Use this free voltage drop calculator to find the voltage drop, percentage drop and voltage at the load for a copper or aluminum cable. It works with American wire gauge (AWG/kcmil) and metric sizes (mm²), for single-phase, three-phase and DC circuits.
How to use the voltage drop calculator
- Choose the wire standard: AWG/kcmil (US and Canada) or mm² (IEC: Pakistan, India, UK, Middle East, Europe). It switches automatically when you pick a US or international voltage.
- Select copper or aluminum and the wire size you plan to use.
- Pick your system voltage. US presets are built in (120 V, 120/240 V, 208Y/120 V, 480Y/277 V, 600 V), plus international ones (230 V, 400 V, 415 V). Choose “Other voltage / DC” to enter any value, such as 12 V or 48 V DC.
- Enter the load current in amps and the one-way cable length from the panel to the load.
- If you run more than one cable per phase, enter the number of parallel sets.
The results update as you type. Change the wire size until the percentage drop is within your limit, or use the wire size calculator to pick the size for you.
Voltage drop formula
For conductors up to about 4/0 AWG or 95 mm², resistance dominates and the drop can be calculated from the conductor resistance:
- Single-phase and DC: VD = 2 × I × L × R
- Three-phase: VD = √3 × I × L × R
- Voltage drop % = VD ÷ V × 100
Where I is the load current (A), L is the one-way length and R is the conductor resistance per unit length. The factor 2 accounts for the outgoing and return conductors; √3 (1.732) applies to balanced three-phase loads.
This calculator uses resistance values from NEC Chapter 9, Table 8 (at 75 °C) for AWG sizes and IEC 60228 values corrected to 70 °C for mm² sizes. For large cables (above about 250 kcmil or 120 mm²) on AC, reactance adds to the drop, so treat the result as a minimum and check with the impedance values in NEC Chapter 9 Table 9 or the cable manufacturer’s data.
Worked examples
Example 1: 120 V circuit, 12 AWG copper, 16 A, 75 ft
R for 12 AWG copper = 1.93 Ω per 1000 ft. VD = 2 × 16 × (75 ÷ 1000) × 1.93 = 4.63 V, or 4.63 ÷ 120 = 3.9 %. Because 120 V circuits have so little voltage to spare, long 120 V runs often need upsizing: 10 AWG (1.21 Ω/1000 ft) brings the drop down to 2.9 V (2.4 %).
Example 2: 12 AWG copper, 240 V, 20 A, 100 ft
R for 12 AWG copper = 1.93 Ω per 1000 ft. VD = 2 × 20 × (100 ÷ 1000) × 1.93 = 7.72 V, which is 7.72 ÷ 240 = 3.2 %. That is slightly above the 3% recommendation. Moving up to 10 AWG (1.21 Ω/1000 ft) gives 4.84 V or 2.0 %.
Example 3: 480 V three-phase feeder, 100 A, 300 ft
With 4 AWG copper (0.308 Ω/1000 ft): VD = 1.732 × 100 × 0.3 × 0.308 = 16.0 V (3.3 %). Moving up to 3 AWG (0.245) gives 12.7 V (2.65 %), inside the 3% recommendation.
Example 4: 2.5 mm² copper, 230 V, 16 A, 25 m
R for 2.5 mm² copper at 70 °C ≈ 7.41 × 1.197 = 8.87 Ω/km. VD = 2 × 16 × 0.025 × 8.87 = 7.1 V (3.1 %). A 4 mm² cable brings this down to 4.4 V (1.9 %).
Allowed voltage drop at common voltages
| System voltage | 3 % limit | 5 % limit |
|---|---|---|
| 120 V single-phase | 3.6 V | 6.0 V |
| 208 V (208Y/120) | 6.2 V | 10.4 V |
| 240 V (120/240 V) | 7.2 V | 12.0 V |
| 277 V lighting | 8.3 V | 13.9 V |
| 480 V (480Y/277) | 14.4 V | 24.0 V |
| 230 V single-phase (IEC) | 6.9 V | 11.5 V |
| 400 V three-phase (IEC) | 12.0 V | 20.0 V |
What is an acceptable voltage drop?
| Standard | Branch circuit / final circuit | Feeder + branch total |
|---|---|---|
| NEC 210.19(A) and 215.2(A) informational notes | 3 % | 5 % |
| IEC 60364-5-52 (Annex G) – lighting | 3 % | – |
| IEC 60364-5-52 (Annex G) – other uses | 5 % | – |
| BS 7671 (public supply) | 3 % lighting, 5 % other | – |
Excessive voltage drop makes motors run hot, lights dim and electronics misbehave, and it wastes energy as heat in the cable. Keeping drop low is one of the main reasons to upsize a conductor beyond its minimum ampacity.
Frequently asked questions
How do you calculate voltage drop in a cable?
Multiply the current by the cable resistance for the one-way length, then multiply by 2 for single-phase or DC circuits, or by 1.732 (√3) for three-phase circuits. Divide by the supply voltage and multiply by 100 to get the percentage drop.
What is the maximum voltage drop allowed by the NEC?
The NEC recommends (in informational notes) a maximum of 3% on a branch circuit or feeder and 5% total from the service to the furthest outlet. These are recommendations, not mandatory rules, except for specific cases such as fire pumps and sensitive electronic equipment.
Does voltage drop depend on wire length?
Yes. Voltage drop rises in direct proportion to the length of the run. Doubling the length doubles the voltage drop, which is why long runs often need a larger wire size than the ampacity tables alone suggest.
Is aluminum wire worse for voltage drop than copper?
Aluminum has about 61% of the conductivity of copper, so an aluminum conductor of the same size has roughly 1.6 times the resistance and voltage drop. Aluminum is usually upsized by about two AWG sizes to match copper.
What is the voltage drop on a 120 V circuit over 100 ft?
For a 15 A load on 120 V over 100 ft one way: 14 AWG copper drops 9.2 V (7.7 %), 12 AWG drops 5.8 V (4.8 %) and 10 AWG drops 3.6 V (3.0 %). Long 120 V runs at full load usually need 10 AWG to stay within 3%.
Should I use one-way or round-trip length?
Enter the one-way length from the source to the load. The formula already includes the return path with the factor of 2 for single-phase or √3 for three-phase.
Related calculators and guides
These calculators give engineering estimates for planning and learning. Final designs must follow your local code (NEC, IEC/BS 7671, PEC or the authority having jurisdiction) and be checked by a qualified electrical engineer or licensed electrician.