Calculating voltage drop on paralleled conductors

[texasmike]

I have a situation where I need to figure the voltage drop on a branch circuit to a 3 phase motor. Because of the distance involved - 2000 feet - the original installation was four (4) sets of conductors in parallel. Now the original motor is being changed out.

How do I figure this? Treat is as combining equal resistances in parallel - figure the drop for one run and divide that result by 4?

Appreciate your input and advice.

 

[tkb]

I think that you add the circular mills of all the parallels and use that as the wire size.

 

[Smart $]

Calculate using one set and one-fourth the current. For example, where the motor load is 30A, calculate the voltage drop using numbers for one set of conductors at 7.5A. The voltage drop is the same for each set.

 

[Ingenieur]

I have a situation where I need to figure the voltage drop on a branch circuit to a 3 phase motor. Because of the distance involved - 2000 feet - the original installation was four (4) sets of conductors in parallel. Now the original motor is being changed out.

How do I figure this? Treat is as combining equal resistances in parallel - figure the drop for one run and divide that result by 4?

Appreciate your input and advice.

yes
figure drop for 1 and divide by 4 or divide the Z for 1 conductor by 4 and calc
same thing

 

[Julius Right]

In my opinion, if the cables run as triplex then you may divide the voltage drop by 4.In the case of single core cables in flat positions-for instance single cable per duct in a duct bank- the reactance depends on phase order and distance between ducts.
It could be a current difference between parallel cables-in amplitude but power factor also.
If the cables run without any free space between-as in a cable tray-the unbalance could be elevated.

 

[winnie]

I think that the cable configuration point in post #5 is important for the run described, in other words: texasmike, more details are needed.

If you have a _single_ run and calculate its voltage drop, and then you duplicate that run so now you have 4 runs that are exactly the same, then the voltage drop will be 1/4 of your original. But by duplicate, I mean everything the same: same conductor size, same conduit, and same arrangement in the conduit.

But, say for example that you run the calculation for 3 conductors in a single conduit, but your installation is 'isolated phase' with 4 conductors per conduit in 3 separate conduits, you could get a significantly different voltage drop, especially at motor starting when power factor is low.

Spacing between conductors does not change the DC voltage drop, but adds inductive reactance to the voltage drop.

-Jon

 

[Sahib]

I can only imagine the cables run as triplex and not as single cables, because the possible voltage unbalance, in the case of single cables due to wrong placement in the field, could damage the motor.