Voltage Drop Calculator | NEC Wire Sizing

Q: How do I calculate voltage drop for 12V DC?

For DC, reactance is zero. Use Vdrop=2LIR/1000V_drop = 2 L I R / 1000Vdrop=2LIR/1000—the 2 accounts for out-and-back. Pull R from NEC Table 8 for your wire size and material. Example: 12 V, 10 A, 20 ft one-way, 12 AWG copper (R ≈ 1.98 Ω/1000 ft): Vdrop≈0.79V_drop 0.79Vdrop≈0.79 V.

AWG	Diameter (mm)	Area (mm²)	Copper R (Ω/1000 ft)
18	1.02	0.82	7.95
16	1.29	1.31	4.99
14	1.63	2.08	3.26
12	2.05	3.31	1.98
10	2.59	5.26	1.24
8	3.26	8.37	0.78
6	4.11	13.3	0.49
4	5.19	21.2	0.31
2	6.54	33.6	0.19
1	7.35	42.4	0.15
1/0	8.25	53.5	0.12
2/0	9.27	67.4	0.10
3/0	10.4	85.0	0.08
4/0	11.7	107	0.06

AWG

Diameter (mm)

Area (mm²)

Copper R (Ω/1000 ft)

1.02

0.82

7.95

1.29

1.31

4.99

1.63

2.08

3.26

2.05

3.31

1.98

2.59

5.26

1.24

3.26

8.37

0.78

4.11

13.3

0.49

5.19

21.2

0.31

6.54

33.6

0.19

7.35

42.4

0.15

1/0

8.25

53.5

0.12

2/0

9.27

67.4

0.10

3/0

10.4

85.0

0.08

4/0

11.7

107

0.06

Voltage Drop Formulas & Quick Reference

NEC Chapter 9 Tables 8 and 9 drive the math. Single-phase uses the 2× factor for out-and-back; three-phase uses √3. Resistive loads use power factor 1.0; motors typically 0.85–0.9.

Formulas & Variables

Single Phase

V_{\text{drop}} = \frac{2 L I (R \cos\theta + X \sin\theta)}{1000}

Three Phase

V_{\text{drop}} = \frac{\sqrt{3} L I (R \cos\theta + X \sin\theta)}{1000}

12V DC (no reactance)

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

Variables

•

L

= one-way length (ft)

•

I

= current (A)

•

R

= resistance (Ω/1000 ft, Table 8)

•

X

= reactance (Ω/1000 ft, Table 9)

•

\cos\theta

= power factor

Power Factor

\cos\theta = 1

for resistive loads (heaters, incandescent). Motors often 0.85–0.9. Lower PF increases the reactance term.

When to Upsize

If drop exceeds 3% on a branch or 5% total, go up a wire size or shorten the run. See the wire size chart in the hub above for AWG resistance values.

Voltage Drop Calculator: NEC Wire Sizing & Distance

Calculate voltage drop for single-phase and three-phase circuits using NEC Table 8 and Table 9. Wire size chart, copper vs aluminum, conduit type. Free wire sizing tool for electricians and DIY.

Wire Size Chart & Voltage Drop

A voltage drop calculator answers: will my wire handle this run? Enter voltage (120, 208, 240, 480 V), phase (1 or 3), material (copper or aluminum), conduit (PVC, aluminum, steel), wire size from the AWG wire size chart (14 through 4500 kcmil), one-way distance in feet or meters, and load current. The tool uses NEC Table 8 for resistance and Table 9 for reactance, so the results match what you'd get with the standard formulas.

3% Branch, 5% Total: NEC Voltage Drop Limits

NEC Informational Notes recommend keeping branch-circuit drop under 3% and total system drop (feeder + branch) under 5%. These aren't enforceable limits—inspectors won't fail you—but exceeding them often leads to dim lights, hot motors, and efficiency loss. Contractors and engineers routinely design to these targets. The calculator flags results in amber when drop tops 3%.

Copper vs Aluminum Wire Sizing

Copper has lower resistance per gauge than aluminum, so you can use smaller wire for the same drop. Aluminum costs less but needs one size up to match; it also demands proper torque on lugs and antioxidant. Conduit choice matters too: steel adds inductive reactance versus PVC or aluminum. Use the calculator to compare wire sizes before ordering.

12V DC, Solar, and Low-Voltage Runs

For 12V DC, RV, solar, or automotive wiring, reactance is zero—use the DC formula. The calculator's power factor of 1.0 covers it. Long low-voltage runs are where drop hurts most: a 0.5 V drop on 120 V is minor; on 12 V it's over 4%. Upsize early for DC and low-voltage runs.

Voltage Drop Calculator FAQ

What is the maximum allowed voltage drop?

The NEC doesn't set a hard limit, but Informational Notes suggest 3% for branch circuits and 5% for feeder plus branch combined. Most electricians aim for these thresholds—going over can cause flicker, motors running hot, and wasted power.

How do I calculate voltage drop for 12V DC?

For DC, reactance is zero. Use

V_{\text{drop}} = 2 L I R / 1000

—the 2 accounts for out-and-back. Pull R from NEC Table 8 for your wire size and material. Example: 12 V, 10 A, 20 ft one-way, 12 AWG copper (R ≈ 1.98 Ω/1000 ft):

V_{\text{drop}} \approx 0.79

How does wire material (Copper vs Aluminum) affect drop?

Aluminum has about 61% of copper's conductivity, so for the same gauge it has roughly 1.6× the resistance. To get similar drop, you typically go one size larger with aluminum (e.g. 10 AWG Al where 12 AWG Cu would work). Aluminum saves money but needs proper lugs and torque.

Does conduit type actually matter?

It does. Steel conduit is magnetic and boosts inductive reactance; PVC and aluminum conduit are non-magnetic. NEC Table 9 gives different X values for each. On long runs or motor circuits, the difference can be noticeable.

How do I reduce voltage drop in a long run?

Upsize the wire, shorten the run, or switch to copper from aluminum. For AC, non-magnetic conduit (PVC or aluminum) lowers reactance. Try different wire sizes in the calculator to see the impact.

What happens if my voltage drop is too high?

Motors pull more current at low voltage and run hotter—life drops. Lights dim or flicker. Gear can brown out. You lose efficiency because power goes to heating the wire. Fix it by larger wire, shorter distance, or splitting the load.

Voltage Drop Calculator | NEC Compliant Wire Sizing Tool

Circuit Basics

Conductor Specs

Load

Results

The Science of Voltage Drop: Why Wire Size Matters

The Water Pressure Analogy

Formula Breakdown

NEC Recommendations: The 3% and 5% Limits

Wire Size Chart: AWG to Resistance