Voltage Drop Calculator


What is voltage drop?

A voltage drop (Vdrop) appears when the voltage at the end of the cable (V2) is lower than at the beginning (V1).

A conductor has some resistance (Rwire). When the current flowing through it causes a voltage decreasing. As the length of the cable increases, its resistance and reactance also increase.

Voltage drop is a problem with long cables in large buildings. Also it’s a problem with electric power distribution. We can also find voltage drop between contacts or terminals. This voltage across the cables provokes power losses. So, the larger the current and the longest cable, more losses we get.

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Voltage drop representation

Why voltage drop is important?

As a voltage drop helps to know the efficiency of the current power supplied to the load. We would get 100% efficiency for zero voltage drop. If we supplied 100W and load got 100W, that would mean there are no losses, but this is not the real life. 100% efficiency is impossible.

For long cables it is important to calculate the voltage drops because potential hazards⚠️. If we underestimate the size of the cable or the material of the conductor, we may have safety problems. For instance, that cable can burn out due to the fact of huge dissipation power when a current flows. This cables may becomes a fuse. 🔥🔥

What causes a voltage drop?

Voltage drop may cause malfunction problems in load due to voltage reduction. Imagine we have a DC lamp that needs, at least, 12V to work , but, due to voltage drop, supplies 10V. This DC lamp may not work or may be flicker. You may claim to the lamp manufacturer because it doesn’t work. Actually it is a cable distribution fault. ADVICE: Don’t complain if you didn’t look at the details.

Voltage drop formula

Using the Ohm’s law , the voltage drop in a wire is equal to the current through it times resistance of the wire such us so:

Vdrop (V) = Iwire (A) × Rwire(Ω)

This formula serves for Direct Current (DC) or for Single phase calculation. To measure the drop at 3-phase Voltage we have to square root of three:

Vdrop (V) =√3 × Iwire (A) × Rwire(Ω)

How to calcule a wire resistance?

The parameters for a resistance calculation are: Wire diameter and cross sectional area.

Wire diameter (D) is obtained from manufacturer datasheet and it comes in mm, inches or AWG. If you don’t have the datasheet it can also measured from wire with a caliper, for instance.

Wire area and diameter

The wire cross sectional area (S) is equal to pi divided by 4 times squared wire diameter (D) as following:

S(mm2) =(π/4)×D2

Once we have the cross sectional area (S), we need to know the resistance value per kilometre of the wire. For this calculation we need the material resistivity of the conductor. The cross sectional area is also needed:

Rwire(Ω/km) = 109 × ρ(Ω·m) / S(mm2)

We get the voltage drop by multiplying the current, resistance and cable length (by 2), such so:

Vdrop (V) = Iwire (A) × Rwire(Ω/km) /1000 (m/km)× 2 × L(m)

How far can you run 12 gauge wire on a 20 amp circuit?

You can run a 12 gauge wire up to 52 feet on a 20 amp circuit. Considering 110V AC single phase and copper as conductor material. The circuit will go over the 3% voltage drop recommended by NEC (National Electrical Code®) if you use more than 52 feet.

How do I fix low voltage in my house?

You need to identify where the problem comes from. You have to measure in different contacts or connections of the wire to know which one has the issue. There may be two main reason to have low voltage at home:

  • Load issue (such as appliance, lamp bulb or electrical equipment broken)
  • Inappropriate dimension of the lines (too small AWG for the wire, too far from source).

If you have the second problem, then try to increase the section of the cable or try to close the load to the source.

AWG chart

The AWG stands for American Wire Gauge and it’s a US standard to define wire size. The AWG was implemented as standard in 1857. AWG does not fit in rounded mm or inches, so it has to be rounded up or down to get the conversion value. The technical formula for converting from AWG to mm is the following: 

AWG conversion formula. AWG to mm

See the following AWG conversion table as a reference.

AWG # Diameter
(mm)
Diameter
(inch)
Area
(mm2)
0000 (4/0) 11.6840 0.4600 107.2193
000 (3/0) 10.4049 0.4096 85.0288
00 (2/0) 9.2658 0.3648 67.4309
0 (1/0) 8.2515 0.3249 53.4751
1 7.3481 0.2893 42.4077
2 6.5437 0.2576 33.6308
3 5.8273 0.2294 26.6705
4 5.1894 0.2043 21.1506
5 4.6213 0.1819 16.7732
6 4.1154 0.1620 13.3018
7 3.6649 0.1443 10.5488
8 3.2636 0.1285 8.3656
9 2.9064 0.1144 6.6342
10 2.5882 0.1019 5.2612
11 2.3048 0.0907 4.1723
12 2.0525 0.0808 3.3088
13 1.8278 0.0720 2.6240
14 1.6277 0.0641 2.0809
15 1.4495 0.0571 1.6502
16 1.2908 0.0508 1.3087
17 1.1495 0.0453 1.0378
18 1.0237 0.0403 0.8230
19 0.9116 0.0359 0.6527
20 0.8118 0.0320 0.5176
21 0.7229 0.0285 0.4105
22 0.6438 0.0253 0.3255
23 0.5733 0.0226 0.2582
24 0.5106 0.0201 0.2047
25 0.4547 0.0179 0.1624
26 0.4049 0.0159 0.1288
27 0.3606 0.0142 0.1021
28 0.3211 0.0126 0.0810
29 0.2859 0.0113 0.0642
30 0.2546 0.0100 0.0509
31 0.2268 0.0089 0.0404
32 0.2019 0.0080 0.0320
33 0.1798 0.0071 0.0254
34 0.1601 0.0063 0.0201
35 0.1426 0.0056 0.0160
36 0.1270 0.0050 0.0127
37 0.1131 0.0045 0.0100
38 0.1007 0.0040 0.0080
39 0.0897 0.0035 0.0063
40 0.0799 0.0031 0.0050

Electrical resistivity

This chart represent the electrical resistivity from different materials. The electrical resistivity is represented by the ρ (rho) and is a measure of how a material opposes the flow current. The unit for resistivity is Ω•m. The lower the resistivity, the more the material permits to flow the electric charge. As you can see cooper is one of the best conductor material because of its low resistivity value. In opposite way, teflon and even water (deionized) will be a strong barrier against the current flow.

Material ρ (Ω•m) at 20 °C
Resistivity
Silver 1.59×10−8
Copper 1.68×10−8
Annealed copper 1.72×10−8
Gold 2.44×10−8
Aluminum 2.82×10−8
Calcium 3.36×10−8
Tungsten 5.60×10−8
Zinc 5.90×10−8
Nickel 6.99×10−8
Lithium 9.28×10−8
Iron 1.0×10−7
Platinum 1.06×10−7
Tin 1.09×10−7
Carbon steel (1010)
Lead 2.2×10−7
Titanium 4.20×10−7
Grain oriented electrical steel 4.60×10−7
Manganin 4.82×10−7
Constantan 4.9×10−7
Stainless steel 6.9×10−7
Mercury 9.8×10−7
Nichrome 1.10×10−6
GaAs 5×10−7 to 10×10−3
Carbon (amorphous) 5×10−4 to 8×10−4
Carbon (graphite) 2.5×10−6 to 5.0×10−6 //basal plane
3.0×10−3 ⊥basal plane
Carbon (diamond) 1×1012
Germanium 4.6×10−1
Sea water 2×10−1
Drinking water 2×101 to 2×103
Silicon 6.40×102
Wood (damp) 1×103 to 4
Deionized water 1.8×105
Glass 10×1010 to 10×1014
Hard rubber 1×1013
Wood (oven dry) 1×1014 to 16
Sulfur 1×1015
Air 1.3×1016 to 3.3×1016
Paraffin wax 1×1017
Fused quartz 7.5×1017
PET 10×1020
Teflon 10×1022 to 10×1024