# Any one have a good landscape lighting voltage drop calculator

##### Member
I'm looking for a low voltage calculator that figures the voltage drop when designing a low voltage lighting system.

I have seen the ones availiable on the internet but I can find one that calculates voltage drop based on the distances from the power source to the first load, then the next load and so forth to the last load or furthest load.

It would seem to me that one could more accuratly know if they have droped to much with a calculator that takes the distance between each fixture into account and not just a total load and a total distance.

#### gar

##### Senior Member
110504-2352 EDT

If you want high accuracy, then it is a very difficult problem to solve because the loads, assuming incandescent lamps, are a somewhat non-linear load. These are between a pure constant resistance and a constant current load.

Some sort of better approximation than total load and total distance is probably what you want. Thus, what is the problem you want to solve? Is it the variation in intensity as you progress along the string or something else?

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##### Member
I would like to be able toplan out landscape lighting jobs with a better idea of what will work, keepingthe light output of the farthest fixtures to acceptable levels.

I have yet to find anythingexcept the ones that generalize the installation. I was hoping for moreaccurate figures when the distances and loads are known. I know I could use thegeneral guidelines that manufacturers have but my experience has been that thenumbers are conservative. I would like to know more accurately what the numbersare to know if a certain situation could be accommodated. Right know I wouldhave to install and test to see if there was room for additional loads or if Iwas already pushing the limits of the layout

Staff member

#### gar

##### Senior Member
110505-1332 EDT

Assume the maximum voltage difference between the brightest and dimmest lights is (120-115)/120 = 4.2% and the lamp current is constant and the same for each lamp, then the solution is moderately simple. The voltage drop to the first lamp is V1 = N*I1*R1 where I1 is the current to one bulb, N is the total number of bulbs (assuming all are identical), and R1 is the resistance from the voltage source to the first bulb. The voltage drop between the first bulb and the second is V2 = (N-1)*I1*R2 where R2 is the resistance between the first and second bulbs, etc.

What do you do with the return path? This doubles the voltage drop if the supply and return wires are the same material and diameter.

If you progressively change wire size and bulb-to-bulb distances, then the individual Rs have to change between segments.

If you change the wiring topology from two parallel wires to a three wire scheme, then you can more easily reduce light intensity variation.

You could create a spreadsheet to do the calculations.

See my photo P9 at http://beta-a2.com/EE-photos.html for an approximation of lamp intensity vs voltage.

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