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I have a new 1500 sqft pole barn that needs electric. I expect to need 100Amp service at the barn which will have a 220V 15A compressor and 220V 25A welder. The rest of the barn will be 110V fluorescent lights, and few if any high amperage tools. My house has 200A service, and I could branch off a 100A breaker from the house breaker box.

My first question is: Can I run 275 ft of underground feeder wire (in pvc, 24 inch burial) to this barn from a 100A breaker at the house? I've done some calculations that suggest #1 aluminum feeders would be good, and the voltage drop should be less than ~3%. But most calculators stop at 250ft, and runs over 250 ft aren't discussed much. I want to ask here because the experience is unmatched. The house has a good 3-wire feed and breaker box. But the house is all electric. I'm willing to go with copper feeders if I have to.

I plan to use two 8ft deep copper grounds at the barn.

I done a lot of research and can't find a definite answer on maximum feeder lengths. BTW our state uses the 2014 NEC codes, but I'm all about safety.

EDIT: Thanks again, let me give this a try. I'll probably not be using all tools or equipment at once. Figure VERY intermittent use of the compressor and welder because I'm not setting up a paint booth. But a 50A car charger is a real future possibility (thanks @jwh20). A large fan, permanently wired (but not always on) could be 8A (so figure 10A), a heater or small A/C (for a very small office) would be about the same 10A because it will be on when I'm in the barn when the compressor kicks in. Lighting would be LED almost certainly just as in my house with no weed being grown. So 1500 sq ft of space needs requires three 20A circuits for lighting, but will likely be less than 5A. Add an extra 10A for everything else, so my provisioning is for 50+10+10+5+10 or ~85A (someday).

Wire type: Using UG-F inside conduit seemed to be the safest, most long lasting option. Because THHN/THWN saves more space within the conduit (thanks to @NoSparksPlease) this decision will likely come down to availability and costs. Thanks also to @Ecnerwal for the tip about putting the grounding rods in the trench, and that 18" deep is enough. I'm not crossing any roads or areas where there will be vehicles crossing.

The ampicity table 310.15(B)(16) under 60C says that at 85A I'm at AWG3, or at 105A I'm at AWG1. So if I use AWG1, I can use a 100A breaker. the ground wire would need to be about #8 (Best guess). But now I need to check the voltage drop.

So you guys are magic or something, because I went to the voltage drop calculator that @Ecnerwal recommended and it was literally already set up exactly for this. Using 275ft of AWG1 aluminum wire, I would have a 4.48% voltage drop, and would have 229.25V at the barn. This seems okay to me (not in Canada I assume) with the main line only 15 ft from my house meter and breaker box.

I know I need to mark an answer... Hard to pick just one but going with @Harper - Reinstate Monica for the process he described, but you all were extremely helpful.

Thanks everyone.

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    Are you basing your voltage drop calculations on actual load, or breaker handle rating? – ThreePhaseEel Oct 20 at 3:51
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    Issues like this are often a judgement call. Will you REALLY be using 100 amps at a time? Running 4/0 AL seems like overkill to me (sorry jwh20). Also, how tolerant would your loads be of more voltage drop? Not sure, but something like a welder can be quite tolerant. You got it right about the 2 ground rods, but be sure to isolate the neutral from the ground and you'll sill need a 4 wire circuit out to the barn. – George Anderson Oct 20 at 11:13
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    Because you're running the feeders through a conduit, you could up-size the conduit to future-proof your installation. – Jeff Wheeler Oct 20 at 12:29
  • @GeorgeAnderson I also question the 100A requirement and noted that in my answer. ;-) – jwh20 Oct 20 at 13:56
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    @phoyt Don't use cable assemblies in conduit, you have to greatly oversize the conduit to size the conduit fill calculation of the cable as a single conductor, and the jacket on UF-b is too sticky. Use xhhw conductors. – NoSparksPlease Oct 21 at 0:57
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Don't panic!

And don't oversize the wires unnecessarily.

250' is a typical distance in AC mains wiring; I suspect you're familiar with calcs intended for low voltage/electronics. For them, 250' is "Here there be dragons" territory :)

Now, my rule of thumb is I don't even bother looking at a voltage drop calc until the distance is over 180'. You must run wire large enough so the wire doesn't catch on fire... but the United States of America is not a "nanny state" and will not force you to upsize wire merely to avoid voltage drop, which isn't even harmful to many loads.

Start with the absolute statutory minimum size needed.

And that starts with provisioning our loads. You have 2 "poles" of 120V each. Each 120V load goes on either pole. 240V loads go on both poles.

You only need to figure for loads that are actually running at the same time. if you have 3 machines of 15 13 and 10A and you'll only ever use one of them at once, you provision 15A. You're probably going to identify "ruling factors" such as one big machine that doesn't have simultaneous use with other machines.

Certain continuous loads must be provisioned at 125% - so if you have a 12A heater you must provision 15A for it.

Here's a slam-dunk example: suppose you need to provision 50A for a Tesla charger, 10A for lights (on one 120V pole) and blah blah blah for a bunch of machines that won't ever exceed 50A. No need for further calculation; the Tesla is the ruling load. 60A (on one leg) is the answer.

Welders require special computation based on duty cycle, and require surprisingly small provision of service. For instance a 66A welder may only need 30A provisioned.

We arrive at a number of amps to provision.

Next, we figure out our wire type. NM-B cable isn't legal outdoors but UF-B cable is (take a mental note: both of those use the 60C column). MH feeder uses the table for the individual wires in it, which are labeled THWN or XHHW or something.

Now, we go to the ampacity table - 310.15(B)(16) which I hear they are moving to 310.16 in the 2020 Code. Find our wire type in the top column. If it's in the 90C column, that's off limits, you must move left to the 75C column. Also, for #14 through #10 wire, the 90 and 75C columns are off limits, you must use 60C for those wire sizes. Move down that column until we hit a number that exceeds our provisioning number.

For example let's say your provisioning number was 66A and we have 75C/aluminum wire. You crawl down the table and hit #4/65A (too small) and #3/75A. There's your answer. However, they don't make 75A breakers, so you round up to the next available breaker size, 80A. Well wait a minute! They don't make 65A breakers either so couldn't we have selected #4 and rounded up to a 70A breaker? Nope, can't do that. You can't plan to use those bonus amps.

I think your real provisioning number will be 40-ish amps, since the welder will be greatly discounted, and there'll be other loads you did not mention. If it's a place you could reasonably park a car, I really like to see people provision 50A for an EVSE, and since very few people run 50A of power tools simultaneously, that greatly simplifies the provisioning calculation. That puts at about 55-59 amps assuming quite a lot of fluorescent or LED lighting.

Anyway, this arrives at the mandatory wire size you must use. You're always free to use bigger wire. However if you do, you can upsize your breaker to suit (offer void in Canada).

NOW, we figure out voltage drop

You compute voltage drop on the actual current flowing realistically -- not even the "provisioning" figure, since some loads require you to provision 125% of their actual load.

You would never compute voltage drop using breaker trip. If you are pulling 100% of breaker trip amperage, you seriously mis-provisioned that service, boy howdy!

And even then, forget 3%. No load will be seriously damaged by 5-6% voltage drop or even 8% (the only number NEC really cares about). Current goes up and down all the time from different machines cycling on and off - A/C pack, heater, compressor, welder... it's not a solid number you can stick to the wall. We don't need a "nanny state" protecting us from voltage drop, least of all at such a conservative 3%.

Anyway, given your expected loads and your desire to use #1 aluminum wire, it'll be a moot point.

Into the calc we go. I crunched the following:

enter image description here

#2 at 3.97% drop. That's good enough.

You're planning to go #1 Al anyway, which gives you 3.28% drop at 59A which is fine for your expected and likely future needs.

What size of breaker, though?

In the US, you are allowed to breaker for the ampacity allowed on the wire (back to table 310.15(B)(16). So your #1 aluminum wire is allowed 100A, so you can breaker for 100A.

In the People's Republic of Canada, the nanny state needs to protect you from voltage drop over 3.00%. However they base this on your actual loads not breaker trip. If your actual loads are un-knowable, then they base it on 80% of breaker trip. For 275' of #1Al, 3.00% happens at 54A. 54A is 80% of 67.5 amps, so your #1 Aluminum would be fine in Canada even with your Tesla + 9A of lights + 8.5A of other loads. In Canada, if you really wanted a 100A breaker with un-knowable loads, that puts you at 2/0 wire (2.98% voltage drop 2/0-275'-240V). But you're not in Canada.

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Your first, and only question that I see, is whether or not you can run 275 ft. of underground feeder wire to your barn. The answer is: Yes, you can provided you use wire large enough so that the voltage drop at the barn is acceptable to your use. As noted in your question, the commonly accepted value is 3%.

For long runs (i.e. large wire sizes) Al wire is significantly cheaper than Cu and the only benefit from using Cu is that you will use a smaller wire size. The overall performance of a properly size Al wire vs. a properly size Cu wire is the same.

I do question your 100A load figure here. Are you going to be running multiple tools at one time? You've mentioned only a welder and a compressor but those together will take only 40A. I can't imagine that you'll be using anywhere near 60A for your lighting.

BTW - you might consider LED lighting instead of fluorescent. The cost difference is not that great any longer and LED use quite a bit less power.

I don't get your 1 AWG figure, however. Using the Southwire calculator at:

Southwire Wire Calculator

returns a recommendation of 4/0 :

enter image description here

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  • Why did you put 100A in the current box? Why did you put 3% in the voltage drop box? – Harper - Reinstate Monica Oct 20 at 16:51
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    @Harper-ReinstateMonica Read the OP! – jwh20 Oct 20 at 17:17
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    @Harper-ReinstateMonica - Apparently you didn't read the OP that well, because that's what OP is asking about – Havegooda Oct 20 at 18:35
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    @Harper-ReinstateMonica - just because OP doesn't plan to use 100A currently doesn't mean you shouldn't provision for it if that's what their breaker is going to support. Who knows what they'll do in the future, or what a future owner might want to do? Laying cable can be expensive. Do it right the first time – Havegooda Oct 20 at 18:41
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    @jwh20 I'm not judging, I'm trying to find an explanation for the current. I'm Dutch. We're not offended by weed. – Mast Oct 20 at 19:31
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For your described loads (if you were actually running welder and compressor at the same time with some assumed lights on) AWG #1 aluminum should be fine (50A-ish load)

I like this voltage drop calculator, it has no artificial limits. There is not (to my knowledge) any limit in code to how far you can go, but there are practical issues that come into play for how you go as far as you go while affording it; but you are within a distance/load range that probably does not support any more creative solution (such as using transformers back-to back to run at high voltage/low amperage) being more affordable than just running wires.

3% is only a guideline (unless you are in Canada, I gather) but for the loads you mention, that should be safely below, and if you draw more than that, the 5% or so drop at 100A won't be a practical problem.

You do need a 4-wire feeder, but the grounding wire can be smaller than the hots & neutral.

Under NEC rules you only need 18" to the top of the conduit for PVC, but you can obviously go deeper if you prefer. Mine are down about 3 feet, as it happens.

If you plan for it while the trench is open, you can set one of your ground rods starting from the bottom of the trench and make it effectively deeper into the soil that way.

Consider adding a communications conduit while you have the trench open.

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  • I don't know about NEC, but in Canada the burial depth (and protection) are partly dictated by whether or not the buried line runs underneath traffic (a driveway, etc). Canadian code is 24" unprotected, 18" deep for conduit/TECK, and +6" for both cases if the run crosses an area where vehicles drive. – J... Oct 20 at 16:06
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    NEC is similar WRT driveways, even deeper for roads IIRC (never had to deal with a road crossing myself, that tends to be utility territory.) – Ecnerwal Oct 20 at 17:32
  • As expected, CEC and NEC are usually pretty similar. The only point being that OP should not take 18" as an absolute - they should consider if the run is crossing traffic. – J... Oct 20 at 17:34
  • if your planning a 100a feeder for that length, id go with four wires: two 1/0 AL for the mains, a 2AWG AL for the neutral (70% rule) and a 6AWG CU for the ground. all in 2-1/2" conduit for future upgradability when you discover you want a plasma cutter and Mill... – mark f Oct 20 at 18:35
  • If a subpanel, should there really be grounding rods at the pole barn, or should the earth grounding go back through the 4 wire to the service at he house, and the grounding there? – mongo Nov 17 at 13:21
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It does seem like your voltage drop is calculated with 50A load, which I think may be a bit conservative, taking liberty on one or two factors usually will get you by, but taking liberty on every calculation leads to trouble, I would increase your wire one or maybe two sizes, 4/0 I think is going a little far.

The NEC only addresses voltage drops in informational notes which aren't enforceable. The informational note recommends 3% for feeders and 5% total including branch circuit. I think those numbers are a little tight, otherwise they would make them enforceable. Consider NEMA motor nameplate is 230v +/-10%.

But my concern is understanding of feeder and branch circuit. Your feeder to the barn is a branch circuit when calculating voltage drop. If your service to your house is underground from a bug in your yard next to your house you could be good calculating your drop as a feeder, but if you are on a shared transformer 100+ away with an overhead 200A service being fed by #1 aluminum and your utility wasn't very generous with tap you could be in trouble. Voltage drop can only be calculated under load, you might want to kick your water heater on, turn on your oven, and all your heat in order to get a worse case scenario that might load your service similar to the load when your barn is in use and take measurements to see if you already are losing 3% on your service. Adding another 50A of actual load in the shop will actually increase the voltage drop at the service.

Also be sure to check Table 300.5 for cover requirements, you may need the 24" you were planning on if subject to vehicle traffic and your local authority thinks your shop is used for commercial or ag purposes. 18" cover only applies to "dwelling related purpose" only.

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