Voltage Drop Calculator: Circuit Volt Loss Over Distance

This voltage drop calculator determines the voltage drop across a circuit, particularly useful for long wire runs. It computes the voltage drop based on input parameters like voltage, current, wire size, conductor type, and circuit distance. Additionally, it calculates the voltage at the load and the percentage voltage drop.

How to Calculate Voltage Drop

Voltage drop calculation is grounded in one of the most fundamental electrical engineering principles: Ohm’s Law. Ohm’s Law posits that the voltage difference across a conductor is directly proportional to the current flowing through it and the conductor’s total resistance.

Voltage drop is determined by multiplying the current and the resistance within the conductor. This concept has led to the derivation of a straightforward formula for computing the voltage drop in a conductor.

You can accurately determine the voltage drop for a specific circuit and select the appropriate wire gauge to ensure the drop remains within acceptable limits.

According to the National Electrical Code, the voltage drop in a feeder circuit should not exceed 5%, and in a branch circuit, it should be limited to 3% or less.

Input Parameters:

Voltage: Enter the source voltage of the circuit.

• DC 12V, 24V, 48V
• Single-phase voltages are 115V or 120V
• Three-phase voltages are 208V, 230V, 240V, or 480V.

Amperes: Enter the maximum current (in amps) expected to flow through the circuit. For motor circuits, use the nameplate Full Load Amperage (FLA) multiplied by 1.25.

Wire Size: Select the wire size in American Wire Gauge (AWG) or kcmil.

Conductor: Choose between common conductor materials like Copper or Aluminum.

Distance: Input the one-way length of the wire in the circuit in feet.

Key Information:

• Conductor Temperature: Calculations are based on a conductor temperature of 75°C.
• Source Reference: NFPA 70, National Electrical Code, Chapter 9, Table 8.

Calculation Formulas:

Calculation Formulas:

Ohm’s Law: Voltage drop (V_d​) is calculated as V_d​=I×R, where I is current and R is resistance.

Single-Phase Circuits: V_d=(2×K×L×I)/Cm​

Three-Phase Circuits: V_d=(1.73×K×L×I)/Cm​​

Where:

• Vd​ = Voltage Drop
• I = Current in Conductor (Amps)
• L = One-way Length of Circuit (Feet)
• Cm = Cross Sectional Area of Conductor (Circular Mils)
• K = Resistance in ohms of 1 circular mil foot of conductor (12.9 for Copper, 21.2 for Aluminum at 75°C)

AWG chart

AWG #	Diameter (inch)	Diameter (mm)	Area (kcmil)	Area (mm2)
0000 (4/0)	0.4600	11.6840	211.6000	107.2193
000 (3/0)	0.4096	10.4049	167.8064	85.0288
00 (2/0)	0.3648	9.2658	133.0765	67.4309
0 (1/0)	0.3249	8.2515	105.5345	53.4751
1	0.2893	7.3481	83.6927	42.4077
2	0.2576	6.5437	66.3713	33.6308
3	0.2294	5.8273	52.6348	26.6705
4	0.2043	5.1894	41.7413	21.1506
5	0.1819	4.6213	33.1024	16.7732
6	0.1620	4.1154	26.2514	13.3018
7	0.1443	3.6649	20.8183	10.5488
8	0.1285	3.2636	16.5097	8.3656
9	0.1144	2.9064	13.0927	6.6342
10	0.1019	2.5882	10.3830	5.2612
11	0.0907	2.3048	8.2341	4.1723
12	0.0808	2.0525	6.5299	3.3088
13	0.0720	1.8278	5.1785	2.6240
14	0.0641	1.6277	4.1067	2.0809
15	0.0571	1.4495	3.2568	1.6502
16	0.0508	1.2908	2.5827	1.3087
17	0.0453	1.1495	2.0482	1.0378
18	0.0403	1.0237	1.6243	0.8230
19	0.0359	0.9116	1.2881	0.6527
20	0.0320	0.8118	1.0215	0.5176
21	0.0285	0.7229	0.8101	0.4105
22	0.0253	0.6438	0.6424	0.3255
23	0.0226	0.5733	0.5095	0.2582
24	0.0201	0.5106	0.4040	0.2047
25	0.0179	0.4547	0.3204	0.1624
26	0.0159	0.4049	0.2541	0.1288
27	0.0142	0.3606	0.2015	0.1021
28	0.0126	0.3211	0.1598	0.0810
29	0.0113	0.2859	0.1267	0.0642
30	0.0100	0.2546	0.1005	0.0509
31	0.0089	0.2268	0.0797	0.0404
32	0.0080	0.2019	0.0632	0.0320
33	0.0071	0.1798	0.0501	0.0254
34	0.0063	0.1601	0.0398	0.0201
35	0.0056	0.1426	0.0315	0.0160
36	0.0050	0.1270	0.0250	0.0127
37	0.0045	0.1131	0.0198	0.0100
38	0.0040	0.1007	0.0157	0.0080
39	0.0035	0.0897	0.0125	0.0063
40	0.0031	0.0799	0.0099	0.0050

Additional Notes:

• The results are estimations and may vary based on wire condition, temperature, connector quality, etc.
• Voltage drop should ideally be less than 5% under full load for optimal performance.
• Voltage drop factors include wire material, size, length, and current carried.
• Ensure proper cable selection and adherence to bundling rules for accurate ampacity and voltage drop estimations.

Tools and Resources:

• Circuit Distance Calculator: For determining the maximum circuit distance based on the desired voltage drop.
• Wire Size Calculator: To calculate the appropriate wire size for your circuit.
• Advanced Wire Size/Ampacity Calculator: For more detailed wire sizing and ampacity calculations.

Related tools:

• Watts to Amps Calculator: DC/AC Wattage to Amps Conversion