Voltage Drop Calculator: AWG or mm², DC, Single-Phase and Three-Phase
By BitsFromBytes Team · Reviewed by Weston Hale · Last updated October 26, 2026
Quick answer: Voltage drop is the voltage your wire turns into heat. For DC and single-phase circuits it equals 2 × one-way length × current × conductor resistance per unit length. Standard annealed copper is defined at 0.017241 Ω·mm²/m at 20 °C, which gives 10 AWG wire 3.277 Ω per kilometre (NBS Handbook 100, Copper Wire Tables).
Voltage drop calculator
Results update as you type; the table below checks every common gauge against your target.
Enter your circuit to see the result.
Resistance model: International Annealed Copper Standard (0.017241 Ω·mm²/m at 20 °C, temperature coefficient 0.00393 per °C) and AWG diameters from NBS Handbook 100; aluminum 2.828 µΩ·cm and 0.0039 per °C from NBS Circular 31. Resistive drop only: AC reactance and power factor are not modelled. Runs in your browser; nothing is sent anywhere.
What does this voltage drop calculator do?
It computes how many volts a cable loses between the source and the load, the percentage of the supply that represents, the voltage left at the load and the watts burned in the wire. Enter the circuit type, voltage, current or power, one-way length, conductor and size. The table under the result runs the same circuit through every common gauge from 4/0 to 30 AWG and flags the thinnest one that stays inside your target. A datasheet option takes the resistance of a specific cable instead. Everything runs in your browser.
How is voltage drop calculated?
Voltage drop follows Ohm’s law: drop = current × resistance of the wire the current flows through. In a DC or single-phase circuit the current goes out on one conductor and back on the other, so the wire length counted is twice the one-way run.
- DC and AC single-phase: Vdrop = 2 × L × I × R′
- AC three-phase (line to line, balanced load): Vdrop = √3 × L × I × R′
- Percentage drop = Vdrop ÷ source voltage × 100
- Heat lost in the wire = 2 × I² × L × R′ (DC and single-phase) or 3 × I² × L × R′ (three-phase)
- Maximum length at your target = (target % ÷ 100 × source voltage) ÷ (2 or √3 × I × R′)
L is the one-way length in metres, I the current in amperes and R′ the conductor resistance in ohms per metre. When you enter power instead of current, the tool divides watts by volts (or by √3 × volts on three-phase), which assumes a power factor of 1.
Where does the wire resistance come from?
R′ is calculated from the conductor’s resistivity and its cross-section, then corrected for temperature: R′ = ρ ÷ A × (1 + α × (T − 20 °C)).
| Constant | Value | Source |
|---|---|---|
| Copper resistivity at 20 °C (International Annealed Copper Standard) | 0.017241 Ω·mm²/m | NBS Handbook 100 (1966), section 1.1 |
| Copper temperature coefficient at 20 °C | 0.00393 per °C | NBS Handbook 100 (1966), section 1.1 |
| AWG diameters, 4/0 to 30 | 460.0 mil to 10.0 mil | NBS Handbook 100 (1966), Table 1 (ASTM B258-61) |
| Hard-drawn aluminum resistivity at 20 °C | 2.828 µΩ·cm (0.02828 Ω·mm²/m) | NBS Circular 31, 3rd edition (1914) |
| Aluminum temperature coefficient at 20 °C | 0.0039 per °C (approximate) | NBS Circular 31, 3rd edition (1914) |
| Foot | 0.3048 m exactly | NIST |
The American Wire Gage defines 4/0 as 0.4600 inch and 36 as 0.0050 inch, with 38 sizes in between set by geometric progression, according to NBS Handbook 100. Each step up in gauge number therefore cuts the cross-section by the same ratio, about 1.26, which is why resistance doubles roughly every three gauges.
The calculator reproduces the handbook’s own tables. For 10 AWG copper it returns 3.277 Ω/km at 20 °C, the value printed in Handbook 100 Table 10, and for 12 AWG it returns 1.589 Ω per 1,000 ft against the handbook’s 1.59.
Why does wire temperature change the result?
Copper’s resistance rises 0.393 % for every degree Celsius above 20 °C. BitsFromBytes Research computes from the handbook coefficient that copper at 75 °C has 21.6 % more resistance than at 20 °C (1 + 0.00393 × 55 = 1.216), and 15.7 % more at 60 °C.
If your cable runs warm — bundled in a conduit, in an attic, or carrying close to its rated current — set the temperature you expect. The tool accepts °C or °F (°C = (°F − 32) ÷ 1.8).
Worked example: a 12 V LED strip 20 ft from its power supply
A 12 V DC supply feeds a strip drawing 5 A through a 20 ft (6.1 m) two-wire run of copper at 20 °C. The target is a 3 % drop, which is 0.36 V on a 12 V supply.
| Copper size | Drop | Drop % | Voltage at load | Heat in wire | Max length at 3 % |
|---|---|---|---|---|---|
| 18 AWG | 1.277 V | 10.64 % | 10.72 V | 6.39 W | 5.6 ft |
| 16 AWG | 0.804 V | 6.70 % | 11.20 V | 4.02 W | 9.0 ft |
| 14 AWG | 0.505 V | 4.21 % | 11.50 V | 2.52 W | 14.3 ft |
| 12 AWG | 0.318 V | 2.65 % | 11.68 V | 1.59 W | 22.7 ft |
| 10 AWG | 0.200 V | 1.66 % | 11.80 V | 1.00 W | 36.0 ft |
Computed by BitsFromBytes Research with the calculator above, from NBS Handbook 100 constants.
12 AWG is the thinnest gauge that meets 3 % here. 16 AWG, a common choice for LED wiring, loses 6.70 % — and if that cable warms to 60 °C the drop climbs to 7.75 %.
Why do low-voltage circuits need thicker wire?
The volts lost depend only on current, length and wire; the percentage depends on the supply. The same 16 AWG run carrying 5 A over 20 ft drops 0.804 V in every case: that is 6.70 % of 12 V, 3.35 % of 24 V and 0.67 % of 120 V. Moving a 12 V project to 24 V at the same power also halves the current, so the drop in volts halves as well and the percentage falls to a quarter.
What voltage drop is acceptable?
The US National Electrical Code (NFPA 70) gives a recommendation, not a requirement: its informational notes to sections 210.19(A) and 215.2(A) suggest sizing branch circuits for no more than 3 % drop at the farthest outlet, and no more than 5 % for feeder and branch circuit combined. The calculator defaults to 3 %.
Low-voltage electronics often need tighter margins than the code note, because the load has its own minimum input. Check the voltage range on the datasheet of whatever sits at the end of the cable and set the target from that.
When should I use the datasheet resistance option?
Use it whenever you have the cable’s datasheet. Real cables are not ideal solid wires: NBS Handbook 100 notes that stranding lengthens each strand, and the ASTM convention it reproduces adds 2 % to the resistance of concentric-lay conductors up to 2,000,000 circular mils. Hard-drawn copper conducts about 2.5 % less than annealed copper, per the same handbook. Choose “Datasheet resistance”, enter the maximum conductor resistance in Ω/km or Ω/1000 ft, and the calculator uses it unchanged; no temperature correction is applied to a datasheet value.
What can’t this calculator tell you?
- Ampacity. A wire that passes the voltage-drop check can still be too thin to carry the current safely. Voltage drop and current rating are separate checks; follow the ampacity tables in your local code.
- AC reactance and power factor. The model is resistive. On large AC conductors, inductive reactance adds drop that this tool does not include, and motors or other non-unity loads change the current. Treat AC results for big feeders as a lower bound.
- Connections. Terminals, connectors, switches and fuses add resistance the tool cannot see.
- Metric sizes are nominal. The mm² option uses the nominal cross-section. A real cable’s datasheet resistance can differ from that nominal-area calculation; the datasheet option is the precise route.
- Aluminum data is old. The aluminum constants come from a 1914 Bureau of Standards measurement of commercial hard-drawn wire. Modern aluminum building-wire alloys differ; use the datasheet option for them.
Data freshness
Last updated: October 26, 2026. Copper constants and AWG diameters from NBS Handbook 100 (National Bureau of Standards, now NIST, 1966); aluminum constants from NBS Circular 31, 3rd edition (1914); foot definition from NIST’s unit conversion factors, page updated February 3, 2026. All retrieved October 26, 2026.
Review trigger: a new edition of the NEC that changes the informational notes on voltage drop, a reported calculation error, or a request to add a conductor material. Corrections are dated at the bottom of this page under our corrections policy.
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FAQ
How do you calculate voltage drop?
Multiply the current by the resistance of the wire it flows through. For DC and single-phase circuits that is 2 × one-way length × current × resistance per unit length; for balanced three-phase circuits, replace 2 with √3. Divide by the source voltage and multiply by 100 for the percentage.
What is the resistance of 12 AWG copper wire?
At 20 °C, standard annealed 12 AWG copper measures 1.59 Ω per 1,000 ft, or 5.21 Ω per kilometre, according to NBS Handbook 100 Tables 5 and 11. At 75 °C the handbook lists 1.93 Ω per 1,000 ft.
Does temperature affect voltage drop?
Yes. Copper resistance rises by 0.00393 per °C above 20 °C, so a run at 75 °C drops 21.6 % more voltage than the same run at 20 °C.
What wire size do I need for 12 V over 20 feet at 5 amps?
With copper at 20 °C and a 3 % target, 12 AWG: it drops 0.318 V (2.65 %). 14 AWG drops 4.21 % and 16 AWG drops 6.70 %. Check the current rating separately.
Sources: National Bureau of Standards, Copper Wire Tables, Handbook 100 (1966); Bureau of Standards, Copper Wire Tables, Circular 31, 3rd edition (1914); NIST, “U.S. Survey Foot: Revised Unit Conversion Factors” (updated February 3, 2026); NFPA 70, National Electrical Code, informational notes to 210.19(A) and 215.2(A). Retrieved October 26, 2026. Header photo: Émile Dionne on Unsplash (Unsplash License).