Cable Voltage Drop Calculator | AWG & mm² Wire Sizing Tool

A cable voltage drop calculator works out how much voltage is lost when electric current travels through a length of wire. It uses the wire's resistance, its length, and the current to find the voltage drop, the power wasted as heat, and the voltage that actually reaches the load.

Every wire has some resistance. When current pushes through that resistance, part of the supply voltage is used up before it reaches the device at the far end. A short, thick wire loses very little voltage. A long, thin wire can lose enough that motors overheat, lights dim, or batteries fail to charge fully.

How to calculate voltage drop

The calculator needs six inputs: cable length, the length unit (feet or meters), the wire gauge, the gauge system (AWG or mm²), the load current in amperes, and the supply voltage in volts. It then returns the voltage drop, the percentage drop, the power lost as heat, and the delivered voltage.

The formula

For a standard two-conductor circuit (one wire out to the load, one wire back), the voltage drop is:

$V_{drop} = 2 \times I \times \frac{R}{1000} \times L$

• $I$ = load current, in amperes
• $R$ = wire resistance per 1000 feet (AWG) or per 1000 meters (mm²), so it is divided by 1000 to get resistance per foot or per meter
• $L$ = one-way cable length, in feet (AWG) or meters (mm²)
• The factor of 2 accounts for both conductors, since current must travel out and back

Power lost as heat in the wire follows:

$P_{loss} = I^2 \times R_{total}$

where $R_{total}$ is the combined resistance of both conductors ($R_{total} = 2 \times \frac{R}{1000} \times L$, using consistent units).

The voltage that reaches the load is:

$V_{delivered} = V_{supply} - V_{drop}$

And the drop as a share of the supply is:

$\%_{drop} = \frac{V_{drop}}{V_{supply}} \times 100$

Wire resistance values

American Wire Gauge (AWG) sizes use resistance figures from NEC Chapter 9, Table 8, given in ohms per 1000 feet at 75°C, the standard operating temperature for common building wire.

Metric (mm²) sizes start from IEC 60,228 conductor resistance limits, which are published at 20°C. Copper resistance rises with temperature, so the calculator adjusts each mm² value to the same 75°C basis before comparing it with AWG figures. The adjustment uses copper's temperature coefficient of resistance, about 0.00393 per °C:

$R_{75°C} = R_{20°C} \times [1 + 0.00393 \times (75 - 20)]$

This raises each IEC value by a factor of about 1.216. For example, 6 mm² wire is rated at 3.08 ohms per 1000 meters at 20°C, which becomes about 3.75 ohms per 1000 meters at 75°C. Using the uncorrected 20°C figure would understate the real voltage drop by roughly 18%.

Length is converted automatically: feet for AWG sizes, meters for mm² sizes, regardless of which unit was typed in. The conversion uses the defined international foot, 1 foot = 0.3048 meters exactly.

Worked example

Residential branch circuit

A 120 V circuit runs 100 feet of 12 AWG wire (1.93 ohms per 1000 feet) carrying 15 A.

• Voltage drop: 5.79 V (4.82% of supply)
• Power loss: 86.85 W
• Delivered voltage: 114.21 V

This exceeds the commonly used 3% guideline for branch circuits. Switching to 10 AWG cuts the drop to 3.63 V (3.02%), still slightly above 3%. Switching to 8 AWG brings it to 2.292 V (1.91%), comfortably inside the limit.

Solar DC circuit

A 48 V DC system runs 50 meters of 6 mm² wire (3.08 ohms per 1000 meters at 20°C, about 3.75 ohms per 1000 meters once corrected to 75°C) carrying 20 A.

• Voltage drop: 7.491 V (15.61% of supply)
• Power loss: 149.83 W
• Delivered voltage: 40.51 V

A drop this large wastes a meaningful share of the system's output as heat and starves the inverter or battery bank of voltage. Moving to 16 mm² wire reduces the drop to 2.797 V (5.83%) and the loss to about 56 W.

Industrial 240 V circuit

A 240 V circuit runs 200 feet of 6 AWG wire (0.491 ohms per 1000 feet) carrying 40 A.

• Voltage drop: 7.856 V (3.27% of supply)
• Power loss: 314.24 W
• Delivered voltage: 232.14 V

This is only slightly over the 3% guideline. Upgrading to 4 AWG reduces the drop to 4.928 V (2.05%).

Effect of wire size on wasted power

Wire size affects heat loss directly, since power loss grows with the square of the current but only linearly with resistance. Over a 50-foot run at 20 A, 10 AWG wire (1.21 ohms per 1000 feet) loses about 48.4 W as heat, while 6 AWG wire (0.491 ohms per 1000 feet) loses about 19.6 W for the same current. The heavier wire saves close to 29 W that would otherwise be wasted, not delivered.

Recommended voltage drop limits

The NEC (National Electrical Code, the wiring standard used across the United States) offers, as an informational note rather than a strict rule, commonly cited limits of:

• 3% for a branch circuit alone
• 5% for a feeder and branch circuit combined

Going over these limits does not automatically break the law, but it can cause dimming lights, motors that run hot, and electronics that behave erratically. IEEE Std 141 gives similar guidance for industrial and commercial power systems.

AWG compared with mm²

American Wire Gauge dates to 1857 and uses a reversed, logarithmic scale: smaller numbers mean thicker wire. A drop of three gauge numbers roughly doubles the cross-sectional area. It is standard across North America.

The mm² system states the copper cross-sectional area directly in square millimeters, so a 2.5 mm² wire has 2.5 square millimeters of copper. It is used through most of the rest of the world.

Limitations

The calculator covers two-conductor DC or single-phase AC circuits only. It does not apply to:

• Three-phase circuits, which use a different formula involving a factor of √3 and generally show a lower percentage drop than single-phase circuits carrying the same power.
• Large AC conductors, above about 1/0 AWG, where inductive reactance adds to plain resistance. NEC Chapter 9, Table 9 lists impedance values for that case.
• Conduit fill and ambient temperature above 75°C, both of which raise the wire's real resistance beyond the standard table values used here.

If the calculated voltage drop equals or exceeds the supply voltage, the circuit cannot deliver the entered current at all; the calculator flags this rather than showing a negative delivered voltage.

Frequently asked questions

What is voltage drop? Voltage drop is the loss of voltage that happens when current pushes through a wire's resistance, similar to how water pressure falls over a long garden hose. Some of the supply voltage never reaches the load.

What voltage drop percentage is acceptable? The NEC's commonly cited guidance is 3% for a branch circuit and 5% for a feeder and branch circuit combined. Beyond that, motors, lights, and electronics can behave poorly.

How can voltage drop be reduced? Use a thicker wire, shorten the cable run, reduce the load current, or, where practical, raise the supply voltage. Thicker wire is the most common fix.

Why does the formula multiply by 2? Current flows out to the load through one conductor and returns through a second one. Both conductors add resistance, so both are counted.

Does voltage drop waste energy? Yes. The same resistance that causes the drop also turns part of the current into heat, following $P = I^2 \times R$. That heat is energy paid for but never delivered to the load.

What temperature are the resistance values based on? 75°C (167°F), the standard rating for common building wire types such as THHN and THWN. AWG values come from NEC Table 8 directly at that temperature. mm² values are converted up from the 20°C figures published in IEC 60,228.

Does this calculator work for three-phase power? No. It is built for two-conductor, single-phase circuits. Three-phase voltage drop needs a separate formula that includes a √3 factor.

References

1. National Fire Protection Association, NFPA 70: National Electrical Code, 2023 Edition — Article 210.19(A) (branch circuit voltage drop), Chapter 9, Table 8 (conductor resistance), Table 9 (AC impedance).
2. Institute of Electrical and Electronics Engineers, IEEE Std 141-1993 (Red Book), Recommended Practice for Electric Power Distribution for Industrial Plants.
3. International Electrotechnical Commission, IEC 60,228: Conductors of Insulated Cables.