Need help with Sub Panel design

Hi all, looking to understand the basics I guess of sub panel design and requirements. Have been doing homework for some time now and after reading numerous posts regarding this topic, I don't get the feeling it is an easy process.

Here is my situation:

We had our home built 3.5 years ago...
200 Amp service to the main panel in finished garage
Want 100 Amp sub panel (future proofing) in basement, about 100' away from panel in garage
Looked at sheet rock demo to expose walls to run that way however I think I want to run 3 - #2 CU THHN, with a #6 THHN ground in conduit on the outside of home. Run would be 15' to 20' of conduit before punching through sill plate to basement. Understand with THHN wire, it would have to remain in conduit all the way to the sub panel location. Want to use an Eaton type CH 100 amp 22 space panel for the sub panel, with 90 amp double pole breaker at main panel.

Have a decent load planned for the basement and I want to ensure that any future "honey do" projects will be accounted for by what appears to be a over-designed project.

Anyway, I am a rookie at best. I have attempted to do some homework on this, but after reading some spirited discussions, I am questioning my approach.

Any responses would be greatly apprieciated!!

 

I have a few comments. First of all, I assume this is an attached garage, correct? Overall, your approach is good. I like using conduit and individual copper THHN/THWN conductors over using aluminum SER cable although SER cable would be acceptable. You have upsized your hot and neural conductors to #2 and also upsized the ground (EGC) to #6. I assume that is because of the length (approximately 100 feet) of the subpanel feeder. The choice of panels is good, Eaton/Cutler-Hammer CH series is a copper bus loadcenter and breakers carry a lifetime warranty. Is this a main lug only panel? That's all you need as long as the garage is attached; no main breaker would be necessary. You are feeding this new subpanel with a 90 amp breaker in the main panel in the garage. Why just a 90 amp, why not a full 100 amp breaker? You didn't mention it, but the subpanel will require an accessory ground bar and the neutral bus WILL NOT be bonded to the panel box. All ground and neutral conductors will be kept separated in this subpanel. Your #6 EGC (ground) will terminate at this ground bar.

The only suggestion I might make would be to investigate routing of the conduit without taking it outside.

 

Yes, it is an attached garage. Yea, i tend to err on the conservative side which is why the upsize. Sorry for my ignorance here but I don't know how to answer your question, "Is this a main lug only panel". As for the main breaker, just thought it would be handy to be able to isolate the basement if needed rather than killing the whole house. (again might be overkill). Charts I am trying to understand show #2 CU THHN is good for 95 amps. Figured the panel is good for 100 amps, wire is good for 95 amps, then PROTECT it all with a 90 amp breaker? My logic might be wrong.

The only way not to do demo of existing is too route it outside. (trying to minimize costs).

 

A main lug panel has no main breaker, just lugs to terminate the feeder. A main breaker is not required since the garage is attached, but code does no prohibit a main breaker. I would use a MLO (main lug only) panel.

Use the 75 degree C column, #2 is rated at 115 amps. I'd probably use #3 (100 amp rated) and a #8 for the EGC (ground) and feed from a 100 amp breaker.

 

For understanding purposes, a supplier sells 3 gauge stranded THHN cable that shows 95 amp allowable amps. Guess I am confused by this...

 

The main breaker panel may be cheaper than a MLO.

 

After some looking at the local supplier (will refrain from name dropping here), the MLO is half the cost than the main breaker panel.

 

Also, forgot to mention, I want to do the all the work I can myself (to save costs), then hire a pro to make the connections....

 

The conection part isn't hard but finding an electrician to do just the connections may be. He'd be risking his license on the hope you didn't screw up. Plus who draws the permit.

 

I may have found a guy that will work with me. Need him to verify my approach and materials. I am able to run the lines and deal with the conduit no problem.

 

Ok, so I looked a little closer at the charts and now see the information I was confused about...
for 100 amp sub panel (MLO), 3 - #3 CU THHN (red, black, white) & 1 - #8 CU THHN (green) would do the trick? If I am making a massive snafu, please someone chime in..Thanks all.

 

The 90 degree column is used for derating purposes. The terminations in the panel are not 90 degree rated so you need to use the lower rated ampacities.

 

Look one column to the left, the 75 degree column, and you'll see #3 is rated at 100 amps. You have the 90 degree column circled, that column is used for derating purposes. By the way, I'd also suggest using 1 1/4" conduit.

Yes

And yes, you'll need someone who will work with you and who can take out the permit and arrange the proper inspections. Hopefully that is something that is possible in your area, if not, you'll have to contract the entire project. In my area, a contractor is prohibited from taking out a permit to cover someone elses work. The penalty if caught is a $500 fine and possible loss of electrical license.

 

geez.....rookies. Yea, i picked the wrong column....should be the 75 degree column...

 

Do I need to concern myself with voltage drop given the run will be around 100' or so? If so, would it be worth upsizing the wire #3 -> #2, with #4 ground?

 

Possibly. In order to determine that, the total load on the new subpanel needs to be known. Do a residential load calculation for that panel and tell us what that shows.