Voltage Drop Calculator

Find the right wire size for any run, from a 12V landscape light to a 240V subpanel.

Project Details
Enter your measurements and requirements

Length is one way, from the panel or transformer to the load. The calculator counts the return conductor.

Results
Calculated estimates for your project

Voltage drop

7.72 V (6.43%)

112.28 V at the load.

3% branch: fail5% total: failYour 3%: fail
Minimum size for target
8 AWG Cu
Max run at this size
46 ft
Ampacity
20 A
Power lost
154 W, about $38 a year
Current
20.0 A

VD = 2 x I x L x R / 1000

= 2 x 20.0 A x 100 ft x 1.93 ohm/kft / 1000 = 7.72 V

% = 7.72 / 120 x 100 = 6.43%

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  • 8 AWG copper wire
  • 6.4% on 12 AWG drop
  • 30 A breaker

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Project plan · built from your numbers

Run a 30 A circuit 100 ft on 8 AWG copper

20 A at 120 V over 100 ft one way. 8 AWG copper keeps the drop near 2.5%. Pull a permit; most areas require one for a new circuit, and an inspector checks the wire size and breaker.

Materials
$558
Time
1 weekend plus inspection
Level
Advanced
7 steps 4 tools 4 materials
  1. 1

    Confirm the load

    Check the nameplate amps or watts. This plan uses 20.0 A.

  2. 2

    Plan the route

    Measure the actual path, not the straight line. Every extra foot adds drop; this plan allows 111 ft.

  3. 3

    Get the permit

    Most jurisdictions require a permit and inspection for a new circuit or subpanel feed.

01 · Your numbers
Wire
8 AWG copper
Drop
6.4% on 12 AWG
Breaker
30 A
Est. materials
$558
02 · The plan

7 steps · 4 tools · 4 materials

4 more steps, the full tool list and a shopping list you can check off as you go.

03 · Finished

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Run a 30 A circuit 100 ft on 8 AWG copper

Finished in 1 weekend plus inspection · $558 DIY

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Voltage drop formula

  1. Find the current. Use the nameplate amps, or watts divided by volts (divided by 1.732 x volts for three-phase).
  2. Measure one-way length. Measure the path the wire takes from the panel or transformer to the load, in feet.
  3. Look up resistance. Take the ohms per 1,000 ft for the wire size and metal from NEC Chapter 9 Table 8.
  4. Multiply. VD = 2 x I x L x R / 1000 for single-phase and DC, or 1.732 x I x L x R / 1000 for three-phase.
  5. Compare with the source voltage. Divide by the voltage and multiply by 100. Aim for 3% or less on a branch circuit, 5% for the whole run.

Single-phase and DC: VD = 2 x K x I x L / CM. Three-phase: VD = 1.732 x K x I x L / CM. K is the resistance of a circular-mil foot of wire: 12.9 for copper and 21.2for aluminum. CM is the wire's area in circular mils (12 AWG is 6,530, 6 AWG 26,240). The calculator uses the Table 8 resistance directly, which gives the same answer.

NEC voltage drop recommendations

NEC 210.19(A) Informational Note No. 4 suggests a branch circuit drop of 3% or less and 5% for feeder plus branch circuit combined. 215.2 says the same for feeders. They are informational notes, so they are not enforced unless your jurisdiction adopts them, but equipment makers design around them: motors run hot and EV chargers slow down on low voltage.

Max run length for 3% drop at 120V and 240V (copper)

One-way length; 'too small' means the wire is under NEC 310.16 / 240.4(D) ampacity for that load
Load14 AWG12 AWG10 AWG8 AWG6 AWG4 AWG2 AWG
15 A, 120 V39 ft62 ft99 ft157 ft244 ft389 ft618 ft
20 A, 120 Vtoo small46 ft74 ft117 ft183 ft292 ft463 ft
30 A, 120 Vtoo smalltoo small49 ft78 ft122 ft194 ft309 ft
40 A, 120 Vtoo smalltoo smalltoo small58 ft91 ft146 ft231 ft
50 A, 120 Vtoo smalltoo smalltoo smalltoo small73 ft116 ft185 ft
60 A, 120 Vtoo smalltoo smalltoo smalltoo smalltoo small97 ft154 ft
Load14 AWG12 AWG10 AWG8 AWG6 AWG4 AWG2 AWG
15 A, 240 V78 ft124 ft198 ft314 ft488 ft779 ft1237 ft
20 A, 240 Vtoo small93 ft148 ft235 ft366 ft584 ft927 ft
30 A, 240 Vtoo smalltoo small99 ft157 ft244 ft389 ft618 ft
40 A, 240 Vtoo smalltoo smalltoo small117 ft183 ft292 ft463 ft
50 A, 240 Vtoo smalltoo smalltoo smalltoo small146 ft233 ft371 ft
60 A, 240 Vtoo smalltoo smalltoo smalltoo smalltoo small194 ft309 ft

Worked examples

20 A shop circuit, 120 V, 100 ft

12 AWG copper: 2 x 20 x 100 x 1.93 / 1000 = 7.72 V, or 6.4%. That fails 3%; 8 AWG drops 2.5%.

50 A EV charger, 240 V, 150 ft

6 AWG copper drops 3.1%. The 3% size is 4 AWG. See the EV charger installation guide.

100 A subpanel to a detached garage, 120 ft, copper vs aluminum

2 AWG copper drops 1.9%; 1/0 aluminum drops 2.0% and usually costs about half as much. Wiring the garage itself is covered in the garage electrical guide.

12 V landscape lighting, 100 W, 80 ft

8.3 A on 12 AWG drops 2.57 V (21.4%); 10 AWG gets it to 13.4%. Valve wiring for a sprinkler system and 12 V recessed lighting follow the same math.

Wire resistance per 1,000 ft (NEC Chapter 9 Table 8)

Sizemm²Circular milsCopper (ohm/kft)Aluminum (ohm/kft)Ampacity Cu 60/75 C
18 AWG0.821,6207.7712.8n/a
16 AWG1.312,5804.898.05n/a
14 AWG2.084,1103.075.0615 / 20 A
12 AWG3.316,5301.933.1820 / 25 A
10 AWG5.2610,3801.21230 / 35 A
8 AWG8.3716,5100.7641.2640 / 50 A
6 AWG13.326,2400.4910.80855 / 65 A
4 AWG21.241,7400.3080.50870 / 85 A
3 AWG26.752,6200.2450.40385 / 100 A
2 AWG33.666,3600.1940.31995 / 115 A
1 AWG42.483,6900.1540.253110 / 130 A
1/0 AWG53.5105,6000.1220.201125 / 150 A
2/0 AWG67.4133,1000.09670.159145 / 175 A
3/0 AWG85167,8000.07660.126165 / 200 A
4/0 AWG107211,6000.06080.1195 / 230 A
250 kcmil127250,0000.05150.0847215 / 255 A
300 kcmil152300,0000.04290.0707240 / 285 A
350 kcmil177350,0000.03670.0605260 / 310 A
400 kcmil203400,0000.03210.0529280 / 335 A
500 kcmil253500,0000.02580.0424320 / 380 A

Voltage drop calculator FAQ

What is acceptable voltage drop?

3% on a branch circuit and 5% total from the service to the farthest outlet, per the informational notes in NEC 210.19 and 215.2. Motors, EV chargers and LED drivers all run better and last longer near those limits.

Is 3% voltage drop required by code?

No. The 3% and 5% figures are informational notes, not enforceable rules, except where a local code adopts them or for specific equipment like fire pumps and sensitive electronics. Inspectors check ampacity; voltage drop is on you.

Aluminum or copper for long runs?

Aluminum has about 61% of copper's conductivity, so it needs about two sizes larger for the same drop (K = 21.2 vs 12.9). For long feeders to a subpanel or detached garage it is usually much cheaper even at the larger size. Use AL-rated lugs and antioxidant compound.

Does length mean one-way?

Yes. Enter the distance from the panel or transformer to the load. The formula multiplies by 2 for single-phase and DC to count the return conductor, and by 1.732 for three-phase.

What wire size for 100 ft at 30 A?

At 240 V, 10 AWG copper drops 3.0% and passes 3%. At 120 V the same wire drops 6.0%, so go to 8 AWG (3.8%).

What is the difference between voltage drop and ampacity?

Ampacity is how much current a wire can carry without overheating (NEC 310.16), and it sets the minimum size. Voltage drop is how much voltage is lost over the length, and on long runs it often makes you go bigger than ampacity alone requires.

What is acceptable voltage drop for LED lighting?

Keep it under 5% for consistent brightness. Up to 10% works with many 12 V landscape fixtures rated for 10 to 15 V, but the far lights will look dimmer.

Should I use 12V or 24V landscape lighting?

24 V carries the same watts at half the current, so it has a quarter of the percent drop on the same wire. Use 24 V for long runs or big loads.

What wire gauge for landscape lighting?

12 AWG for most runs. 100 W at 12 V over 80 ft of 12 AWG drops 21.4%; 10 AWG gets it to 13.4%. Use a multi-tap transformer to boost the voltage on long runs.

Why do my LED strips get dimmer at the end?

Thin strip copper drops voltage along its length. Inject power at both ends or every 16 ft, or use 24 V strips.

How do I wire landscape lights to minimize voltage drop?

Use a hub or T layout instead of a daisy chain, so the main cable runs to the middle of the fixtures, and split long runs into separate home runs from the transformer.

Does wire type affect voltage drop?

Yes. Aluminum drops about 1.6 times as much as copper at the same size. Stranded and solid copper are close enough to treat the same.

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