Unless your kiln is tiny, it's going to need more than a 20 A circuit. If it's running at the same time you have other stuff going you may have shorted yourself.

You can not run THHN underground. It's not approved for wet areas (underground is always wet). Fortunately, most THHN these days is also rated THWN which is wet rated.

THWN #4 copper has an ampacity of 85 amps. The sub panel rating is largely immaterial, make sure you use an appropriate sized breaker on the feed. If 70 Amps is correct, your sizing for the voltage drop looks good.

Your ground is too small if you have larger than a 60 A breaker on the feeder.

Unfortunately 3 #4 and 1 #8 is on the hairy end. If your conduit is RMC, ENT, or sched40 PVC you are within limits, if it's Sched 80 pac or EMT you've exceeded it.

If your units have the choice of 120 or 240, 240 will have less voltage drop (because it's fewer amps which is the driving factor). Most of the motors work better on 240 anyhow.

In order to run a 100 amp service you would need to assume the you will be using 100 amps and size according to that. With that distance to keep the voltage drop under 3% which is the max allowed you will need to run 2-#2 coppers for the hots, 1-#4 copper for the neutral, and 1-#6 copper for the ground. This puts your conduit fill at 44% which exceeds the allowable 30% so you will need to upgrade the conduit from 1" to a min of 1-1/4" which will put the fill at 25% which is under the allowable 30%. You can go larger to make the pull easier if you want. You will also need a ground rod with a min of #6 bare solid running to it unless its already there. You want to size this correctly if you don't the voltage drop could damage the equiptment hooked up to it.

"In order to run a 100 amp service you would need to assume the you will be using 100 amps and size according to that"

it's a feeder, you can it size according to table 310.15(B)(6)

100 amps = #4 copper
GEC = #8 copper (250.66)
EGC = #8 copper (250.122)

(not accounting for voltage drop which I'm too lazy to do the calculation right now)

Again, the only issues I see is:

I'm concerned he shorted his calculations. Kilns tend to start at 20A and work their way up.

Assuming 70A is accurate, #4 is fine both a far as ampacity and voltage drop. #6 would barely squeak by on ampacity but the voltage drop may be an issue.

If he uses a feeder breaker bigger than 60A, he must use a larger ground than #10.

Computing the conduit fill on 3 #4's and a #8 ground is right at the limits. It depends what the conduit material is if 1" conduit will suffice.

OK guys, thanks for the responses. I have some more info.

The conduit is flexible grey (I don't know what that is called).

I have to replace the breaker on my main panel (it was ripped out) so I guess I can pick whichever size I want.

The kilns are glass kilns and the one I have is small and says 20-amps is fine. The one I want says 20-amps is fine too but with 240. I'm not sure how that changes things.

I see how it makes sense to upgrade quite a bit to account for future changes and upgrades. I was just hoping to reuse the same conduit so I don't have to dig in this clay soil. The downside would be spending a bunch of money on a wire that will be wasted if and when I do upgrade. If the answer is that I have to dig, then I do.

Sounds like my choices are:
1. Squeak by with 3 #4's and 1 #8 in my conduit
2. Dig. Bigger conduit. 2 #2's, 1 #4, 1 #6
Does that sound right?

You might be OK, unfortunately I only guessing what sort of flexible gray conduit you might have. LFNC probably is ok with that fill as well. You should ask your electrician what the conduit actually is. He should know how to compute the fill (there are calculators online or for his smart phone as well as tables in the back of the code).

Yes 4 is good for 100 amp but not with the voltage drop don't try to confuse people

You can get away with #4's if you want just use a 70 amp main at the sub panel instead of 100. So it would be 3-#4 and 1-#8 that will fit in 1" conduit. You want to get a main lug panel and put a 70 amp breaker in it with a breaker tie down to feed the panel. Fee it into the breaker don't use the lugs

The breaker at the sub panel is largely immaterial. The important breaker (which must be sized for the feeder wires involved as well as what size the ground must be) is the one feeding this panel (in the main panel or wherever the feeder originates).

There's not even required to be a breaker in the sub panel.

...that is to say there's no requirement that there be a single "main" breaker in the sub panel. Obviously breakers are required for the branch circuits.

Well obviously he would put the breaker in the main Panel as a 70 I didn't think I needed to write every detail. The breaker in the sub panel is for the 6 throws of the hand rule. It is a detached building if he does not use the main he will only be able to put up to 6 throws of the hand in the panel meaning 6 breakers. Single pole counts as 1 and 2 pole counts as 1. Typically a 100 panel will have at least 18 space in order to legally utilize all the spaces you need to have a main breaker which creates 1 throw of the hand to kill the power. And that is the purpose of the main breaker at the sub panel

You do NOT need to use a breaker as the disconnect.

Your information is incomplete and silly.

There is no reason to put a 70A breaker in the sub panel. He's free to put a 100A main breaker there even if it is fed by 14g wire for the feeder. The feeder is protected by the breaker at the feeding panel.

He's quite possibly legal with 3-#4 and 1-#8 depending on what sort of conduit he has. My best guess from his description is he has LFNC-A and that would result in less than a 40% fill.

I don't think you understand the purpose of the breaker in the panel. If he doesn't have a main disconnect he can only install up to 6 breakers. I guess he could just get a main breaker panel, probably would cost the same. If its LFNC-A the wires will fit hopefully your right

"keep the voltage drop under 3% which is the max allowed "

Allowed by what? The NEC? The NEC does not recognize voltage drop as a safety issue. It only talks about it in the fine print notes. Not to say it's not something to be taken in account, but it is not actually "code" and is not enforceable.

At 100 amps 138 feet 240v I come up with needing #3 feeders accounting for voltage drop. Anyone else actually do the calculation?
Your average home depot and the like probably isn't going to stock #3 but any good supply house will.

pull 3 #3's, and 1 #8 for the equipment grounding conductor, all of which you can pull in a 1" conduit assuming it's not schedule 80 PVC the entire run (which I highly doubt). You could downsize the neutral if need be also, but it's not needed since it will fit in your 1".

Yes I did the calculation but #3 isn't always easy to find so I went to #2 ok tough guy and besides we are passed that he wants to feed it for 70 amp

"There's not even required to be a breaker in the sub panel."
I have to disagree with you there Ron, the OP said the shop is 138 feet away from the house which I take to mean detached building. Not only is a disconnect required but a grounding electrode will be required as well. The disconnect does not have to be in the sub panel but it does need to be on the building.

This post was edited by joefixit2 on Mon, Mar 10, 14 at 22:34

It could be argued that 310.15(B)(7) does not apply for a workshop. True it is a residence but it is a stretch to say it meets the qualifications of 310.15 IMHO.

Also let's not forget that voltage drop is calculated for the expected load, not the circuit size/ capacity.

This post was edited by joefixit2 on Mon, Mar 10, 14 at 22:52

JOE The disconnect doesn't need to be a BREAKER.
Read what I wrote. I'm trying to get NICK off his stupid assumption that the sub panel breaker sizing has any bearing on the situation. He told the hapless original user that he should back feed a 70a breaker which is just plane dumb. The OCD is back in the feeding panel.

He can either use the 100A main breaker in his proposed panel, six handle throws of branch circuit breakers if he has that few, or he can just use a switch,

Good catch on the grounding system requirement. I assumed that he already had one since he was just upgrading an existing building service.

"Yes I did the calculation but #3 isn't always easy to find so I went to #2 ok tough guy and besides we are passed that he wants to feed it for 70 amp"

Tough guy? Sorry for calling you out for giving bad advice and information. I have never had an issue finding #3. Are we just guessing at people's expense now? I don't think people can find #12 easily so you better pull in 750kcmil....
If he's going with 70 amp now the wire size would be even smaller so you're still wrong.
You're just babbling and grasping at straws here to try to make what you've said justified. There's several posts where you're flat out wrong.

Look all I'm trying to say is 3 is very hard to find where I at only 1 out of 5 supply houses carry it and do you really think Home Depot would have 400 feet of it. 3 is an odd size and is hard to find don't be so dramatic about pulling 750 for a 12, really. And as far as the main beaker goes why would you put a seperate disconnect in and spend extra money when all you need to do it put a main breaker in the panel. And if you actually read what I wrote I was said
"Yes I did the calculation but #3 isn't always easy to find so I went to #2 ok tough guy and besides we are passed that he wants to feed it for 70 amp"
So I know we are fusing it at 70 amps. All he needs is 3-#4's and a #8. If you think he should waist his money on the diconnect and the time installing it when all he needs is a panel with a main breaker, then that's fine your just an idiot.

you guys need to switch to decaf.

First, thanks to all of you for the advice. I really do appreciate all the input. I feel smarter already.

Also, I think you are right about the kiln issues. The one I have is a tiny glass kiln that recommends 20 amps. But upon further investigation, future kilns that I like need 30 and 50 amps. So that sucks. I am thinking now that making this work in this conduit is a pipe dream. I am thinking I'm going to have to dig to avoid spending a bunch of money to NOT get what I want in the end.

So that being said, if I am digging anyway, I might as well do it right. So what does that look like? The perfect scenario would be what? 3 #2 wires and a #8 to a 100-amp panel with about 18 breakers? And pulled through a 1.5" conduit? What would an awesome shop wiring set-up look like?

Thanks again,
Kristi

Who said squat about #3? #4 is more than adequate for his load and the voltage drop. He can put an 100A panel in at his shop and still protect the feeder with a lesser ampacity.

All you are doing is introducing information that is extraneous at best, incorrect in some aspects, Nick.

Ronnatalie you read my last post I said the same to use #4

Basically : #4 at 70amp and 138'= 2.41% that is under 3% so it will work.
: In the main panel there will need to be a 70amp breaker.
: In the shop you will need a main disconnect, either a stand alone disconnect or a panel that has a main breaker, I usually use a main breaker panel, it's less to buy and less work.
: Dont forget the ground rod if there isn't one already, you can use 8 wire, I typically don't go any smaller then #6 on the rod but you can do what you want it's not a big price difference you should really only need 10'.
: This will all fit in the pipe you have at 33% fill, the max allowable fill is 40%.

"the allowable 30%." (from previous post)

"max allowable fill is 40%." (from today's post)

Go back to apprentice school please

Who said anything about #3? I did Ron. To be honest I skimmed through this thread and did a quick voltage drop calc at the distance and 100 amps which came to #3. Super sparky Nick here was suggesting #2 at the time. He's flip flopped so many times though that I'm totally lost even actually going back through and reading the entire thread now.
My suggestion to the OP is get a real electrician to actually come and look at things and not trust people on the internet. Granted some here are qualified, others... not so much.

Nick you are confusing the GEC with the EGC. The GEC cannot be smaller than #6 copper and if not physically protected it cannot be smaller than #4. The EGC can be #10 only up to 60 amps, then #8 is required. See 250.66(A) and 250.122 (table)

Sorry joe I thought that was right what you said I was just taking into consideration what hexus said earlier
"it's a feeder, you can it size according to table 310.15(B)(6)
100 amps = #4 copper
GEC = #8 copper (250.66)
EGC = #8 copper (250.122)"
That's what I get for listening to someone else, it makes me look stupid. I knew it was six in never in my life have ever ran less or have even bought eight bare. And as far as the fact that I said 30% earlier and now I said 40% that was just a typo guess ill go back to school and maybe learn how to type better. Or maybe ill write a letter to apple and tell them to make the letters bigger on the iPhone so I don't make mistakes, what do you think hexus? I guess you don't make mistakes, oh wait nevermind you made one about the ground.