Voltage drop

[tomspark1]

I work with a guy I met just recently. He insists that because voltage drop doesn't affect the first 100' or so, he can use #12 for that section then up the size to #10 for the far section. I did a spit take when he said it then realized he was serious. Try as I might I couldn't convince him this was a specious argument. Anybody have a simple explanation I can use to help him see the " light "?

 

[hillbilly]

I work with a guy I met just recently. He insists that because voltage drop doesn't affect the first 100' or so, he can use #12 for that section then up the size to #10 for the far section. I did a spit take when he said it then realized he was serious. Try as I might I couldn't convince him this was a specious argument. Anybody have a simple explanation I can use to help him see the " light "?

There is voltage drop in the "first 100" feet.

In fact, there is voltage drop in each and every foot of a electrical circuit.....every inch actually.

Just because you can't measure it with a conventional meter doesn't make it go away....it may be insignificant, but it's still there.

So....The voltage drop is a culmination of the conductor material, it's cross section, length, applied voltage, current and temperature.
(These are my words, I'm sure someone else could say it better.)

To say that the first 100' doesn't matter is incorrect.
Installing a piece of #12 for the first 100' and then increasing to #10 for another 100' will cause the circuit to have greater voltage drop than if the circuit is run completely in #10.

All you have to do is add up the total resistance and use Ohm's law to see that this is true.

Just my opinion
steve

 

[barbeer]

When sizing for voltage drop you upsize the other way, larger wire near OCPD and transition to smaller as load diminishes(assuming there is more than one load). If you are running a single piece of equipment or feeding a panel I would strongly think of running larger conductor the entire way.

 

[charlie b]

He insists that because voltage drop doesn't affect the first 100' or so. . . .

He is saying this wrong. The amount of voltage drop will not be very high, if the run is 100 feet or so, and therefore you need not worry about it. But the voltage will drop along each and every inch of that distance (as Steve has already said).

. . . he can use #12 for that section then up the size to #10 for the far section.

He is right. He can do this. And if he does, the total voltage drop would be (1) less that it would have been, if he used #12 for the entire run, and (2) more than it would have been, if he used #10 for the entire run.

If you want to take this back to basic theory, consider the ?first part of the run? and the ?rest of the run? as two resistors in series. You can have (1) a ?#12 resistor? in series with another ?#12 resistor,? or (2) a ?#12 resistor? in series with a ?#10 resistor,? or (3) a ?#10 resistor? in series with another ?#10 resistor.? In all three cases, you will get some voltage drop in the first resistor and some more VD in the second resistor, and they will add up to a ?total VD? for the combined set of two resistors.

 

[charlie b]

When sizing for voltage drop you upsize the other way, larger wire near OCPD and transition to smaller as load diminishes. . . .

Yes, that is frequently the way things are done. But it would not make any difference, in the situation described by the OP.

You are talking about feeding the first load in a series of loads, using the larger wire first, then feeding other loads further downstream, using the smaller wire. In that instance, you do gain an advantage, in that you get better voltage at the first load, and better voltage at each succeeding load, by using the larger wire first. But the OP is talking about running two lengths of wire before hitting the first load. In that circumstance, it does not matter whether the larger wire comes first or second.

 

[LarryFine]

But the OP is talking about running two lengths of wire before hitting the first load. In that circumstance, it does not matter whether the larger wire comes first or second.

You're absolutely correct, of course, but there's something inside me that still makes me want to plug in the heaviest cord closest to the source. :roll:

 

[mivey]

I work with a guy I met just recently. He insists that because voltage drop doesn't affect the first 100' or so, he can use #12 for that section then up the size to #10 for the far section. I did a spit take when he said it then realized he was serious. Try as I might I couldn't convince him this was a specious argument. Anybody have a simple explanation I can use to help him see the " light "?

How about:
Since the first section is not a superconductor, it has resistance and voltage drop issues just like all of the other non-superconductor sections.:roll:

 

[infinity]

Bottom line is that it doesn't really matter which part of this circuit will have the #10 or #12 conductors if both lengths are the same. Put the #10's first, fine. Put the #12's first that's fine too. Personally I like the wire to get smaller as I go away from the source but it's not required.

 

[ceb58]

I had a good one this last weekend. Lady calls needing a recp. in horse pasture, going to have a shelter with tack room built. They need power for their tools and after the build is built I come back and wire for lights and recp. for fan. I measure off distance from house 570 ft. Come back to the good old free voltage drop calc. here on Mike Holt's web site. #6 for a 120v 20a circ. She had heart failure when I gave price but dug deep in to her pockets

 

[LarryFine]

I measure off distance from house 570 ft. Come back to the good old free voltage drop calc. here on Mike Holt's web site. #6 for a 120v 20a circ.

I wonder if it would have paid to run 240v and stepped down to 120v at the shelter?

 

[hillbilly]

I had a good one this last weekend. Lady calls needing a recp. in horse pasture, going to have a shelter with tack room built. They need power for their tools and after the build is built I come back and wire for lights and recp. for fan. I measure off distance from house 570 ft. Come back to the good old free voltage drop calc. here on Mike Holt's web site. #6 for a 120v 20a circ. She had heart failure when I gave price but dug deep in to her pockets

You could have installed 100A Mobile Home Feeder for less money and had more power.:smile:

#6 cu, 120V, 20A, 570ft.....I calculate almost 11V drop.

#2 Al, 120V, 20A, 570ft...I calculate around 6V drop.

Add extra savings if you ran the copper in conduit, as the Al feeder can be direct buried.

steve

 

[mivey]

I wonder if it would have paid to run 240v and stepped down to 120v at the shelter?

Hmmm, that would be worth looking at. You might could drop to a #10 (even #12 if you push it)?

As a reference, I'm looking at 120 volts at 16 amps with #6 for 570 feet gets you 111 volts. I'm assuming this is adequate for the needs.

For 240 volts at 9 amps (assuming some inefficiency) you could use #10 to get a voltage of 228 (114 on 120v basis).

FWIW, for 240 volts at 9 amps (assuming some inefficiency) you could use #12 to get a voltage of 220 (110 on 120v basis).

I'm not sure what kind of load the tools would have but the starting current is an issue but I'm assuming it was ok with the #6.

 

[mivey]

You could have installed 100A Mobile Home Feeder for less money and had more power.:smile:

#6 cu, 120V, 20A, 570ft.....I calculate almost 11V drop.

#2 Al, 120V, 20A, 570ft...I calculate around 6V drop.

Add extra savings if you ran the copper in conduit, as the Al feeder can be direct buried.

steve

You know he used #6 UF, right?:grin:

 

[weressl]

I work with a guy I met just recently. He insists that because voltage drop doesn't affect the first 100' or so, he can use #12 for that section then up the size to #10 for the far section. I did a spit take when he said it then realized he was serious. Try as I might I couldn't convince him this was a specious argument. Anybody have a simple explanation I can use to help him see the " light "?

This is not a practical example, but you'll get the gist of it.

Lets say you calculate your voltage drop on an entire run of 300' cable to be 3.5% for a #12 run. A #10 run would give you a 2.5% run for the same length. Then you run the calculation for a combination of the two and find that 225' of #12 and 75' of #10 would give you the exact 3% you're looking for. This would save you the cost differential of the 225' between the #10 and #12, but you add the cost of the splicing.Of course you would think that one would put the #10 closer to the solurce, but if you are pulling it as one length, pre-splice, from the laod, you may want to pull the #12 in first and drag the #10 on the end.

 

[charlie b]

I wonder if it would have paid to run 240v and stepped down to 120v at the shelter?

I wondered the same thing. With the price of copper these days, it might have been worth at least doing a cost comparison.

 

[barbeer]

Like I planned it.........a great article showed up in the weekly IAEI newsletter from EC&M:

http://ecmweb.com/training/electrical_basics/electric_calculating_series_voltage/

 

[LarryFine]

Then you run the calculation for a combination of the two . . .

I wouldn't choose making a splice over using the larger size the whole way . . .

. . . if you are pulling it as one length, pre-splice . . .

. . . especially making a splice in the conduit!

 

[e57]

What about a MWBC, #12, and #10 the neutral the whole way....
This way you get lower neutral current when lightly out of balance, and lower return restistance when heavily out of balance...

I know that doesn't help the op explain the guys ill's, but helps the problem itself....

 

[hillbilly]

You know he used #6 UF, right?:grin:

No I didn't, where did you see that?
6/2?

I'll still bet that the #2 Aluminum cable is cheaper....even with the extra conductor:smile: .

steve

 

[mivey]

No I didn't, where did you see that?
6/2?

I'll still bet that the #2 Aluminum cable is cheaper....even with the extra conductor:smile: .

steve

I just made it up (why not 6/2 UF w/g?), but I thought it would be fairer to compare cheaper method with cheaper method.

If someone ever gets around to it, we might get an actual cost comparison including a step-down transformer. I sure haven't felt like it. Maybe when I get back to town.