Voltage Drop Calculator

This is a calculator for the estimation of the voltage drop of an electrical circuit. The "NEC data" tab calculates based on the resistance and reactance data from the National Electrical Code (NEC). The "Estimated resistance" tab calculates based on the resistance data estimated from the wire size. Click the "Other" tab to use customized resistance or impedance data, such as data from other standards or wire manufacturers.

Wire material
Wire size
Material of conduit
Power factor (PF)

Wire material
Wire size

Wire impendence
or resistance

Voltage
Phase
Number of conductors
Distance (one-way)
Load current	Amps

When electrical current moves through a wire, it is pushed by electrical potential (voltage) and it needs to surpass a certain level of contrary pressure caused by the wire. The voltage drop is the amount of electrical potential (voltage) loss caused by the contrary pressure of the wire. If the current is alternating, such contrary pressure is called impedance. Impedance is a vector, or two-dimensional quantity, consisting of resistance and reactance (reaction of a built-up electric field to a change of current). If the current is direct, the contrary pressure is called resistance.

Excessive voltage drop in a circuit can cause lights to flicker or burn dimly, heaters to heat poorly, and motors to run hotter than normal and burn out. It is recommended that the voltage drop should be less than 5% under a fully loaded condition. This can be achieved by selecting the right wire, and by taking care in the use of extension cords and similar devices.

There are four major causes of voltage drop:

The first is the choice of material used for the wire. Silver, copper, gold, and aluminum are among the metals with the best electrical conductivity. Copper and aluminum are the most common materials used for wires due to their relatively low price compared with silver and gold. Copper is a better conductor than aluminum and will have less voltage drop than aluminum for a given length and wire size.

Wire size is another important factor in determining voltage drop. Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. In American wire gauge, every 6-gauge decrease doubles the wire diameter, and every 3-gauge decrease doubles the wire cross sectional area. In the Metric Gauge scale, the gauge is 10 times the diameter in millimeters, so a 50 gauge metric wire would be 5 mm in diameter.

Still another critical factor in voltage drop is wire length. Shorter wires will have less voltage drop than longer wires for the same wire size. Voltage drop becomes important when the length of a run of wire or cable becomes very long. Usually this is not a problem in circuits within a house, but may become an issue when running wire to an outbuilding, well pump, etc.

Finally, the amount of current being carried can affect voltage drop levels; an increase in current through a wire results in an increased voltage drop. Current carrying capacity is often referred to as ampacity, which is the maximum number of electrons that can be pushed at one time – the word ampacity is short for ampere capacity.

The ampacity of a wire depends on a number of factors. The basic material from which the wire is made is, of course, an important limiting factor. If alternating current is being sent through the wire, the speed of alternation can affect ampacity. The temperature in which the wire is used can also affect ampacity.

Cables are often used in bundles, and when they are brought together, the total heat which they generate has an effect on ampacity and voltage drop. There are strict rules about bundling cables which must be followed for this reason.

Cable selection is guided by two main principles. First, the cable should be able to carry the current load imposed on it without overheating. It should be able to do this in the most extreme conditions of temperature it will encounter during its working life. Second, it should offer sufficiently sound earthing to (i) limit the voltage to which people are exposed to a safe level and (ii) allow the fault current to trip the fuse in a short time.

Voltage drop calculation

Ohm's Law is a very basic law for calculating voltage drop:

V_drop = I·R

where:

I: the current through the wire, measured in amperes

R: the resistance of the wires, measured in ohms

The resistance of the wires is often measured and given as length-specific resistance, normally in the unit of ohms per kilometer or ohms per 1000 feet. Also, the wire is round-tripped. Therefore, the formula for a single-phase or direct current circuit becomes:

V_drop = 2·I·R·L

The formula for a three-phase circuit becomes:

V_drop = √3·I·R·L

where:

I: the current through the wire

R: the length-specific resistance of the wires

L: the one-way length

Typical AWG wire sizes

American Wire Gauge (AWG) is a wire gauge system used predominantly in North America for the diameters of round, solid, non-ferrous, electrically conducting wire. The following is a list of typical AWG wires and their sizes:

AWG	Diameter		Turns of wire		Area		Copper resistance
AWG	inch	mm	per inch	per cm	kcmil	mm²	Ω/km	Ω/1000ft
0000 (4/0)	0.4600	11.684	2.17	0.856	212	107	0.1608	0.04901
000 (3/0)	0.4096	10.404	2.44	0.961	168	85.0	0.2028	0.06180
00 (2/0)	0.3648	9.266	2.74	1.08	133	67.4	0.2557	0.07793
0 (1/0)	0.3249	8.252	3.08	1.21	106	53.5	0.3224	0.09827
1	0.2893	7.348	3.46	1.36	83.7	42.4	0.4066	0.1239
2	0.2576	6.544	3.88	1.53	66.4	33.6	0.5127	0.1563
3	0.2294	5.827	4.36	1.72	52.6	26.7	0.6465	0.1970
4	0.2043	5.189	4.89	1.93	41.7	21.2	0.8152	0.2485
5	0.1819	4.621	5.50	2.16	33.1	16.8	1.028	0.3133
6	0.1620	4.115	6.17	2.43	26.3	13.3	1.296	0.3951
7	0.1443	3.665	6.93	2.73	20.8	10.5	1.634	0.4982
8	0.1285	3.264	7.78	3.06	16.5	8.37	2.061	0.6282
9	0.1144	2.906	8.74	3.44	13.1	6.63	2.599	0.7921
10	0.1019	2.588	9.81	3.86	10.4	5.26	3.277	0.9989
11	0.0907	2.305	11.0	4.34	8.23	4.17	4.132	1.260
12	0.0808	2.053	12.4	4.87	6.53	3.31	5.211	1.588
13	0.0720	1.828	13.9	5.47	5.18	2.62	6.571	2.003
14	0.0641	1.628	15.6	6.14	4.11	2.08	8.286	2.525
15	0.0571	1.450	17.5	6.90	3.26	1.65	10.45	3.184
16	0.0508	1.291	19.7	7.75	2.58	1.31	13.17	4.016
17	0.0453	1.150	22.1	8.70	2.05	1.04	16.61	5.064
18	0.0403	1.024	24.8	9.77	1.62	0.823	20.95	6.385
19	0.0359	0.912	27.9	11.0	1.29	0.653	26.42	8.051
20	0.0320	0.812	31.3	12.3	1.02	0.518	33.31	10.15
21	0.0285	0.723	35.1	13.8	0.810	0.410	42.00	12.80
22	0.0253	0.644	39.5	15.5	0.642	0.326	52.96	16.14
23	0.0226	0.573	44.3	17.4	0.509	0.258	66.79	20.36
24	0.0201	0.511	49.7	19.6	0.404	0.205	84.22	25.67
25	0.0179	0.455	55.9	22.0	0.320	0.162	106.2	32.37
26	0.0159	0.405	62.7	24.7	0.254	0.129	133.9	40.81
27	0.0142	0.361	70.4	27.7	0.202	0.102	168.9	51.47
28	0.0126	0.321	79.1	31.1	0.160	0.0810	212.9	64.90
29	0.0113	0.286	88.8	35.0	0.127	0.0642	268.5	81.84
30	0.0100	0.255	99.7	39.3	0.101	0.0509	338.6	103.2
31	0.00893	0.227	112	44.1	0.0797	0.0404	426.9	130.1
32	0.00795	0.202	126	49.5	0.0632	0.0320	538.3	164.1
33	0.00708	0.180	141	55.6	0.0501	0.0254	678.8	206.9
34	0.00630	0.160	159	62.4	0.0398	0.0201	856.0	260.9
35	0.00561	0.143	178	70.1	0.0315	0.0160	1079	329.0
36	0.00500	0.127	200	78.7	0.0250	0.0127	1361	414.8
37	0.00445	0.113	225	88.4	0.0198	0.0100	1716	523.1
38	0.00397	0.101	252	99.3	0.0157	0.00797	2164	659.6
39	0.00353	0.0897	283	111	0.0125	0.00632	2729	831.8
40	0.00314	0.0799	318	125	0.00989	0.00501	3441	1049