4GA Cu feeders for 100A?

[mwchemy]

Hello all, longtime reader, first post. Hoping I can get your help in clearing up my choice for feeder conductors.

I'm adding a panel (flush) in my attached, finished garage, next to the main (flush). I need to go into and up an insulated stud bay. Total feeder length is about 4-5ft. Right now I've got a 100A breaker for the additional panel. Shouldn't need near that amount of juice currently, but I've already got the breaker. Here are my choices for feeders as I see it:


#1) QTY(4) 4GA copper THHN, I think (free, b/c I have extra laying around)

- do these need to be in conduit?
- can I use the 90c ampacity if run through insulation?

city inspector (over the phone) said this would be OK for 100A, FWIW.
I believe they're using the 2008 NEC

(I realize this should be a slam dunk, but can't determine if code allows this...also, may not have the same inspector actually signing off on the work)

#2) 2-2-2-4 Al SER ($1.59/ft)
- if I understand correctly, this does not need to be in conduit, but should be limited to the 60c ampacity of 75A. If going this route, I would probably buy a 60A breaker due to cost of $7 vs $22 for a 70A.

60A should be fine for my needs now, but don't want to open up the drywall again and upgrade feeders if they increase in the future.


My preference would be to run my existing individual 4GA copper through the insulation w/o conduit to my existing 100A breaker, assuming it's legit.

What do you think?

Thanks in advance!

 

[Gooch]

Hello all, longtime reader, first post. Hoping I can get your help in clearing up my choice for feeder conductors.

I'm adding a panel (flush) in my attached, finished garage, next to the main (flush). I need to go into and up an insulated stud bay. Total feeder length is about 4-5ft. Right now I've got a 100A breaker for the additional panel. Shouldn't need near that amount of juice currently, but I've already got the breaker. Here are my choices for feeders as I see it:


#1) QTY(4) 4GA copper THHN, I think (free, b/c I have extra laying around)

- do these need to be in conduit?
- can I use the 90c ampacity if run through insulation?

city inspector (over the phone) said this would be OK for 100A, FWIW.
I believe they're using the 2008 NEC

(I realize this should be a slam dunk, but can't determine if code allows this...also, may not have the same inspector actually signing off on the work)

#2) 2-2-2-4 Al SER ($1.59/ft)
- if I understand correctly, this does not need to be in conduit, but should be limited to the 60c ampacity of 75A. If going this route, I would probably buy a 60A breaker due to cost of $7 vs $22 for a 70A.

60A should be fine for my needs now, but don't want to open up the drywall again and upgrade feeders if they increase in the future.


My preference would be to run my existing individual 4GA copper through the insulation w/o conduit to my existing 100A breaker, assuming it's legit.

What do you think?

Thanks in advance!

THHN needs to be in a raceway. #4 is ONLY good for 100A if it's a residential dwelling(home) service. In your situation #4 is good for 85A Since this isn't a standard size breaker, you can put it on a 90A

If you are good with 60A, just get some 6-3 romex don't waste your time fighting 2/2/2/4 just for 60A.

 

[pattenp]

NM-B (Romex) needs to be sized on 60 deg, period. SER if run in insulation is sized from 60 deg, if not in insulation then 75 deg. THHN in conduit is 75 deg regardless of insulation.

You may find the 2-2-2-4 Al SER per foot is as cheap or cheaper than 6/3 copper per foot. The 2-2-2-4 SER will give you a little more headroom on the amps.

 

[mwchemy]

I vaguely recall that there was some allowance/variance/interpretation that a main panel to another panel run could be considered under the "residential service feeder" definition and thus the inspector Ok'ing the 4GA copper? However, if it needs to be run in conduit and derated, then even that isn't OK, right?

Sounds like the easiest (for my needs), guaranteed spec install is purchasing the 2-2-2-4 Al SER and 60A breaker.

I might even come out ahead if I return the 100A

Thanks!

 

[pattenp]

Nope. To fall under the definition as a Service it needs to supply the entire load to the dwelling. Using it to feed an auxiliary panel is a branch feeder so different rules apply. The 2-2-2-4 Al SER run in your insulated wall needs to be sized on 60 deg @ 75A, which you can over current protect @ 80A.

I vaguely recall that there was some allowance/variance/interpretation that a main panel to another panel run could be considered under the "residential service feeder" definition and thus the inspector Ok'ing the 4GA copper? However, if it needs to be run in conduit and derated, then even that isn't OK, right?

Sounds like the easiest (for my needs), guaranteed spec install is purchasing the 2-2-2-4 Al SER and 60A breaker.

I might even come out ahead if I return the 100A

Thanks!

 

[Gooch]

I vaguely recall that there was some allowance/variance/interpretation that a main panel to another panel run could be considered under the "residential service feeder" definition and thus the inspector Ok'ing the 4GA copper? However, if it needs to be run in conduit and derated, then even that isn't OK, right?

Sounds like the easiest (for my needs), guaranteed spec install is purchasing the 2-2-2-4 Al SER and 60A breaker.

I might even come out ahead if I return the 100A

Thanks!

while 6-3 romex might cost a bit more, it is MUCH easier to work with, for 60A IMO it's stupid to use SER.(especially if it's 5 feet)