. They let you compare common sizes and operating points without replacing the interactive calculation or its assumptions.
| Size | 10 A | 15 A | 20 A | 30 A | 40 A | 50 A | 60 A | 100 A |
|---|---|---|---|---|---|---|---|---|
| 14 AWG | 6.28 | 9.42 | 12.55 | 18.83 | 25.11 | 31.39 | 37.66 | 62.77 |
| 12 AWG | 3.95 | 5.93 | 7.90 | 11.85 | 15.80 | 19.75 | 23.71 | 39.51 |
| 10 AWG | 2.49 | 3.73 | 4.97 | 7.46 | 9.94 | 12.43 | 14.91 | 24.86 |
| 8 AWG | 1.56 | 2.34 | 3.13 | 4.69 | 6.25 | 7.81 | 9.38 | 15.63 |
| 6 AWG | 0.98 | 1.47 | 1.97 | 2.95 | 3.93 | 4.92 | 5.90 | 9.83 |
| 4 AWG | 0.62 | 0.93 | 1.24 | 1.85 | 2.47 | 3.09 | 3.71 | 6.18 |
| 3 AWG | 0.49 | 0.74 | 0.98 | 1.47 | 1.96 | 2.45 | 2.94 | 4.90 |
| 2 AWG | 0.39 | 0.58 | 0.78 | 1.17 | 1.56 | 1.94 | 2.33 | 3.89 |
| 1 AWG | 0.31 | 0.46 | 0.62 | 0.92 | 1.23 | 1.54 | 1.85 | 3.08 |
| 1/0 AWG | 0.24 | 0.37 | 0.49 | 0.73 | 0.98 | 1.22 | 1.47 | 2.44 |
| 2/0 AWG | 0.19 | 0.29 | 0.39 | 0.58 | 0.78 | 0.97 | 1.16 | 1.94 |
| 3/0 AWG | 0.15 | 0.23 | 0.31 | 0.46 | 0.62 | 0.77 | 0.92 | 1.54 |
| 4/0 AWG | 0.12 | 0.18 | 0.24 | 0.37 | 0.49 | 0.61 | 0.73 | 1.22 |
| Load | Size | 120 V | 240 V |
|---|---|---|---|
| 15 A | 14 AWG | 38.23 ft | 76.47 ft |
| 20 A | 12 AWG | 45.56 ft | 91.12 ft |
| 30 A | 10 AWG | 48.28 ft | 96.56 ft |
| 40 A | 8 AWG | 57.59 ft | 115.19 ft |
| 50 A | 6 AWG | 73.23 ft | 146.46 ft |
Ampacity must be checked separately.
The drop column is independent of source voltage. K = 12.9 ohm-cmil/ft for copper at 75°C. Check ampacity separately.
Sources and assumptions
K = 12.9 ohm-cmil/ft for copper, 21.2 for aluminum. L is the one-way length in feet. CM is the conductor area in circular mils (NEC Chapter 9, Table 8). Calculator formulas are published with their units so a technician can check a result instead of treating it as a black box. Where a code table, manufacturer specification or field measurement is required, use the applicable local source. Copper voltage-drop tables use K = 12.9 ohm-cmil/ft at 75°C.
Last reviewed . Spot an error? Email support@tiermenu.com.
FAQ
Is the length one-way or round trip?
One-way. The single-phase formula already multiplies by 2 to account for the return conductor.
Why is my three-phase drop lower than single-phase?
Three-phase uses 1.732 instead of 2 because current is shared across three conductors that are 120° apart.
Does this pick a wire size that meets code?
No. To check ampacity and voltage drop together, use the wire size calculator, then confirm terminal ratings, ambient temperature and fill derating.
What K value should I use for aluminum?
21.2 ohm-cmil/ft is a common value at typical operating temperature. Use the manufacturer's resistance data if you need more precision.
Is 3% a code requirement?
In the NEC it appears in informational notes, which are recommendations. Some local codes and energy codes make it mandatory, so check your jurisdiction.
Using a reference result in the field
A calculator is most useful when the inputs match the job being checked. Where a field asks for a length, confirm whether it is one-way or total; where it asks for current, confirm whether the load is continuous or intermittent. The unit label beside the result is part of the result; do not carry a number into another calculation without checking its units.
Reference rows are provided for comparison, not as universal design limits. They use the values and conditions stated with each table. A different conductor material, temperature, power factor or load type can change a field result. Where the page calls out a code check, equipment rating or manufacturer table, that check remains part of the job.
Use the interactive inputs for the exact conditions at hand, then review the displayed formula and assumptions. If a value looks unexpected, change one input at a time and compare it with a known point in the reference table. Keep the measured readings and job-specific notes with the work record so another technician can understand how the result was obtained.
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