calculating underground wire size.

am planning on running 100 amp service underground to a tool shed from the main feeder at the meter socket. there is a utility pole in the yard that the power come in off the road. then there is a wood board set up with several electrical feeder boxes going to the grain bin fans, dryers, and stirrators. i have a 100 amp box there that is not being used. i measured the run at it is about 135ft including 10 ft tails extra on each end. i am going to use the plastic conduit. i plan on running two 110v feeds, a neutral and a ground wire. (as a side not, the only ground is a rod driven at the pole where the neutral is tied to. i cant find my book on sizing wire, but i got a couple different answers from tables i found on line. assuming 135 feet at 100 amp service, one table says 8awg copper, the other says 6 awg copper. neither table gives a size for the ground wire. what size would be good for the feeds and neutral, what size for the ground wire, and what size conduit, 1 1/4 or 2 inch. i think 2 inch would be easier to pull the wire thru. reason for the underground is there is an overhead that feeds the shed, but the wire gets real close when putting the auger up on the bins, and i want to eliminate that issue.

I think you're going to need #2 copper for that. Definitely 2" conduit.

I believe #8 is good for about 40 amps, and #6 for about 60.

Can you just run the existing over head wire in new conduit?

no, the overhead wire is from the early to mid 1970's, i want to replace with all new, and i also need to run a separate ground wire as they didnt use one back then. just 2 hots and a neutral in the pole barn. its an old morton bldg.

Here is what mine says.

wow, the charts i found were way off. i thought it seemed a little light. what size ground wire would be needed?

Glennster, good question and typically you will get a ton of opinions as the topic is electricity. That being said, its been years since I practiced electrical power distribution engineering and Im rusty as an old nail on the NEC, but FWIW I will offer my opinion and if I?m wrong others can add to this and/or correct my mistakes. NO WARRANTY? Where fire or life safety is concerned consult professional electricians and electrical engineers, local authority, your utility company and the NEC or any applicable codes (if any exist in your area) and don?t risk your life on opinions posted here mine included


1) Its not surprising your service only has a driven ground rod. When I practiced the NEC required us to bond the incoming service entrance Neutral TO ALL READILY AVAILABLE GROUNDING ELECTRODES and in a field at a pole where no other grounding electrodes were ?readily available? what?s referred to as a ?made electrode? such as a rod driven into the earth sufficed. In our jurisdiction we drove one rod and if it passed no more were required but if not we were required to drive a second rod.

2) I take your post to indicate your new remote panel will be treated as a SUB PANEL for 120/240 Volt Single Phase Three Wire Service ???? right ??? under the latest code, if you want 120 and 240 at the remote sub location, you need to run FOUR wires, two Hots L1 & L2,,,,,,,,a Neutral,,,,,,,an Equipment Grounding Conductor.

3) At the remote Sub Panel (assuming that?s your case) you need to maintain separate and insulated Neutral and Equipment Ground Busses NOT any single common Neutral/Ground Buss and DO NOT bond the Neutral Buss to the Ground Buss.

4) I preferred to use THWN insulated conductors in underground conduit.

5) Per the chart below #3 Copper THWN is rated for 100 Amps,,,,,,,#2 Copper THWN 115 Amps

6) In order to calculate Voltage Drop you need to know the current, the wire size and the wire length. I don?t know the current but to reduce voltage drop even if #3 has sufficient ampacity I would use the bigger # 2 Gauge 115 Amp rated conductors. In the event based on current and wire length Voltage Drop was excessive, I would bump the wire up to #1 Gauge.

7) Per the chart for 2 inch PVC conduit there?s absolutely no problem running four # 2 Conductors and even if its oversized it makes for easier wire pulling plus room for expansion. The Equipment Grounding Conductor need not be as big as the Hots and Neutral but I didn?t take the time to look up its size. Maybe some other gent can do this??

8) Best I recall direct bury in earth is okay for proper rated PVC conduit UNLESS its under a road or driveway in which case it needs concrete encased.

9) NOTE sure if direct burial rated USE cable or UF is used or aluminum conductors in conduit versus copper you can save money BUT THAT WASN?T YOUR QUESTION you indicated copper cable and conduit so I take you at your word and its how I would have specified it when I was a designer.

Looks like Four (but Ground can be downsized) # 2 or # 1 (if voltage drop is excessive) THWN Insulated Copper Conductors inside 2 inch PVC buried conduit ran to a 100 Amp (Main Breaker equipped is what I would use) SUB PANEL with separate insulated and isolated Neutral and Ground Busses should suffice HOWEVER consult the NEC and Utility to make sure and be safe.

John T Long retired and rusty EE

I'm getting #2 THHN for the lines and neutral, #6 for the ground.

1 1/4" PVC conduit.

Steve, you posted "#6 for the ground" thanks for taking the time to look that up, I was too lazy lol, seems small I usually oversized the ground regardless, but that was just me...??.



John T

thank john and steve. i will go with the #2 feeds and neutral and the number 6 ground. yes the panel in the barn will be a sub panel and the neutral and ground will be separated.

I think you want two hot wires for 220 and a neutral. I think the ground should be a ground rod(or 2) at the shed end. Check with local inspector to be sure.

Dave

For 100 amp service to my shop, a distance of 125 feet, I ran four #2 aluminum USE cables. I did direct burial, only using conduit where the cables entered and exited the ground. This was very economical and has worked out well.

As others have said, you need four conductors, two hot, one neutral and one ground. Mark the ground with green tape and neutral with white tape. Ground and neutral must be kept separate at the outbuilding. You'll need two ground rods at the outbuilding, driven several feet apart.

It is extremely rare to run underground in conduit anymore, I would use direct burial.

(quoted from post at 19:14:55 01/03/20) Can you just run the existing over head wire in new conduit?

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No offensive but after seeing this and the grounding question question . It will take less time , cost less money , work better and be safer than a home brew job. To hire a professional .

Direct burial cables need service more often than conductors in a conduit . If the conductors fail,
Just pull in new set of conductors.
Conduit is cheap .

Menards has this, rated for 150 amps. But if you put it in a conduit you loose a little ampacity, right?

2/0-2/0-2/0-1 Stranded Aluminum Mobile Home Feeder

(quoted from post at 22:22:22 01/03/20) It is extremely rare to run underground in conduit anymore, I would use direct burial.

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how did you arrive at that statement?

i wish i had your money b+d to hire everything out. to get an honest contractor to come out and actually do the job is dang near impossible. i have been
trying to get a roofer out to replace a tool shed roof at the other farm since july. 5 different contractors were called, not one showed up to even give an
estimate. funny, every body wants the money, nobody wants to work. hire a professional, no thanks, i will do it myself and be done with it.

Glennster, I predicted there would be several answers and you got some answers even if NOT to your question, however that's typical and all the fine gents here are trying to help which is why this is a great site with great people. Gotta love this forum...??..

Some observations:

1) You stated " i am going to use the plastic conduit."

THEREFORE MY answer addressed that and DID NOT try to change your mind and use Direct Burial UF or USE cable. Sure it may be easier and cheaper HOWEVER for any future upgrades or if a conductor were to go bad YOUR CHOICE is a good one in my opinion. FWIW Buried PVC Schedule 40 Conduit with individual THWN conductors is how I would have specified it when I was practicing, even if YES direct bury rated cable may be cheaper and easier THATS YOUR CHOICE.

2) You stated "and what size conduit, 1 1/4 or 2 inch. i think 2 inch would be easier to pull the wire thru"

You asked about 1 1/4 or using 2 inch conduit as it would be easier to pull wire thru
EXACTLY RIGHT The chart I posted shows you can get by with much smaller conduit HOWEVER I have to agree with YOUR
CHOICE of using bigger then required conduit


3) You posted " i plan on running two 110v feeds, a neutral and a ground wire."

THATS CORRECT for modern NEC a feed to a "sub panel" for 120/240 Volt Single Phase Three Wire requires YOUR CHOICE of
Two Hots L1 & L2, Neutral, Ground... As posted below use a panel with separate and insulated Neutral and Ground
Busses...??. YOU DONE GOOD

4) You asked about wire size. The chart I posted indicated #3 Copper was good for 100 amps, # 2 for 115 Amps. My choice
would have been the higher then needed 115 amp rated #2, but if voltage drop was excessive Id up it to #1 Wire.

5) You asked about ground wire size. I didn't take the time to look that up but Steve posted #6. When I designed in that
situation I typically (per the NEC) sized the Ground SMALLER then the Phase and Neutral Conductors but usually not as
small for #6 on a 100 amp service. Consult the NEC notttttttttt me lol

6) When I practiced each buildings electrical service required grounding. At your sub panel you will need at least one
ground and (subject to NEC or any applicable codes in your area) perhaps two. In our day in our jurisdiction one
sufficed it passed but if not we drove another CHECK LOCAL AUTHORITY AND UTILITY AND PROFESSIONAL ELECTRICIANS
AND ENGINEERS


Good question, good choices Glennster, even if more then required yayyyyyyyyyyyy

God Bless all here and Happy New Year

John T

Youre welcome, WISE CHOICES

John T

X-2

Direct burial versus in conduit.

With individual conductors (not the twisted mobile home cable, nor the jacketed direct bury cable) if one conductor should fail, you can replace only the bad conductor(s).

Or start over with replacing all of the cabled choices (direct buried, in conduit or overhead).

Pouring more gas on the fire...

It seems to me that all the conduit does, is hold water and make the cables fail sooner. Even if the conduits start and end, inside of a building, they will sooner or later fill with water which is then trapped forever over the cables.

Etherway... good luck on the pull.

If you are looking to save money, direct burial aluminum will save a lot of money, and is a lot less labor.

I do not see the folks that talk about pulling the wires out and repairing, as it is super easy to repair UG conductors, just dig that place up and put repair slice in. The UG will probably be good for at least 30 years, which will outlive most of us on here.

I forgot where you live, maybe some of us can help.

I would run PVC pipe and pull Thhn through it then it is replaceable with no digging. All copper.

As a former power distribution design engineer IM WITH YOU (an experienced electrician) DR. It seems some prefer direct burial and THATS THEIR CHOICE more power to them and it can be cheaper IF THATS THE PRIMARY ISSUE ????. HOWEVER Glennster made HIS choice to use Conduit so I say let him do as HE chooses and am glad to advise him and answer his questions the best I can and NOT try to change his mind lol.

John T Hope to see you in Florida ???????

I had a situation once where I ran a continuous conduit from a pole to a garage, after about 6 years the conduit accumulated water or condensation and when it froze it broke the wires. I pulled new wires in the conduit and then poured some anti-freeze in it. That is why conduit is not always a good idea, especially in an area with a high water table.

If I ever have to run a water pipe underground , it will be in a 3 or 4 inch sewer pipe too. We have rocks here & I lost 2 different runs of my water pipe,. One is to the shop so now I don't have water there.

> I do not see the folks that talk about pulling the wires out and repairing, as it is super easy to repair UG conductors, just dig that place up and put repair slice in.

The catch is it's usually not easy to figure out exactly where the failure occurred. Worst case scenario you just dig a new trench and replace everything.

I suspect that most failures of direct burial service occur in rocky ground or when the cables weren't buried deep enough to keep critters from chewing them up. Running heavy-gauge wire through a hundred feet of conduit doesn't sound like fun, but I suppose it's the safest way to go.

I'll be there .I have some meters to show you !

i like the conduit idea as the power pole sits a little ways from the grain bins, and when hauling in the tractors will drive over the yard where the underground will be. an 8300 dualed up pulling two full 750 bushel gravity wagons gets a little on the portly side. i will prolly pour concrete to protect the conduit.

(quoted from post at 08:41:50 01/04/20) i wish i had your money b+d to hire everything out. to get an honest contractor to come out and actually do the job is dang near impossible. i have been
trying to get a roofer out to replace a tool shed roof at the other farm since july. 5 different contractors were called, not one showed up to even give an
estimate. funny, every body wants the money, nobody wants to work. hire a professional, no thanks, i will do it myself and be done with it.

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This why I have pretty much given up on doing side jobs. Keep finding these home brew wiring jobs . Don't want to jeopardize my license by being called in to cleanup a mess.

Good plan, when I practiced I recall underground PVC required concrete encasement under roads and driveways.

John T

At the huge Naval industrial complex where I was a power distribution design engineer we had thousands of feet of buried conduit with THWN conductors and even if they contained water (pretty common) we NEVER had a problem. Those conductors with that thermoplastic insulation weren't harmed by water inside the conduit. HOWEVER where there was direct buried cable (also in water or wet soil) NOW THATS WHERE MANY PROBLEMS HAPPEN GRRRRRRRRRR sharp rocks or shifting or improper burial can cause all sorts of headaches. Many lay persons or farmers etc used direct bury cable but many of the professionals and electricians and engineers went with buried conduit and THWN or THHN insulated conductors.

Like so many other engineering decisions there are trade offs and cost differences and advantages and disadvantages when deciding what method to use. So long as either is NEC approved that's a good thing and a persons own free choice which I strongly support.

For direct burial we instructed our electricians to dig the trench,,,,,,,,put in a layer of sand,,,,,,lay the cable,,,,,,,,add another layer of sand,,,,,,,,, add a strip of warning tape. HOWEVER many lay persons or farmers just dug a trench and laid the cable, maybe not deep enough, then wondered why they had a failure ??????

Hope this helps, best wishes and Happy New year

John T

I ran a underground service across the road to my shop using triplex in conduit, we're required to have two ground rods 8 ft apart at the pole and at each building we run to. I will not direct bury even a little 30 amp circuit.
We ran a electric service underground direct bury several hundred feet to a well pump years ago, after a number of years the wire worked it's way up till it was less than a foot under the ground. We plowed it into a couple of times and my brother nicked the wire in several places discing the field one year, what a nightmare, finally ended up replacing that section.
I'd run 2 1/2" conduit and up size the wire to handle 200 amps, should you ever build on or add a shop in the shed and need a bigger service the wire is already there.

> We ran a electric service underground direct bury several hundred feet to a well pump years ago, after a number of years the wire worked it's way up till it was less than a foot under the ground. We plowed it into a couple of times and my brother nicked the wire in several places discing the field one year, what a nightmare, finally ended up replacing that section.

Hmm. Sounds like it was run pretty shallow; I'd think you'd want to be down four feet or more under tilled ground. But wouldn't cable in conduit also work its way up in the same conditions? After all, cable in conduit is less dense than direct-buried cable.

450 FWIW you and I think alike.

You posted "I'd run 2 1/2" conduit and up size the wire to handle 200 amps, should you ever build on or add a shop in the shed and need a bigger service the wire is already there. "

The deal is once you already dig the trench, its not that much more money to install bigger conduit which can make for easier wire pulling PLUS allow for expansion if you later upgrade the service. THWN conductors in deep enough buried conduit (even with water that always gets inside) never gave us any problems in the many years I designed at our huge facility. Per code when and where I practiced where we came up out of the ground and up the side of a building we used RIGID Conduit.

Fortunate for Glennster he's already installing over sized conduit for his needs and choices so he's fine.

Nice sparky chatting with you, take care and best wishes

John T

(quoted from post at 08:05:41 01/05/20) >
Hmm. Sounds like it was run pretty shallow; I'd think you'd want to be down four feet or more under tilled ground. But wouldn't cable in conduit also work its way up in the same conditions? After all, cable in conduit is less dense than direct-buried cable.

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This was back in the 60's using a pipe laying attachment welded to a tractor pulled subsoiler so 18-24" depth, below frost line and deeper than any standard tillage equipment went, burying 4 ft down wasn't going to happen unless it was done by professionals at at cost we could not afford. Also here in KY if one went very deep you'll hit rock and you don't want wire laying on rock.
I was always told that electricity caused wire to vibrate and the vibration was what caused the wire to work it's way up, my thinking is the conduit absorbs the vibration plus any water that gathers in the conduit was additional weight to keep it down, but it could just be the soil we have as I've never understood how a football size stone can work it's way to the top of the ground for my mower to hit, but that cast wheel weight I lost off of a disc 30 years ago hasn't been seen since.
I'm not a licensed electrician or electrical engineer but I have helped install a good bit of 3 phase electric was well as my own single phase services. I can tell you that here no electrican will direct bury any electrical wiring nor will the inspectors pass it, all must be in conduit.

When and where I practiced I saw BOTH conduit and direct bury used, however as I already said much (NOT saying all) commercial and that done by electricians and specified by us electrical power engineers was in conduit, although many (NOT saying all) farmers or lay persons (maybe with or without inspection requirements) went the direct bury route. Still, in our jurisdiction REMC utility direct buried USE cable from their transformer to homes, but they did it properly at sufficient depths including sand fill under and over with warning tape. There can be advantages as well as disadvantages to BOTH methods but both can if done properly meet most codes. A MATTER OF CHOICE

John T Long retired and rusty electrical engineer SO NO WARRANTY LOL

john , i like the idea of the warning tape, i was thinking on using 1/2 rebar about a foot above the conduit, ( i have a butt load of rebar) to mark the line. i do keep maps of anything underground on the farms, water lines, field tiles, septic ect. sure helps a lot. id like to get somekind of hand held portable gps unit where i can get co ordinates and write them down on the map. beats pushing a tile probe! and once again john, thank you for your sharing your knowledge with everyone here. before i start a project, i like to ? run it up the flagpole? and gather as much information so i can make an informed decision. thanks again john! btw, i am prolly going to bury the conduit about 3 feet underground to avoid any problems with grain wagons going over it.

You're welcome and you and the good Krusemeister dude should come on down to Flywheelers some time. There are other Illinois dudes down here like my old buddy tractor puller Jimmy from St. Joe.

John T Hoosier neighbor

i would love to come down and hang with all you guys!! as soon as my health improves more, i hope to do it. had an ultrasound last week, still have blood clots in my left leg, my lung capacity is about 35 %. lupus is a horrible disease. i am doing a lot better than when i was in mayos in march. almost bought the farm up there. they are great!!!

> This was back in the 60's using a pipe laying attachment welded to a tractor pulled subsoiler so 18-24" depth, below frost line and deeper than any standard tillage equipment went, burying 4 ft down wasn't going to happen unless it was done by professionals at at cost we could not afford. Also here in KY if one went very deep you'll hit rock and you don't want wire laying on rock.

I'm guessing that 18-24 inch depth worked out to be closer to 12 to 18 inches. And of course if you hit the cable with a disk, it must have been shallower than that.

The wire was buried 18-24 inches deep but over the years it worked it's way back up, in some places it was less than 6 inches under the ground when we dug it up.
The water line was laid with the same equipment, we had a leak in that 40 year old line back in the summer, it was 16" down to the top of the line.

glennster if you have a smart photo or know someone that does, you can use it's gps app to map the location of anything you want.
I use a free app called My GPS Coordinance, I got for fire service to direct medical helicopters to accident scenes but have found it handy for many other uses.