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We are looking at installing a 100-amp sub panel to a barn and the wire run from the 200-amp main panel is 600ft we will probably end up using the whole 80amps continuous, the run will be all outside and I have the ability to trench it and use schedule 40 pvc conduit.

I'm reading I need to up my amps by 20% for every 100ft so would I be correct to say wire sizing has to be done for 220amps? is it possible to reduce the wire size for the run by stepping up from 240v to 480v and then back down at the other end if so how do you calculate what transformers you need and wire sizing.

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  • Are most of loads 240V or 120V?
    – manassehkatz-Moving 2 Codidact
    May 10 at 3:58
  • there will be 3 240v loads and the rest 120v
    – Darren Watson
    May 10 at 5:46
  • Size of those loads?
    – manassehkatz-Moving 2 Codidact
    May 10 at 11:04
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    Please do some major self-education or hire competent help. It's fairly clear that you are deep into not knowing what you don't know. That can be both dangerous and expensive.
    – Ecnerwal
    May 10 at 13:17
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    A great deal of LED lighting can easily be a 240V load, since 85-277 volt inputs are common (not universal, but commonly available.)
    – Ecnerwal
    May 10 at 14:54

2 Answers 2

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The "20% per 100'" rule of thumb you found there isn't very good. It's excessive for short distances and insufficient for long distances, and this is the latter. Anyway, Voltage Drop Calculators exist so I consider the rule to be functionally obsolete. Remember to select aluminum because it won't, and 3% is a salesman's suggestion.

However, you say you will be running this continuously near 80A. That sounds like something a novice would say - but you may well know better than I. OKAY... think about it. What does 1% voltage drop cost you? 80A you say? 1% of 80A is 192W or 4.6 kWH/day. 1681 kWH/year and at 12 cents a KWH, $200/year. Over 10 years, $2000. Obviously if your load is partial-day, (e.g. a big solar array that really only hits its number 1/3 of the time)... you should factor accordingly. If I were you, I would actually run several Voltage Drop calcs between 1% and 6%, and compare the cost of wire for each scenario, vs the cost of wasted electricity, and see what makes sense for you.

If your loads are predominantly 240V, remember that voltage drop is your free choice and not mandated by NEC in any way (except you can't plan for voltage drop over 10-ish percent, for fault clearing reasons deep in Article 250 arcana) . As such, nobody's going to write you up for using the minimum legal wire size of #1 aluminum for neutral. To get voltage drop on the neutral, run through the voltage drop calc again, and give it neutral "imbalance current"... and then, divide by 2 (since it's giving you round trip voltage drop, and you only need one-way).

is it possible to reduce the wire size for the run by stepping up from 120v

Wait. Did you say 120V???? Remember that thing I said about novices? It's movie time: go watch this Technology Connections video on how our power is actually 240V. If that video is news to you, then you should probably ask another question, giving detail about your loads and asking how to figure balancing and sizing. This could be a total game-changer on wire cost.

Is it possible to reduce the wire size for the run by stepping up from 120v to 480v and then back down at the other end?

Absolutely. However this is serious "pro tier" stuff. And you'll have to get lucky on Craigslist to really save that much money; otherwise transformers this size are not cheap and may well cost more than the heavy wire, since aluminum is our very best friend at these large feeder sizes.

So yes, you obtain two transformers of appropriate kVA (VA is a funny way of saying watts, and k means thousand). For 80A @ 240V that sounds like 20 kVA to me. The typical popular transformer you're likely to find on Craigslist is 240/480V on one side and 120/240V on the other side. You wire the transformers up "back to back" primaries facing each other, so the primaries jumpered to 480V are connected directly to each other. This is where you must be very careful with construction methods so the conduit is literally going straight down out of the transformer and pad, a continuous uninterrupted run to the other transformer pad and straight up into the other transformer. No going through extra boxes, no pull boxes, no intermediate stops - you really, really do not want amateurs trying to open up boxes and mess with this, because it ain't what they think it is. If they open up the transformer enclosure itself that is plainly labeled "240/480V", then they deserve their Darwin Award, but give them no other temptation. Keep it very simple, neat and pro.

What about overcurrent protection for this 480V circuit? It is provided at the source by the 240V breaker feeding it.

Now you are doing your voltage drop calculation on 50A @ 480V and you may find more pleasant wire sizes come up. Whether you save enough on the wire to justify buying two transformers, I cannot guess.

Again this is pro-tier stuff, and is an easy way for a sophomoric K.I.A. (know-it-all, but the other thing too) on the wrong side of the Dunning-Kruger curve to win a Darwin Award just to save a few bucks on wire. Pro advice is essential here. There are lots of missable details.

At the secondary at the shed, you jumper the 120/240V side for 120/240V split-phase. Feed that into a MAIN panel with a 100A breaker. This will be a main panel since it is a seprately derived service. Neutral is bonded to the ground to the ground rods at the first disconnect past the transformer secondary.

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  • Mind you, the 80A @ 240V continuous will cost about $20K itself at your 12 cents/kWh guesstimate. $33K in my neck of the woods. Perhaps a couple of $2K 25KVA transformers aren't that big of a deal in the grand scheme of things.
    – Ecnerwal
    May 10 at 15:02
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    @Ecnerwal: But adding the transformers isn't saving any of the cost of power actually delivered to the load, so only the value of lost power is relevant.
    – Ben Voigt
    May 10 at 15:39
  • yes, I'm a novice when it comes to long runs and transformers, 120v was a typo sorry it was late and doing a bunch of stuff, I've been looking at 240v to 480v transformers would I be correct that using these would drop half the amps in the wiring and therefore allow us to run smaller gauge?
    – Darren Watson
    May 10 at 16:45
  • Yes, doubling the voltage will for a given cable size reduce power loss in the cable by a factor of four. Alternatively you can reduce the cable size by a factor of 4 while keeping losses in the cable the same. But the transformers may be quite expensive, and the transformers themselves will have losses.
    – Peter Green
    May 10 at 19:20
  • If you can find a 25KVA 240/120 to 600V transformer, that would cut your current even more for the same power delivery, on "standard" wire (600V). On a quick look the biggest one I found was 15KVA, while 240/120 to 480/240 were available in 25KVA. I would heed the "really sort out your actual loads" advice, though - that might allow use of a smaller service overall, if you are over-guesstimating your actual loading.
    – Ecnerwal
    May 10 at 21:33
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If you are to follow the national electric code you are only allow to have a maximum voltage drop of 5% from the service entrance to the furthest outlet/switch in your barn. 600' carrying 100 amps is going to cost some serious money for the wire. If you use copper you can make the run with 4/0 and have under a 3% voltage drop from one end of the wire to the other. If you use aluminum you will need 300 mcm to carry that 100 amps.

300 mcm aluminum is about 2800 bucks for 1000' spool. you are going to need 2 spools to make up the 2 hots, for the neutral you should be able to use 4/0 but if you plan on using the majority of that 100 amps I would go with 300 MCM for the neutral as well. Check with your local building codes as far as the ground is concerned, You may have to run a ground or you might be able to ground the panel at the barn.

for 4/0 copper you are looking at almost 6,000.00 for a 1000 ft.

You are going to want several pull boxes along that 100 feet. You can do a single run or you can bug the wire in the pull boxes if you want to piece the wire together. My suggestion is to set up a rack to hold the spools and pull all the wires through at the same time and if possible no splicing. You are going to need a pretty heavy rope to pull the wire through and also a tractor or some kind of strong off road vehicle (ATV is not gonna cut it) to pull the wire through. You are also going to want to use a pulling lubricant for wire.

How far is the barn from the utilities? is you house the closest or is there maybe a street that is closer? I ask this because you can have the electric company put a drop in and a new service to the barn. I don't know what the cost would be it all depends on distance so it might be worth looking into.

I also believe schedule 40 pipe is not the right thing to use for this application. schedule 80 is what you should be using. again you will want to check with your local building codes. I also believe you will need 2.5" diameter conduit at a minimum.

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  • 1
    The OP is definitely going to need a ground wire or metal conduit to substitute. It's been years since 3-wire 240/120V subpanel feeds were allowed by the NEC.
    – DoxyLover
    May 10 at 4:59
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    Why would anyone use copper for a run like that? NEC does not specify 5% or any other allowable voltage drop. It has suggestions but allows you to make the call at your discretion. CEC requires 3% at actual load. I'm also not thrilled with the advice of "buy a 1000' spool and throw 400' of it away". No one does that.
    – Harper - Reinstate Monica
    May 10 at 5:20
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    @DarrenWatson yes, for more than a decade, (NEC 2008 finally killed 3-wire sub-panel feeds) unless you find that deal on suitable transformers, (making it a "separately derived service" and only needing two wires between the transformers) and the transformers suitable for your load are... not cheap. An alternative method is to simply have the utility provide a separate service drop to the barn. That has its own associated costs but also simplifies some aspects.
    – Ecnerwal
    May 10 at 13:15
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    If you need 600 feet of wire, you order 600 feet of wire. Buying 1000 feet and throwing away 400 is not cost efficient, from any reasonable supplier. Just don't need 602 feet if you only ordered 600...install the conduit, measure the actual run length, and add a small percentage before ordering wires.
    – Ecnerwal
    May 10 at 13:38
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    As far as ground, the issue is that ground serves two different purposes. For lightning/etc. the ground rods will do the job. For fault issues you need the ground wire everywhere. In any case, that's what code requires for new installations.
    – manassehkatz-Moving 2 Codidact
    May 10 at 14:21

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