Need advice on wire gauge and conduit, etc

[sdbowers]

Hi there.

I'm currently building a steel building on my property (half hobby shop, half RV garage). I'm planning to run 100 amp service to a sub panel from my 200 amp main service in my house. (want to make sure I have plenty of power for small welder, plasma cutter, air compressor, RV plug-in, etc).

I've searched for these answers but haven't found a definitive answer with a high degree of confidence - usually just get the "I think you could use..." - I'm looking for: "You could definitely use xyz and here's why". hehe

I'm out in the county - they use NEC 2008 for code.

1. The run is a total of about 90 feet from main panel to sub panel. What gauge of wire do I need, accounting for voltage drop?

2. I will be running underground - can I use metal conduit and stay 12 inches under, or do I have to go 18 inches and go with pvc?

Would love to hear thoughts from any of you who have done this. What worked? What did not work? What would you do differently?

thanks!

 

[sberry]

You need 24 inches and the most economical is number 2 alum good for 90A. I done several of these and used a 60 to feed them all,,, never had a call back for a tripped breaker, never. Had a couple that were busy, my Bud ran a mechanic shop 30 years from it with 5 hp comp.

 

[3rdgendslmech]

I wouldn't use metal conduit just for the sake of only having to dig 12 inches. I'd be worried about it rotting out. Over the weekend I dug and laid about 50' of 1 1/2 pvc for my building. It wasn't too bad of a job. 4" trench shovel a pick mattock and a post hole digger to pull out the loose material. Going to hopefully finish the run this weekend. I'm pulling 3 runs of #2 Al and a #4al for ground xhhw wire.

 

[tapered-pin]

Why wouldn't you just direct bury the feeder using 1/0 Aluminum?


HERE is a calculator, btw


Here's another calculator that says 2/0 will work (lowering the insulation value to 60 degrees)


service/feeder isn't cheap..

 

[ard]

Personally I will NEVER use direct bury. Just me.

Sched 40 PVC, 18 inches.

I can rent a trencher and do 100 feet an hour, give or take. Cover it up by hand.

#1/0 AL will give you 100A.

 

[sdbowers]

Thanks for the tips! Appreciate it.

 

[sdbowers]

Can you tell me why I should or shouldn't use direct bury cable? What are the pros and cons? Sorry if this is a "newb" question. Just trying to get educated. I'd rather ask a "dumb" question and look stupid than not ask and make a "dumb" mistake. thanks guys! Really appreciate your input.

 

[tapered-pin]

Personally I will NEVER use direct bury. Just me.

why not?

 

[75gmck25]

My opinion is that direct bury does not save you much initial labor because it has to be buried deeper, and once you get it in place you are stuck with whatever wiring you installed.

If you bury 2" (or larger) schedule 40 conduit with LBs to use for pull points, you only need to go to 18", and if there is ever an issue in the future, you could pull the wire out and install something else. It just gives you flexibility.

I used 2-2-2-4 AL mobile home feeder to my detached garage, which allows you to use up to a 90 amp breaker at the main panel (I used a 60 amp breaker since it was cheaper, and readily available). I rented a trencher to make the 18" deep trench, and used 2" PVC conduit. I used a 24 space panel in the garage, since it was not much more expensive than a smaller panel, and it uses the same breakers as my Square D QO main panel.

So far I only have four circuits in my garage - 2 circuits for GFCI bench receptacles, one circuit for outside GFCI receptacles, and one circuit for lights.

Bruce

 

[tapered-pin]

I guess my point is that if you're renting a trencher, why not use direct burial cable..
*it's not like the trencher cares whether it is digging an 18" trench vs a 24" trench
*RARELY (I've never seen an issue in 20 years of commercial construction that wouldn't have also been an issue in conduit) does anything "go wrong" with it.
*it's cheaper, since you dont have to buy the PVC at all (and when you do, you have to purchase sweeping 90s and/or pull boxes.)
*one DIY kind of man can run the feeder without any specialized equipment like a cable puller.
*using pvc conduit doesn't really guarantee that you'll be able to re-use the conduit if "something goes wrong" with the feeder (like a crushed conduit or the PVC being filled with water/dirt during a flood)..


I'm not pretending that direct burial cable doesn't have its drawbacks as well, but for someone who is running his own feeder/subpanel, it's much less complicated..
(especially if there isn't any specialized landscape or hardscape between the panel and the subpanel)

 

[ard]

why not?

My neighbor and I built in the same year.

He has had a failure of his well line AND his main PoCo line. Both direct burial.

Poco paid for his main line, but it was under his asphalt driveway- he got to replace

He paid for the well line. Replaced with PVC conduit.


In my 45 years of doing electrical work, I just prefer the idea of a protective, inert conduit to isolate from rocks, motion, chafing, etc. Plus you can pull out old conductors and replace, if upsizing or additional conductors are needed. Also, I have a common practice of dropping empty conduits into open trenches - 'for future use'.

Again, lots or arguments around this. No one 'right' answer. My personal opinion. The cost differences are not material, one way or the other.IMO

 

[alfredeneuman]

*it's cheaper, since you dont have to buy the PVC at all (and when you do, you have to purchase sweeping 90s and/or pull boxes.)

90 feet of the proper size PVC conduit, elbows included, will cost you less than $65 and last a lifetime.
It's Cheap Insurance

 

[tapered-pin]

In my 45 years of doing electrical work, I just prefer the idea of a protective, inert conduit to isolate from rocks, motion, chafing, etc. Plus you can pull out old conductors and replace, if upsizing or additional conductors are needed. Also, I have a common practice of dropping empty conduits into open trenches - 'for future use'.

I agree professionally, and if he were paying someone to install the feeder, I would agree with you.

I do LOTS of things personally when I'm working on a project that i do differently when I'm being paid to do the work for someone else

Again, lots or arguments around this. No one 'right' answer. My personal opinion. The cost differences are not material, one way or the other.IMO

it comes down to DIY vs hiring a professional.
there are hundreds of thousands (millions?) of miles of direct burial conductor installed in the US with never a problem. My brother had 3ea 2" conduits run from his house to his garage (two were utilized and a spare) and all three have had to be replaced since they were originally installed (6 yrs ago).

90 feet of the proper size PVC conduit, elbows included, will cost you less than $65 and last a lifetime.
It's Cheap Insurance

that's MY point, it isn't necessarily going to last a lifetime.

it also isn't necessarily insurance as it's a HUGE deal to get the wire out of conduit if you ever need to do so..

 

[alfredeneuman]

.
there are hundreds of thousands (millions?) of miles of direct burial conductor installed in the US with never a problem.

So you're saying every man, woman,and child in the US has >1000' of direct burial with no problems?

 

[ard]

it's a HUGE deal to get the wire out of conduit if you ever need to do so..

lol.

 

[alfredeneuman]

it's a HUGE deal to get the wire out of conduit if you ever need to do so..

If you believe that then you're doing it wrong

Have you ever tried to pull the conductors out of a direct burial cable?

 

[tapered-pin]

So you're saying every man, woman,and child in the US has >1000' of direct burial with no problems?

Every single shopping center/mall/stadium parking lot, in the US has direct burial conductors for it's site lighting...

every neighborhood with underground lines uses direct burial for the distribution and service to the homes..

I ASKED if it was millions of miles, I KNOW it's over a hundred thousand miles.

If you believe that then you're doing it wrong

Have you ever tried to pull the conductors out of a direct burial cable?

Have you ever tried to remove a 75' service out of a conduit that is crushed in one spot?
Have you ever tried to remove a 40' service out of a conduit that is filled with dirt/sand?

 

[alfredeneuman]

Every single shopping center/mall/stadium parking lot, in the US has direct burial conductors for it's site lighting...
every neighborhood with underground lines uses direct burial for the distribution and service to the homes..
I ASKED if it was millions of miles, I KNOW it's over a hundred thousand miles.

I've done extensive site lighting, both new and as a service tech, in all of these places (except for stadiums), and ALL were done in conduit.....
I've also done some underground distribution work for residential utilities (12-33KV, the company I worked for was a SC Edison approved contractor), and they required conduit too. The utilities then install the conductors later.

(Maybe things are done differently in GA)

 

[tapered-pin]

I've done extensive site lighting, both new and as a service tech, in all of these places (except for stadiums), and ALL were done in conduit.....
I've also done some underground distribution work for residential utilities (12-33KV, the company I worked for was a SC Edison approved contractor), and they required conduit too. The utilities then install the conductors later.

(Maybe things are done differently in GA)

I've seen conduits as well, but most of the work I've seen professionally and personally is direct burial.

 

[alfredeneuman]

You should not have used word "every" then.

 

[flan]

Does the panel in the garage need a main breaker

 

[sberry]

Basically for purpose of this conversation,,, if it has more than 6 spaces or breakers and if it is in a detached garage it needs a main.
You can use a 6 space main lug for service entrance. There will be some argument but the code doesn't use the word sub,,, this is basically a service to a detached structure from an existing service. Needs a disconnecting means and its own grounding electrodes.

 

[flan]

Basically for purpose of this conversation,,, if it has more than 6 spaces or breakers and if it is in a detached garage it needs a main.
You can use a 6 space main lug for service entrance. There will be some argument but the code doesn't use the word sub,,, this is basically a service to a detached structure from an existing service. Needs a disconnecting means and its own grounding electrodes.

Cool. It’s probably best to get a main panel and put it out there then.

 

[sberry]

Put a main breaker, 20 spaces.

 

[wyliesdiesels]

I've seen conduits as well, but most of the work I've seen professionally and personally is direct burial.

My local customer owned Co-op got screwed by a direct burial cable salesman.

Decades later after installing the cable it is starting to fail all over town.

They are in the process of replacing every bit of it.

Conduit is cheap insurance.

Your examples of crushed conduits is not a good one. If the conduit is crushed, you couldnt use it for new wire anyways.

And if theres debris in the conduit, theres methods of getting it out.

The truth is, the earth moves, and rocks will nail the direct burial cable.

Why are you such a cheapskate to the point of not wanting to use conduit?

 

[Bert_]

I install a bunch of direct burial, very few failures. I see lot's of stuff out there that was put in in the 70's and 80's with no failures to date.

90% of the problems I see are from people digging and hitting a wire, in that case conduit won't help a bit. It's usually easier to simply splice a direct buried wire rather than repull a bunch of wire though conduit. If the ends are exposed it is literally a 15 min fix with direct bury. If I had to repull wire in conduit it's going to take at absolute minimum a half day to get the new wire and probably a few hours to repull and make connections.

Not to mention once you get over 100 amps or more than a couple bends the wire can be nearly impossible to pull without equipment.

A year ago or so I did a run of about 450' of 350 Urd. Conduit would have added about a grand to the cost. Not to mention the equipment it would take to pull that size wire that far. On the extreme off chance that it has a failure from a rock or something I can dig it up and fix it in a couple hours or less.

 

[tapered-pin]

Why are you such a cheapskate to the point of not wanting to use conduit?

lol, I'm a lot of things, cheap isn't one of them.

the guy wants to do the work himself, conduit IS NOT insurance, it's a time saving method, nothing more.

I dont care what y'all do, im only letting OP know the most cost effective way to run the service..

conduit is only in place to protect the conductors (direct burial has a more expensive casing to protect the conductors), it is NOT a raceway similar to walkerduct or the like.

 

[Bert_]

lol, I'm a lot of things, cheap isn't one of them.

the guy wants to do the work himself, conduit IS NOT insurance, it's a time saving method, nothing more.

I dont care what y'all do, im only letting OP know the most cost effective way to run the service..

conduit is only in place to protect the conductors (direct burial has a more expensive casing to protect the conductors), it is NOT a raceway similar to walkerduct or the like.

We are talking about a complete conduit system from one panel to the other. It is most definitely a raceway, you install the conduit first then pull wire into it.

 

[alfredeneuman]

(direct burial has a more expensive casing to protect the conductors)

More expensive than what?

 

[alfredeneuman]

it is NOT a raceway .

The NEC definition is:
Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing.......

 

[7635tools]

I install a bunch of direct burial, very few failures. I see lot's of stuff out there that was put in in the 70's and 80's with no failures to date.

90% of the problems I see are from people digging and hitting a wire, in that case conduit won't help a bit. It's usually easier to simply splice a direct buried wire rather than repull a bunch of wire though conduit. If the ends are exposed it is literally a 15 min fix with direct bury. If I had to repull wire in conduit it's going to take at absolute minimum a half day to get the new wire and probably a few hours to repull and make connections.

Not to mention once you get over 100 amps or more than a couple bends the wire can be nearly impossible to pull without equipment.

A year ago or so I did a run of about 450' of 350 Urd. Conduit would have added about a grand to the cost. Not to mention the equipment it would take to pull that size wire that far. On the extreme off chance that it has a failure from a rock or something I can dig it up and fix it in a couple hours or less.

I highly doubt that. A 450’ run, and what your going to know exactly where the fault is located. No way. By the time you locate the line and then the fault and dig it up and repair it you’re into that more than a couple hours.


Sent from my iPhone using Tapatalk

 

[alfredeneuman]

There are cable fault finders made that can pinpoint underground faults in an instant.
(I've even heard of a hillbilly way involving removing a wire from the distributor cap from a vehicle, hooking the suspected faulty wire to the cap and using a poorly tuned AM to find it, but I wouldn't recommend it)

 

[Norcal]

There are cable fault finders made that can pinpoint underground faults in an instant.
(I've even heard of a hillbilly way involving removing a wire from the distributor cap from a vehicle, hooking the suspected faulty wire to the cap and using a poorly tuned AM to find it, but I wouldn't recommend it)

What if your vehicle does not have a distributor?


All jokes aside, would agree with it not being recommended.


Unless your burying cable in undisturbed Vina Loam soil, I would not be comfortable w/ direct buried cable unless it was bedded in sand. Vina loam is rock free, and easy digging soil.

 

[tapered-pin]

We are talking about a complete conduit system from one panel to the other. It is most definitely a raceway, you install the conduit first then pull wire into it.

not in the commercial sense..

More expensive than what?

multi conductor cable that is NOT rated for burial, duh...

The NEC definition is:
Raceways include, but are not limited to, rigid metal conduit, rigid nonmetallic conduit, intermediate metal conduit, liquidtight flexible conduit, flexible metallic tubing, flexible metal conduit, electrical nonmetallic tubing, electrical metallic tubing.......

as I noted above, not a raceway in the commercial sense of the term.
(large unobstructed interstitial space readily accessible for the distribution of electrical conductors).
by that reference mc flex cable is considered a raceway and there isn't a serious electrician on the planet that looks at MC cable and Walkerduct as being the "same thing".. NEC uses the term to qualify housings for the purpose of heat buildup and NOT for the purpose of easily distributing cabling through "hidden or finished" assemblies.

This board is FILLED with people who've watched too much HGTV, spend too much time on the internet, and lack real world application/installation of the assemblies to which they speak.

 

[Norcal]

MC cable is not a raceway it is a cable just as NM is, you cannot remove nor replace conductors contained in the factory jacket of the cable assembly.

 

[alfredeneuman]

This board is FILLED with people who've watched too much HGTV, spend too much time on the internet, and lack real world application/installation of the assemblies to which they speak.

This board is also FILLED with people with questions to ask. IMO it's a good place to get answers. Plenty of qualified people populate these boards to answer those questions.
If you are talking about me, I've been an electrician continuously since 1970, retired in 2016.
You've never stated your experiences or trade.

Where did you get your definition of "Commercial Sense", copy and paste directly from the Walkerduct catalog?

 

[tapered-pin]

Where did you get your definition of "Commercial Sense", copy and paste directly from the Walkerduct catalog?

20 years as a GC licensed in 18 states.

 

[ard]

SO much fail here......

 

[wyliesdiesels]

A general contractor... that explains it all...

 

[Norcal]

A general contractor... that explains it all...

. . Thanks for the chuckle, this thread needed it.