I am not certain I completely understand voltage drop. I’ve used the tables from Rex Caudwell’s Q&A piece in FH#137 to calculate voltage drop for 120 volt circuits. Given the same wire guage and type, circuit distance and load, the voltage drop for a 240 volt circuit is halfed as I understand it. I need to run a line from the power pedestal to a cabin which is 300 feet away and which will have a 240 volt panel. The electrical load at the cabin will be about 50 amps. I’m trying to size the wire with no more than a 5% voltage drop (as recommended by Caudwell). So should I be looking at the percentage drop for 120 volts or 240 volts? Thanks for any advice.
Also, for those who might be concerned, I have a fair bit of experience with residential wiring and have permits for this work.
Replies
Use 240vac in your calculations. Also, if you google on 'voltage drop calculator', there are a number of online calculators out there where you can simply plug in the numbers and click the button to get results.
Voltage drop has to do with wire size, wire length, and current. A wire of a given material (aluminum or copper) and diameter has a given resistance per foot. Ohms law tells us that the voltage from one end of that wire to the other will be the total resistance times the current. Since total resistance is resistance per foot times feet, you get something like this:
I = current (don't ask me why it's not C or A -- standards is standards)
Rf = resistance/foot
F = feet
So voltage drop Vd = I * Rf * F.
The convenient thing about 240V vs 120 is that, since power = voltage times current, it only takes half as much current to get the same amount of power at the far end of the wire. So we can replace I in te above equation:
P = total power requirement
Vs = supply voltage (120 or 240)
I = P / Vs
Voltage drop is then Vd = (P / Vs) * Rf * F
So you can see that doubling Vs halves Vd, for a given power level.
Those formulas apply to a 240 volt load. With that in mind, you had better make sure the load at your cabin's panel is evenly distributed or you will have a 300' long 120 volt extension cord.
This is a bit of an exaggeration to make a point, but those formulas are mainly used to calculate voltage drop to an appliance or tool that operates at 240 volts. You can have a 240 volt panel at the cabin, but if the current draw is heavily weighted to one leg of the circuit, you will experience much more than the calculated percentage of drop.
Well, yes and no. Presumably, if you're designing for, say, 10KW, you can never draw all that on one leg -- at most 5KW on each.If you draw 5KW on one leg and nothing on the other, the voltage drop across that "hot" leg is exactly the same as if you were drawing 10KW evenly split between the two legs (since voltage drop is a function of current, and the current will be the same in both cases).However, an oddity is that, since in the 5KW case all the "return" current goes through the neutral, you have the same amount of voltage drop (perhaps it should be called "voltage rise") in the neutral as on the "hot" leg. So the total voltage drop as seen by a 120V load is twice the drop on one wire.If, on the other hand, you add an additional 5KW load on the other leg, it causes the neutral current to go to zero, so the voltage drop as seen by a 120V load is only the drop on one wire -- increasing the load reduces the voltage drop!In practice, though, if you more or less randomly distribute the 120V loads between the two legs the system will be sufficiently balanced. You do the voltage drop calcs so that, worst case, the voltage drop will be "acceptable". In real life you'll rarely approach that worst case.(You can help this a lot by making large loads -- electric heating, AC, etc -- be 240V, so that the 120V loads are small in comparison.)(Or, if it really is likely that you'll have large unbalanced loads, it may make sense to go to a larger wire. You could just increase the neutral size, but it makes more sense to increase both, or just the hots.)
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
It sounds to me that he wants to be as conservative as possible with the wire, but have the system function properly, which is understandable.
The problem that I could see happening here is that he is sizing the wire for a very small panel load. 300' is a pretty good distance to be running line voltage to a new panel and you don't want to be too conservative with the wire sizing.
Once he exceeds the 25 amp draw on one leg of his panel (which is not hard to do), he will start to experience more voltage drop than he anticipated when he was doing his calculations.
I think your last bit of advice about upsizing a bit is good advice.
It never hurts to use larger wire if it fits the budget, but remember that voltage drop is calculated based on the anticipated load, not necessarily the size of the service or the breaker panel. The original poster is wiring up a cabin, which infers there aren't a ton of electrical loads...maybe some lights and a radio and a few other things. On the other hand if he has electric heat or an electric water heater or an electric stove it may not be enough. Of course, this is all speculation without more information.
First, thanks to all for making some of these issues clearer to me. I think I understand the voltage drop calculations and as they apply to real life situations. I had wondered about the fact that a cabin with a relatively light load to begin with could see a significant voltage drop on a single leg when I crank up my 1200 watt amps (which are 110v) to play Bach's D minor Toccata and Fugue.
But for those who are interested here are the real facts of this little endavor. I actually have a small shop between the meter and the cabin. The first leg of my circuit is about 123' and the wire size is 4/0 aluminum. The next leg is about 233' and for that I have on hand 2 guage aluminum which is not yet in the ground. These are actual circuit runs not just the distances between structures so they include the additional lengths to the panel and meter. My estimate of 50 amps seems reasonable for what is in the cabin.
The shop is a single guy operation with no tool greater than 20 amps. Add another 5 amps for lights and you get a load of 25 amps. So by my cacluations I have a 0.6 volt at the shop and a 10.9 volt drop at the cabin for a total of 11.5 volts, a 4.81% drop off of 240 volts at my meter.
So before I rent the trencher and finish the deal, any final thoughts or suggestions?
I would go rent the trencher! Worst case scenario is...you will eventually modernize to the point where the lights will get dimmer and you will have a more candle-like atmosphere in the cabin.
One thing to check is what the local voltage situation is to start with. This would involve checking the voltage a couple of dozen times, from early morning until late, over several days, to find out what the lowest voltage is and what the range is.If the voltage spends much time below 115, and especially if it sees wide variations (more than 10-15 volts) then doing more on this end to reduce voltage drop is probably a good idea.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
I've checked it twice now and both times the output was exactly 120 v on the pedestal mounted GFCI. I'll try it again at some other times.
Check especially early in the morning -- milking chore time. Lots of times electric heat will come on then and cause some major loads.
People never lie so much as before an election, during a war, or after a hunt. --Otto von Bismarck
One thing to check is what the local voltage situation is to start with. This would involve checking the voltage a couple of dozen times, from early morning until late, over several days, to find out what the lowest voltage is and what the range is.
That is very important. It is quite amazing what you find sometimes. The area we work in is served by three different power companies and often has voltages varying by as much as 10 to 15 volts from one place to the other..or at different times of the day.
I'm not quite sure if that is a result of the transformer, load on the line to a certain area or what. I always wonder if the results would be the same in a year or so. I'm thinking that some of it is due to daily demand, much like the drop in water pressure at 6 o'clock in the morning when everybody in the neighborhood is in the bathroom?
Bigger is always better??
When I was a kid growing up in the '50s, we lived seven miles from town at the end of a rural electric line. The voltage was often too low to run our TV, so we had to buy a voltage stabilizing transformer to keep the (black and white) picture from shrinking beyond recognition. The thing weighed well over 50 lbs and only ran the TV. If you have anything that is critical for voltage, they probably make a solid state device that would act similarly. Of course, doing the wiring to avoid the voltage drop in the first place would be preferable.
Although we live somewhat in the bonnies, we do have modern utilities (even got DSL). But the kicker for us was that when we first decided to improve the land, we had to get the electric in before we built because the installation rates were going to skyrocket. Cost us $2500 to run a line about a 1/4 mile off the main road and that price was about to go up 4 fold. So it was only after the electric was on the site that we finally decided where to build. We had the service put pretty much in the middle of our land (about 20 acres on the flat where we could build) and so that became the center for which we planned around. If I knew then what we would end up building, then I would have moved the power another 100'. So all things considered, we didn't do too bad with our placement. My shop, our barn and the main house are reasonably close but the cabin ended up on the edge of it all. And that's the way it goes.