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# Voltage drop/ Cable size selection

## Voltage drop/ Cable size selection

#1
08-04-15, 06:53 AM
Member
Join Date: Aug 2015
Location: Australia
Posts: 4
Voltage drop/ Cable size selection

Hello,

I have a question regarding the voltage drop calculation for a cable.

How do we select the length of the cable?- Is it the one way distance between the source and the equipment or do we need to consider the return path?

Can someone please explain this in detail and whether it applies to both AC and DC systems?

Thank you.

#2
08-04-15, 07:04 AM
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Join Date: Aug 2015
Location: Puerto Rico
Posts: 8
#3
08-04-15, 07:34 AM
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Join Date: Mar 2006
Location: USA
Posts: 29,711
Is it the one way distance between the source and the equipment
Yes, one way distance for most charts. The chart should say.
Can someone please explain this in detail and whether it applies to both AC and DC systems?
DC systems have a greater drop over distance.

#4
08-04-15, 10:46 AM
Member
Join Date: Mar 2010
Location: USA
Posts: 4,501
Raw voltage drop calculations give you the number of volts lost in a given piece of wire. The number of volts lost is the same regardless of the circuit voltage, all other things (amperes, etc.) being equal.

Using raw voltage drop calculations you use the round trip distance. For cable running to a shed or dock or other structure don't forget the portion of the run from the location where the cable enters the main building over to the breaker panel.

THe number of volts dropped (lost; dissipated) in a length of wire is equal to the number of amperes flowing at that moment times the resistance of that length of wire. This formula can be used as-is for both DC systems and all common AC (60 Hz or less) systems.

For lower voltage systems the same number of volts lost represents a greater percentage of the supply voltage. To stay within the usually suggested 4% or less voltage drop ou may need to do some extra manual calculations resulting in a fatter wire needed compared with a higher voltage system. For a feed with both 120 volts and 240 volts (typical and often the only practical house to shed feed) it is customary to figure voltage drop for 240 volts at the house panel rated breaker amperage.

Last edited by AllanJ; 08-04-15 at 11:18 AM.