Subpanel for shop CU or AL?

[AndyCBR]

Hello,

Looking for some input on wiring my attached garage with a subpanel.

I have a newish house that was built (by others) with little room for expansion in mind. It has a 16 space main panel and most of the house runs off of a 125A sub panel from there. The main is filled with most of the 2 pole circuits and a generator interlock breaker. The good news is I have a 200A service so I am fine on power but short on free space in panels.

The 125A panel (closer to shop) is loaded up with the maximum tandems (only bottom 5 allowed) except for two spaces. While physically possible to install 2 more tandems and gain 2 spaces I don't view that as a good option for wiring a sub panel for my shop.

So I am planning to run about 120 feet in the attic all the way back to the main for the shop sub panel which only has two spaces free.

Loads in the shop will be a 1 or 2 stage compressor (3-5 HP). I'll also need a 2 pole space for a future mini split. I'm OK on receptacles/lighting for now with what exists.

Which brings me to my question. What wire would you run?

6/3 with ground for a 60 amp subpanel would work but might be pushing the limits on voltage drop for that length.

For the same money I can run 2-2-2-4 Aluminum and probably have 90 amp capacity for future use. I suspect the thicker aluminum will be as easy to run as the thinner copper.

I can't see myself using 90 amps but I sure as heck don't want to run through the attic to the main again at a later date.

Seems to me like the AL is the way to go. Any caveats to using the AL conductor in this application? Anything special to terminate to the panels or breakers?

Apologies for the longwinded post.

 

[Mustang51js]

Do you know what the tandom breakers do now, most of the time you can combine a few circuits to free up space. Copper is better but for price go with aluminum

 

[pattenp]

If you go the 120ft I'd use Al SER and go with the 2-2-2-4. Be sure to use Noalox paste on the wire ends at the terminations.

 

[theoldwizard1]

If you go the 120ft I'd use Al SER and go with the 2-2-2-4. Be sure to use Noalox paste on the wire ends at the terminations.

AL SER is a good choice if you have a long interior run. IIRC, SER must be run in conduit if underground.

If you have a long under ground run, you could switch to MHF in a outside Jbox just above where it goes underground. The MHF CAN be direct buried, but it requires a conduit above ground. Use a Schedule 80 sweep when coming out of the ground.

Then again, it might not be worth the effort/cost to switch cables. Extra Jboxes, splices, etc.

 

[AndyCBR]

Do you know what the tandom breakers do now, most of the time you can combine a few circuits to free up space. Copper is better but for price go with aluminum

There are some spaces labeled "Lights" that are tandem circuits. I could combine circuits if I had to and free up a space or two but I still think the 125A panel is a bad place to put a 60A breaker based on how full it is. It would also be physically difficult to get large wire into the panel from the top based on large number of conductors entering the box.

 

[AndyCBR]

AL SER is a good choice if you have a long interior run. IIRC, SER must be run in conduit if underground.

If you have a long under ground run, you could switch to MHF in a outside Jbox just above where it goes underground. The MHF CAN be direct buried, but it requires a conduit above ground. Use a Schedule 80 sweep when coming out of the ground.

Then again, it might not be worth the effort/cost to switch cables. Extra Jboxes, splices, etc.

This would all be in the attic except for 5 feet or so where I would use a 1-1/2" PVC conduit to get to the load center down the soffitt and brick wall on the exterior of the house. The main is on the outside wall next to the meter.

 

[wyliesdiesels]

AL SER is a good choice if you have a long interior run. IIRC, SER must be run in conduit if underground.

If you have a long under ground run, you could switch to MHF in a outside Jbox just above where it goes underground. The MHF CAN be direct buried, but it requires a conduit above ground. Use a Schedule 80 sweep when coming out of the ground.

Then again, it might not be worth the effort/cost to switch cables. Extra Jboxes, splices, etc.

SER CANNOT and SHOULD NOT be run underground!

 

[theoldwizard1]

SER CANNOT and SHOULD NOT be run underground!

OOppss !! Thanks for the correction !

 

[AndyCBR]

Went with the AL SER and it was $0.40/ft cheaper to boot.

Had to derate to the higher column per NEC for being in the walls/conduit in 2 spots so only good for 75A per code. No 75A breaker so a 70A is what I used.

Other issues were had to search a little for some mega staples and the size of the wire required a long radius 90 at the panel outside (was no way that wire was going to turn in an LB and would have definitely challenged the 2"minimum radius).

 

[wyliesdiesels]

Went with the AL SER and it was $0.40/ft cheaper to boot.

Had to derate to the higher column per NEC for being in the walls/conduit in 2 spots so only good for 75A per code. No 75A breaker so a 70A is what I used.

Other issues were had to search a little for some mega staples and the size of the wire required a long radius 90 at the panel outside (was no way that wire was going to turn in an LB and would have definitely challenged the 2"minimum radius).

being in walls and conduit DOESNT require derating. Being in insulation does depending on the total length the SER is in the insulation.

 

[AndyCBR]

being in walls and conduit DOESNT require derating. Being in insulation does depending on the total length the SER is in the insulation.

When the walls in question are insulated that would require derating. I am pretty sure the requirements are pretty strict for this and only allow less than 10 feet "in insulation" to stay in the higher ampacity column.

Besides we get very high attic temps here in La. It should really be in the more conservative column anyway in my mind for my install.

[emoji106]





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[wyliesdiesels]

If u have high attic temps, then u shouldve done another deration for that as well...

 

[AndyCBR]

If u have high attic temps, then u shouldve done another deration for that as well...

Thanks for the advice but I'll sleep well with the derating I have already done! I'll never see these loads anyway.

There's high attic temps in most of the country depending on which day you pick.






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