I tried reading through the Google newsgroup archives to see if maybe this

question had been asked and answered already, but as soon as I got to an old

post by Altavoz it became too painful to continue. So, here goes.

If you're running a motor load at the end of a length of wire, it seems easy

enough to use a table or a program to determine the appropriate gauge of

wire to use to keep the voltage drop within a desired range. But how do

you calculate what to use if the wire gauges change midway through the run?

Here's an example. Let's say you have a three horsepower, 240 volt, single

phase motor at the end of 300 feet of 10 gauge wire. From what I have come

up with when plugging those values into some of the online voltage drop

calculators, that's pretty much the maximum you'd want to do in order to keep

below a 5% voltage drop. But what if you have 200 feet of wire leading up

to the point where that 300 foot run of 10 gauge begins? It's easy enough

if you were using 10 gauge the whole way, but what if you had 200 feet of

2 gauge, then 300 feet of 10 gauge? How does that change things?

The above example is taken from a real-world installation of a submerged well

pump where the pump is 300 feet down, and there's a 200 foot run from the

meter to the junction box for the well pump. I'm wondering because not only

would it answer some questions about the efficiency of the well pump, but

also because in the future there would be maybe a 200 foot run from the

meter to a subpanel for a shop that would be running motors on lathes, mills,

etc. Granted, there wouldn't be a 300 foot run of cable from a lathe/mill

motor back to the subpanel, but I'm assuming the calculations would be done

the same way, just using different values for distances. Any bones that

could be thrown to a guy who was never very good at math would be much

appreciated.

- posted
16 years ago