Info: Calculating Bus Cable length against voltage drop

T.D. · Joined: 18 Feb 2010 Posts: 14

Hi,

maybe for someone the following considerations are helpful:

I had the issue of having a given cable (given R/m) and bus
members connected to them in equal distance to each other.
Since taking the total current and muliplying it with the total R
leads to huge - and false - drop results, I evaluated the formula
for that:

dist_step/2 * (x^2+x)*I*R = Vdrop@x

with:
dist_step : is the distance between each member
x: is the member where the voltage drop is calculated
I: current consumption per member (all the same)
R: resistance per meter (cable)

E.g. having 50 members with 20mA each and 2m space between;
R = 80mOhm/m leads to a drop of 4.1V at member 50 (100m total
distance). Distance at x is certainly given by x*dist_step.

Compare: Itotal=1A, Rtotal(100m) = 8Ohm => Vdrop=8V,
which would be more than pessimistic.

best regards,
T.