Question 1

What is voltage drop and why does it matter?

Accepted Answer

Voltage drop is the reduction in voltage along a conductor due to its resistance. NEC recommends limiting it to 3% on branch circuits and 5% total (feeder plus branch). Excessive drop causes lights to dim, motors to overheat and run inefficiently, electronics to malfunction, and heating elements to deliver less power. A 240V circuit with 8% drop only delivers 220V to the load - enough to burn out a motor over time.

Question 2

How do I calculate voltage drop?

Accepted Answer

Use Vd = (2 x K x L x I) / CM for single-phase, or Vd = (sqrt(3) x K x L x I) / CM for three-phase. K is 12.9 for copper and 21.2 for aluminum at 75 deg C. L is the one-way distance in feet, I is current in amps, and CM is the circular mils of the wire. Example: 20A on 12 AWG copper (6530 CM) for 100 ft one-way: (2 x 12.9 x 100 x 20) / 6530 = 7.9V drop, or 6.6% on a 120V circuit - too much.

Question 3

How long of a wire run is too long?

Accepted Answer

It depends on current, voltage, and wire size. Rough guidelines at 3% drop, 120V: 12 AWG copper is good for 15A up to about 50 ft, 10 AWG handles 20A to 75 ft, 8 AWG carries 30A to 100 ft. At 240V the same gauges cover twice the distance because voltage drop percentage is halved. For long runs to a garage or barn, upsize by 1-2 gauges to compensate or run at higher voltage if possible.

Question 4

Should I upsize wire for voltage drop?

Accepted Answer

Yes, when calculated drop exceeds 3%. Upsizing is the simplest fix - going from 12 AWG to 10 AWG reduces voltage drop by about 37%. Alternatives include running at higher voltage (240V instead of 120V halves the drop), reducing the load, or splitting the circuit. For EV chargers, solar, and long runs to outbuildings, wire upsize is standard practice even when the base NEC ampacity would allow a smaller conductor.

Question 5

Is copper or aluminum better for long runs?

Accepted Answer

Aluminum is cost-effective for long runs of large-size feeders (1/0 AWG and above). Because aluminum has about 61% the conductivity of copper, you need to upsize about two AWG steps. For service entrance cables over 100 ft, aluminum can be 40-50% cheaper than copper with acceptable voltage drop. Use anti-oxidant paste on terminations and torque to spec. For branch circuits under 60 amps, copper is standard due to easier connections and smaller conduit.

Question 6

What is circular mils?

Accepted Answer

Circular mils (CM or kcmil) is a unit of wire cross-sectional area. One circular mil is the area of a circle with a 1 mil (0.001 inch) diameter, equal to about 0.0005 mm^2. A solid round wire with diameter d mils has d^2 circular mils of area. 12 AWG is 6530 CM, 4/0 AWG is 211,600 CM, and large service cables are often specified in kcmil (thousands of circular mils). Most NEC tables and the voltage drop formula use CM as the standard.

Question 7

How do I convert between AWG and mm^2?

Accepted Answer

Approximate equivalents: 14 AWG = 2.5 mm^2, 12 AWG = 4 mm^2, 10 AWG = 6 mm^2, 8 AWG = 10 mm^2, 6 AWG = 16 mm^2, 4 AWG = 25 mm^2, 2 AWG = 35 mm^2, 1/0 AWG = 50 mm^2, 4/0 AWG = 95 mm^2. For voltage drop calculations, 1 mm^2 = 1973.52 circular mils. Most of Europe and Asia use mm^2 sizing with standard values 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120 mm^2.

Gauge	mm2	V-Drop	%
14 AWG	2.08	12.56V	10.47%
12 AWG	3.31	7.9V	6.58%
10 AWG	5.26	4.97V	4.14%
8 AWG	8.37	3.13V	2.6%
6 AWG	13.3	1.97V	1.64%
4 AWG	21.1	1.24V	1.03%
3 AWG	26.7	0.98V	0.82%
2 AWG	33.6	0.78V	0.65%
1 AWG	42.4	0.62V	0.51%
1/0 AWG	53.5	0.49V	0.41%
2/0 AWG	67.4	0.39V	0.32%
3/0 AWG	85	0.31V	0.26%
4/0 AWG	107.2	0.24V	0.2%

AWG	Circular Mils	mm²	Typical Use
14	4,107	2.08	15 A lighting
12	6,530	3.31	20 A outlets
10	10,380	5.26	30 A dryer
8	16,510	8.37	40 A range
6	26,240	13.3	50-60 A sub-panel
4	41,740	21.2	85 A feeders
2	66,360	33.6	100 A service
4/0	211,600	107	200 A service

Gauge	mm2	V-Drop	%
14 AWG	2.08	12.56V	10.47%
12 AWG	3.31	7.9V	6.58%
10 AWG	5.26	4.97V	4.14%
8 AWG	8.37	3.13V	2.6%
6 AWG	13.3	1.97V	1.64%
4 AWG	21.1	1.24V	1.03%
3 AWG	26.7	0.98V	0.82%
2 AWG	33.6	0.78V	0.65%
1 AWG	42.4	0.62V	0.51%
1/0 AWG	53.5	0.49V	0.41%
2/0 AWG	67.4	0.39V	0.32%
3/0 AWG	85	0.31V	0.26%
4/0 AWG	107.2	0.24V	0.2%

Voltage Drop Calculator

Common Circuits

Wire Specification

Circuit Details

Drop Visualization

Formula

Smallest Gauge That Passes

Drop by Wire Size

Understanding Voltage Drop

Single-Phase Voltage Drop

Three-Phase Voltage Drop

AWG to Circular Mils

NEC Recommended Drop Limits

Frequently Asked Questions