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I am running a wire 300+ feet. I'll need to place a transformer every 100 feet that would maintain voltage/current, therefore I wouldn't get any drop so that I can maintain 110vac 20 amp. Which transformer should I use?

Edit: I'm powering a shed.

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    I may be missing something, but I'm really not clear on how/why a transformer every 100 feet would stabilize things. Why not just use a pair of transformers to bump up to a higher voltage and down at the end, which will allow you to use smaller wire while keeping voltage drop to a minimum? Also, all that for just one 110V/20A circuit? – manassehkatz Jun 17 at 23:46
  • What are you trying to power at the other end? General receptacles? Outdoor lighting? Some specific load? – ThreePhaseEel Jun 18 at 0:02
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    Unless this is almost never anything but lights and a cell phone charger, seriously consider going to 240V. If you bump up to 240V then for 4 conductors (hot/hot/neutral/ground) instead of 3 (hot/neutral/ground), you get double the power. Even at just 20A that would let you have 2 circuits - one for lights and one for receptacles. Go even just a little higher and you could have a subpanel that powers a whole bunch of stuff. – manassehkatz Jun 18 at 1:03
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    When someone asks you "what do you need to power there" and given examples, you do not say "a shed". Try something like "a circular saw and some lights" .... "a dehumidifier" ... "a level 2 EV charger" ... "a PC w/ 850 watt power supply and laser printer" ... etc. – Harper Jun 18 at 5:38
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    Whoever told you you needed a transformer every 100’, I wouldn't listen to their advice anymore. – Harper Jun 18 at 5:42
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Straight up

You cannot afford to fall below 110V. Ok.

Supply at your house is supposed to be 120V so we have a little room to play with.

If you use 8 AWG cable, voltage drop will be limited to 8.29V on a full 20A load, giving 111.71V. That meets your criteria. If actual pulled amperage is less, voltage drop will be proportionately less, and voltage will be higher.

If that is not good enough, you can run thicker wire. Any bigger than #8 and you should be looking at aluminum wire, e.g. a #1 Al cable will create a 1.46V drop, giving 118.54V.

Transformer

Now if you power the circuit off a 240V breaker, you can do your transmission at 240V and power a 240->120V transformer. A 5 KVA transformer will more than suffice, and is often available on Craigslist for as little as $100.

Nominally you assume 2:1 on the transformer, but you can jumper the transformer for slightly more or less if transmission voltage drop warrants it.

Anyway, since your feed voltage is 240V, you have half the voltage drop and 1/4 the power loss.

14 AWG wire will suffice. At a full 20A on the 120V side (10A at 240V), it will drop 16.08 volts off 240V (8.04V on 120), giving 111.96 volts at your plug.

Two transformers

You can use 2 transformers back to back to kick the transmission voltage up to 480V, 575V or even 600V, and voltage drop will be considerably less still. However, we can't use wire smaller than #14, and #14 wire is getting the job done as above. So this is a bad idea given the higher hazard in the higher voltage section.

Wait. I thought 110 was normal, I meant no drop at all

Then install an "always online" type UPS at the shed. The online UPS always fabricates AC power through its onboard inverter, so it's always making it to the spec you paid for. The supply side of this online UPS will have an input tolerance; wire thick enough wires to satisfy that, and if at all possible, use 240V for thinner, cheaper wires.

But I wanted to compensate with transformer taps

Doesn't work. You can't hold voltage steady by selecting a single transformer tap, because voltage drop is proportional to actual load. Whatever tap you select, voltage will go up and down with load, you're only affecting the range bracket. You can get transformer-based equipment that will automatically "select-a-tap" as needed to keep voltage within spec, it will step up to the next tap as voltage sags etc. But this is not perfect, there will be some sawtoothing.

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Adding transformers will not "stabilize" anything, I have no idea where you got that idea, but it's wrong. Sometimes people step up the voltage to a higher level (like 480V) and back down again for a long distance run so that the current is less, because less current results in less voltage drop since voltage drop is a function of I (current) Squared / R (resistance, in this case of the wire itself).

But you can ALSO just decrease the R by using larger gauge wire, which would cost you a LOT less than adding transformers. There are on-line Voltage Drop calculators that will give you the size of wire you need. According to one that I found, to have no more than a 3% VD on a full 20A load at 120V, you would use #4 Cu wire or #2 Al.

  • Less current means lower voltage drop but it also means you need smaller wire to begin with. That can make a huge difference - e.g., 4 AWG vs. 10 AWG for one rough calculation based on 300' 120V vs. 480V. – manassehkatz Jun 18 at 1:01
  • Right. But what is the cost to buy, install and provide primary and secondary protection for two 480V to 120V transformers and what are the long term losses in those transformers going to cost in wasted electricity over their lifetime, compared to just installing larger conductors? Close to the same amount of work for the conductors either way, but no costs for the transformers, OCPDs, required disconnects and conduit and mounting of all of that superfluous equipment. – JRaef Jun 18 at 1:40
  • I suspect you are right in this particular instance, because it is not super long distance and very low current - just a single 20A circuit for now. Harper has written this up a number of times - what kind of transformers you need, advantages and disadvantages, etc. If OP was looking at a 60A 240V subpanel rather than a single 20A 120V circuit then the difference in wire cost would be more significant. – manassehkatz Jun 18 at 1:42
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    Yes, it would change significantly if the circuit load were higher. By the way, if run as a 240V circuit, it would drop down to 6ga Cu wire, but you would run 3 conductors + ground because you would need a Neutral and 2 hots. Still, 6/3 W/G would be easier to run than #2 SER. – JRaef Jun 18 at 1:50
  • Sorry for the confusion, I meant Maintaining. – Alex Zhuravel Jun 18 at 4:11

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