# Voltage Drop

Voltage Drop is a measurement of the amount of voltage dropped through the length of a circuit given the circuit conductor size and material and the conditions under which the circuit is constructed and operated. The voltage drop calculated is the percent voltage loss from the originating source voltage. The U.S. National Electric Code establishes a maximum percentage loss acceptable for certain circuit uses.

$CM=100K_{p}frac{2K_{m}LI}{VDE}$

$VD=100K_{p}frac{2K_{m}LI}{CME}$

$E=100K_{p}frac{2K_{m}LI}{CMVD}$

$L=frac{CMVDE}{100K_{p}2K_{m}I}$

$I=frac{CMVDE}{100K_{p}2K_{m}L}$

Where VD is the percent voltage drop, Kp is a constant for the circuit phase (single, 3-phase, etc.), L is the length of the run, Km is a constant for the conductor material (i.e., copper, aluminum, etc.), I is the circuit load current, CM is the size of the conductor in circular mils, and E is the source voltage.

The following tables are used for the constant values (defaults in bold):

Material (Km)
Description Value
Copper 12.9
Aluminum 21.2

Note that the approximate K constants used here are for uncoated copper (Cu) and compact aluminum (Al). The formula used to derive the exact value is:

$frac{RCM}{1000}$

Where R is the wire resistance in ohms per 1000 feet and CM is the wire size in circular mils. Both of these values are taken from NFPA 70 Table 8.

Size (CM)
Description Value
18 AWG 1620
16 AWG 2560
14 AWG 4110
12 AWG 6530
10 AWG 10380
8 AWG 16510
6 AWG 26240
4 AWG 41740
3 AWG 52620
2 AWG 66360
1 AWG 83690
1/0 AWG 105600
2/0 AWG 133100
3/0 AWG 167800
4/0 AWG 211600
250 kcmil 250000
300 kcmil 300000
350 kcmil 350000
400 kcmil 400000
500 kcmil 500000
600 kcmil 600000
700 kcmil 700000
750 kcmil 750000
800 kcmil 800000
900 kcmil 900000
1000 kcmil 1000000
1250 kcmil 1250000
1500 kcmil 1500000
1750 kcmil 1750000
2000 kcmil 2000000
Phase (Kp)
Description Value
Single phase 1.000
3-phase 3-wire 0.866
3-phase 4-wire 0.500