[curiousB]

I am going to add a sub panel to an unfinished area of my basement for the relocation of my airconditoners and future expansion. The existing primary panel is located in a finished area of the basement so it is difficult to route circuits to.

A few data points; Cook County, IL so mandatory EMT, 200A main panel circa 1995.

I have a 3/4” abandoned EMT conduit run (60’ long) from the main panel to the unfinished area which saves me a lot of work trying to route a conduit run in a finished area. I’d like to maximize the amperage of the feeder to the sub panel. Based on conduit fill I see I am limited to two #4 or four #6 wires in the conduit. Since I am using EMT, the pipe will be ground so I need three wires in the conduit. Each phase plus neutral. I see if I run three #6 THHN I can run 75 A current at 90 degrees. I plan a lug style sub panel without a main breaker. 12 or 16 circuits. Bonding screw will be removed.

So my questions are:

1) ok to use 90 degree rating for THHN for a feeder ( in EMT) to a sub panel?
2) Can I round up the 75A rating for the THHN wire to 80A for the breaker at the main panel feeding the sub or do I round down to 70A? I know 70A 2P breakers are available but not sure 80A is very common.
3) if I had another conduit run to the unfinished area could I up the gauge to #4 and run two #4 wires in the 3/4” and the third #4 in another conduit run? Or does the feeder set have to be in the same raceway? Not sure I want to do this anyway but wondering if the feeder set must be intact the entire run.

Any other ideas how I maximize the current to the sub panel based on my unusual constraints. Running 1 1/4” EMT would be very difficult due to finished drywall ceilings....








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[joed]

1. Not sure but I think no.
2. see #1
3. Can't do it. All conductors for a circuit must run in the same raceway.

 

[curiousB]

I just looked up breaker and it is only rated to 75 C. So does that bump me down to 65A for the #6 THHN as the breaker becomes the limiting factor on current?


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[jbfan]

I just looked up breaker and it is only rated to 75 C. So does that bump me down to 65A for the #6 THHN as the breaker becomes the limiting factor on current?


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Yes. The 90 degree rating is for derating only.
You are limited to the 60 or 75 degree rating based on all the ratings of the termination points.
You should be able to go to a 70 amp breaker.

As joed stated, all conductors must be in the same conduit.

 

[rjniles]

With copper THHN IN 3/4 EMT, you can run 3#6 and 1#8 ground and stay within fill. Use the 70 amp breaker. Better than relying on the existing conduit run and not knowing the condition of the conduit connections.

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[Missouri Bound]

If you have more than 6 breakers in that sub panel you need a main breaker in the sub panel.

 

[jbfan]

If you have more than 6 breakers in that sub panel you need a main breaker in the sub panel.

No you don't. The main is in the same building and that is the only place for the breaker feeding the subpanel.

 

[surferdude2]

I've always been allowed to round up to the next trade size breaker. I think you would be safe going with a 70 amp breaker on #6 thhn feeders. Make all those EMT connectors and couplings up snug tight and use insulated grounding bushings on both ends with #6 jumpers (so you don't have to buy a short piece of #8) to the panel ground bar. Do it like that and you'll have no problems.

I have never done any work in Cook county, Ill. so it's a good idea to run this by an inspector since I have had some municipal code jobs that didn't allow any application under 100 amps to use any rating other than 65° C. That would run you down to 60 amps. Worth checking before buying the breaker.

EDIT: Just saw your later post saying the run isn't accessible so go with the ground wire advice as offered by rjniles

3/4" insulated grounding bushing

 

[curiousB]

With copper THHN IN 3/4 EMT, you can run 3#6 and 1#8 ground and stay within fill. Use the 70 amp breaker. Better than relying on the existing conduit run and not knowing the condition of the conduit connections.

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Good point. I think I’ll run the green #8 as well. Can’t hurt to have it and as you state a loose EMT connection could mean a doubtful ground which is compounded by these runs being buried in walls and ceiling so not something I can check easily.


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[surferdude2]

How many degrees of bends does that 3/4 emt run have? It's going to be a pretty full conduit now that you are including the grounding conductor so you will want to have some yellow 77 or equal on hand if it has more than a couple of 90's.

 

[curiousB]

Thx. Fortunately there is a pull box midway down the run, three 90s to the first 20-25’ then a straight run the rest of the way. I think I have some conduit lube somewhere.


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[surferdude2]

The pull box will help but a full fill run of any length is challenging if you haven't done some of them before. I don't know your experience level so I'll assume any advice will be accepted without offense. I'll assume you will try it your way and find that it's harder than you thought. The advice below will be valuable if that happens. If your way works, we'll all be happy and you can ignore all the following. Perhaps it may be useful to someone else. :wink2:

First off, getting a steel fish tape through a short run with three 90's in it can be difficult, depending on how the 90's are oriented to each other and how far apart they are. Fish tape resist going around a laterally pointed 90 immediately after having went through a previous 90 that pointed the other way. If you have any oriented that way, you may need to blow a mason's pull string in the conduit to use for pulling in a larger pull rope. Compressed air is needed for this. The discharge hose of a shop vacuum may work if no compressor is available. Tie a small tuft cut from a plastic shopping bag, fold it to form a bow-tie configuration that would fit barely snug into the conduit and tie the string to the middle of it. Start that bow-tie into the conduit and blow it through the other end.

On runs like this, I use poly pull tape but 3/16 or 1/4 poly pull rope will also work. You can pull the rope in with the mason's line or use the fish tape to pull it in if you didn't need to use the mason's line method outlined above. Using a fish tape for crooked hard pulls of this type of pull is doing it the hard way since getting the tape through the crooked conduit is hard enough but making and remaking the head is also more challenging. You will also risk ruining a fish tape on a hard pull like this since it can put a permanent bend in the tape by pulling it that hard around the bends and emt connector thread hubs.

Keeping the process as easy as possible is worth doing. Hooking this many larger wires to a fish tape or pull line makes it challenging to keep the head as small as the body of the pull. The trick there is to strip about 12" of insulation from all wires and then cut away the outer layer of strands so the remaining wire cores can be folded back to create an attachment loop. Loosely twist these folded wires against themselves and tape them all the way to within 2" of the loop eye you will be leaving to thread the pull rope through. The finished head should be taped as smooth as possible to prevent any abrupt edges and be no larger than the body of the following wires. Smaller is even possible if you do it right.

That makes for an easy as possible pull and allows for easy removal and reattaching the pull rope at the middle of the pull. Keep the rope knot as small as possible so it doesn't become part of the problem.

Using wire lube is important... don't scrimp. It requires having a helper feed and push while you pull the other end in a coordinated manner. You can't do this alone.

I'll include a picture of some of the pull tape I actually use that I'd donate to you if were local.

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[curiousB]

Just thought I'd put together a note on how I arrived at feeder sizing, ampacity and so on for others. The issue of conduit fill, fill derating, conductor thermal rating all comes into play.

 

[surferdude2]

Looks good to me... the 90° C. refusal was correct and the upsizing to the nearest available breaker size is commonly allowed. You should be able to run this through and get a permit to go forth. I'd be surprised if there was any objection from plan check.

Even so, a freehand sketch to an inspector before applying might be a good idea. The only possibility of a snag may be that he may required you to expose the existing conduit if you can't prove it was installed under an earlier permit. Be prepared to show proof if you have it. If any of it is exposed, be sure it has proper support so he will be more likely to accept the rest of it that he can't see.

 

[curiousB]

Well finished this install so here’s a couple pictures.

In general went pretty well. Pulling wires took some effort but I did get 4 conductors in the 3/4” conduit. Three #6 and one #8 for ground. 70A breaker feeding the sub.

I got a little carried away on grounds. I ran two independent EMT conduit runs to the panel and used ground bushings with conductor to ground bus. One conduit was the feeder, the other was a nearby receptacle box. These were in addition to the insulated #8 in the raceway.

I do have some concern the screws for the wires on the feeder breaker traveled all the way in.

I would have thought a 6awg conductor would be well fitting in a 70A breaker.




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[Missouri Bound]

First off, getting a steel fish tape through a short run with three 90's in it can be difficult, depending on how the 90's are oriented to each other and how far apart they are.

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What I have done in the past is use a vacuum to pull a string through the pipe.
You can easily pull 100' that way, then tie it to your fish tape and pull it back. It's an extra step but it makes getting the tape through a breeze.
It works the best on PVC but I have used it with success on metal conduit as well.
It doesn't take much to get the string through since the vacuum actually keeps the string centered in the pipe.

 

[curiousB]

That’s what I did. Some masons string and a small wad of a plastic grocery bag. Worked great then I used the string to pull the beefy conduit pull cord I used. Then the conductors.






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[surferdude2]

Only a couple minor suggestion on what is a job well done. I'd cut the feeder wires back to a mere down-over-down and land length. The equipment grounding conductor can also be shortened to land on the first strip it comes to.

Then a couple of more serious item. The inequality of the terminal screw depths on the 70 amp breaker is bothering me. Loosen both of those up and dress the wires so they enter straight and retighten them. Inspectors don't like wires to appear to be trying to pull against the lugs. I try to get them dressed so they are at zero tension and look like it.

When retightening, they should normally both go down in depth about the same amount considering it's the same size wire. If they still don't do that and you're sure they are snug, all's well.

You did go to the max on bonding... inspectors love that so good for you! You can work on my crew any time!

 

[Oldmaster]

FYI :You can only upsize to next standard breaker if there is no standard breaker that matches you conductor ampacity. In your situation since 65 amps is not a standard breaker size you can go up to 70 amp. NEC lists standard breaker sizes.

 

[curiousB]

..."The inequality of the terminal screw depths on the 70 amp breaker is bothering me. Loosen both of those up and dress the wires so they enter straight and retighten them. Inspectors don't like wires to appear to be trying to pull against the lugs. I try to get them dressed so they are at zero tension and look like it.

When retightening, they should normally both go down in depth about the same amount considering it's the same size wire. If they still don't do that and you're sure they are snug, all's well."...

What do you think about stripping the copper double the length and folding it over on itself to get the bulk up? I did check the breaker is rated for as low as #8 and I am using #6 but it just doesn't seem right.


Any other (approved) products or sleeves they make to bulk up a conductor for better fit in a lug?