Voltage Drop, Cable size and Equipment lugs

[3brothers]

I have a 96A load at 480V that is 1500' away from the source. I can increase the cable size to 4/0 which will reduce the VD to 3.225%.
But then the equipment on both the line side and the load side cannot accept any cable larger than a #1 or possibly a 1/0.

If I reduce the size of the cable at the beginning of the run and the end of the run, for instance use an in-line-splice reducer from #4/0 to #1 for the first and last 12" of cable, will this effect my voltage drop calculations?
Or will it still be at 3.225%?

 

[Dennis Alwon]

Reducing that little bit will not make a significant difference

 

[Dennis Alwon]

They do make these

 

[winnie]

To amplify what Dennis Alwon says: of course having a bit of smaller size wire will increase voltage drop...but for a few inches at either end of a long run the change in voltage drop is not significant. Even 10 feet of smaller wire at each end won't make a significant difference in the voltage drop of a 1500 foot run.

I'd suggest that rather than fighting to use as much 4/0 as possible, you use the smallest suitable wire at the terminations, for as long as is convenient, and then transition to the 4/0. There is no reason to fight bending 4/0 in a tight enclosure, change over to the 4/0 in a comfortably sized junction box.

By 'not significant' I specifically mean that the calculated difference in voltage drop will be smaller than the errors in calculation because of unknowns such as wire manufacturing tolerance or conductor ambient temperature.

-Jon

 

[JFletcher]

Welcome to MH forum. Switching to #1 or 1/0 the last few feet will not significantly affect your voltage drop.

 

[3brothers]

Thank You all!

Thank You all!

The answers are what I thought, but I had a hard time explaining why to a co-worker. I tried to explain to him that voltage drop is only a function of distance, but I could not explain what physically happens with the electrons and neutrons that ride on the copper to cause this and why if the source amperage and voltage do not change why does the voltage change with distance.

It's not necessary for me to find out what happens, life will go on with out me knowing, but just out of curiosity, I figured if anybody could give and answer that a non-engineer could understand and explain to some seasoned Journeymen and Foreman, it would be on this forum.

Thank You for you replies and Thank You ahead of time for any response to part 2 of my question.

 

[GoldDigger]

The wire has a non zero resistance. That causes a voltage from one end to the other that is proportional to both the current and the length of the wire.
As for the electrons, basically they bump into atoms in the wire and that slows them down. To keep them moving at the speed required to maintain the current takes a voltage difference between the two ends of each piece of wire.
Going back to the old water analogy, there is drop in pressure from one end of the pipe to the other when water is flowing.
That pressure drop will be the same no matter what overall pressure you start with.

mobile

 

[Barbqranch]

You calculated the voltage drop across 1500 feet of wire. Now do the same for 1490 feet of the same wire, and also do it for 10 feet of your #1 at the 96 amp load. Add the two voltage drops together, and you will get the result of adding the smaller wire at the ends.

 

[GoldDigger]

You calculated the voltage drop across 1500 feet of wire. Now do the same for 1490 feet of the same wire, and also do it for 10 feet of your #1 at the 96 amp load. Add the two voltage drops together, and you will get the result of adding the smaller wire at the ends.

And a hint for the really lazy person:
Forget about calculating for 1500' and 1490'. Just calculate for the 10' of smaller wire and that will be an upper bound on the increase in voltage drop.

 

[ggunn]

The answers are what I thought, but I had a hard time explaining why to a co-worker. I tried to explain to him that voltage drop is only a function of distance, but I could not explain what physically happens with the electrons and neutrons that ride on the copper to cause this and why if the source amperage and voltage do not change why does the voltage change with distance.

It's not necessary for me to find out what happens, life will go on with out me knowing, but just out of curiosity, I figured if anybody could give and answer that a non-engineer could understand and explain to some seasoned Journeymen and Foreman, it would be on this forum.

Thank You for you replies and Thank You ahead of time for any response to part 2 of my question.

Voltage drop is a function of total series resistance. Case 1: what is the resistance of 1500' of 4/0? Case 2: what is the resistance of 1490' of 4/0 plus the resistance of 10' of #1? Vd=IR. Do the math and show him.

 

[Smart $]

Voltage drop is a function of total series resistance. Case 1: what is the resistance of 1500' of 4/0? Case 2: what is the resistance of 1490' of 4/0 plus the resistance of 10' of #1? Vd=IR. Do the math and show him.

Pictures help.

Basic one-wire-size calculation depicted on left.

On right, calculate with the reduced-size ends as R1a, R1c, R2a, and R2c, and the long, larger-size run as R1b and R2b.