Landscape Lighting

Calculating Voltage Drop across a Wire

Star-Tip 1021

Gardening Tips for successful and beautiful Landscapes and Gardens

* gauge of wire has these values of Resistance ”R” and Constant “C”

10 GA -> R = .001020 -> E/R = 11,764 C = 5,800

12 GA -> R = .001588 -> E/R = 7,547 C = 3,700

14 GA -> R = .002525 -> E/R = 4,752 C = 2,300

16 GA -> R = .004016 -> E/R = 2988 C = 1,500

Watts (on cable) x Length of wire run = Voltage Drop on wire

Constant of wire

Voltage drop should not exceed 1 volt on a 12 v system; Use V = 1 as a standard.

----------------------------------------------------

If you know the Wattage (W) and Constant (C) of the wire,

and desire the Length of run (L):

L = C ¸ W

----------------------------------------------------

If you know the Wattage (W) and Length of run (L),

And desire to know what Gauge wire; use wire gauge with constant greater than:

C = L x W

Ex: 50 watt lamps, with 100 foot run

C = 50 x 100 = 5,000 = 10GA (C= 5,800; greater than 5,000)

----------------------------------------------------

If you know the Constant (C) of the wire and the Length of run,

But desire to know how many watts you can supply:

W = C ¸ L

Ex: 12 GA wire (C = 3,700) with 50 foot run

W = 3,700 ¸ 50 = 74 watts

----------------------------------------------------

If you choose to use a wire gauge with too small a Constant, or attempt to push more power across the wire run than calculations indicate, the result could be dangerous or expensive. If the wire over heats, it can start a fire. If certain bulbs do not receive enough power (due to excessive voltage drop) the bulbs will burn our prematurely.