90 Amp Subpanel Question

[Hamball44]

I'm looking to do something similar to the thread below with a slight twist, but didn't want to hijack that thread.

Detached Garage 90A or 100A Subpanel

This is a post similar to others...but taking a slightly different spin. I want to run a 100A SubPanel to a new detached garage. From 200A Main Panel to 12x12 Junction Box where the LB enters the garage is 45' run through the attic, all inside the current garage. From 12x12 Junction Box...

www.garagejournal.com

When we built our house a few years back, I had the electrician run 100 amps to the outside of the house knowing I would eventually build a detached garage. Looking at my power needs, I think 90 amps will be more than sufficient. I plan on running 2-2-2-4 AL MHF in 2" PVC conduit from the 100 amp distribution box (not sure if that's the correct terminology) on the outside of the house approximately 50' to my detached garage.

I realize I will need to have a 90 amp breaker in the subpanel, as well as changing the existing 100 amp breaker on the outside of my house to 90 amps. The question I have not been able to find an answer to is do I also need to change the 100 amp breaker on my main panel?

Also, is #6 CU wire sufficient for connecting the ground rods to the subpanel?

 

[ycgoat]

If your wire is rated for 90*C it should be good for 100 amps, but I believe you will need a #4 ground to the ground rods.
Where in Central Va are you, I am in Yorktown and just put a shop in Lunenburg County outside Blackstone.

 

[wyliesdiesels]

I'm looking to do something similar to the thread below with a slight twist, but didn't want to hijack that thread.

Detached Garage 90A or 100A Subpanel

This is a post similar to others...but taking a slightly different spin. I want to run a 100A SubPanel to a new detached garage. From 200A Main Panel to 12x12 Junction Box where the LB enters the garage is 45' run through the attic, all inside the current garage. From 12x12 Junction Box...

www.garagejournal.com

When we built our house a few years back, I had the electrician run 100 amps to the outside of the house knowing I would eventually build a detached garage. Looking at my power needs, I think 90 amps will be more than sufficient. I plan on running 2-2-2-4 AL MHF in 2" PVC conduit from the 100 amp distribution box (not sure if that's the correct terminology) on the outside of the house approximately 50' to my detached garage.

what size wire did he run to this box?

I realize I will need to have a 90 amp breaker in the subpanel,

no actually you dont need to do that. the main breaker in the subpanel is simply acting as a disconnect. the feeding breaker in the main service panel is what protects the wire....

as well as changing the existing 100 amp breaker on the outside of my house to 90 amps. The question I have not been able to find an answer to is do I also need to change the 100 amp breaker on my main panel?

back up. so he ran a feeder to a breakered disconnect? totally overkill. I would remove the disconnect and just splice onto the MHF.

yes the feeding breaker needs to be changed to 90a

Also, is #6 CU wire sufficient for connecting the ground rods to the subpanel?

yes

If your wire is rated for 90*C it should be good for 100 amps, but I believe you will need a #4 ground to the ground rods.
Where in Central Va are you, I am in Yorktown and just put a shop in Lunenburg County outside Blackstone.

lots of incorrect info here.

breakers are not sized @ 90° C ampacity. the 90° column is for derating purposes only. also, when #2 AL is used for a branch feeder, it is rated @ 90a not 100a...

GEC for rods does NOT need to be #4....

 

[ycgoat]

Ok [B]wyliesdiesels[/B], I stand corrected #6 is correct for their ground rod connection (#4 is for a 200 Amp service), but my 2017 code book Table 310.15(B)(16) for 90* AL RHW2 shows 100 amps. What do you see that I do not.

 

[Hamball44]

Wyliesdiesels, thanks for the response, this is very helpful!

Yes, it is currently in a breakered disconnect (couldn't remember the term for it). From what I remember, the County made them do that to pass inspection. Doesn't really surprise me at all if that is incorrect after dealing with the permit officials while building the detached garage.

I'm unsure of the wire size between the main panel and the disconnect, I didn't see any markings on the wire within the disconnect, but it appears to be #1 or 1/0 AL. I have not opened the main panel to check yet.

 

[wyliesdiesels]

Ok wyliesdiesels, I stand corrected #6 is correct for their ground rod connection (#4 is for a 200 Amp service), but my 2017 code book Table 310.15(B)(16) for 90* AL RHW2 shows 100 amps. What do you see that I do not.

go back and reread what i said. I already explained why you cant use the 90° column for breaker sizes.

 

[wyliesdiesels]

Wyliesdiesels, thanks for the response, this is very helpful!

Yes, it is currently in a breakered disconnect (couldn't remember the term for it). From what I remember, the County made them do that to pass inspection. Doesn't really surprise me at all if that is incorrect after dealing with the permit officials while building the detached garage.

I'm unsure of the wire size between the main panel and the disconnect, I didn't see any markings on the wire within the disconnect, but it appears to be #1 or 1/0 AL. I have not opened the main panel to check yet.

well i didnt say it was wrong, I said it was overkill. but your AHJ may have local WRITTEN amendments that requires the disconnect. I would check with them first to see if they do have written amendments. if not, then i would get rid of it as its not necessary. the breaker in the main service panel will suffice.

having said that, #1 AL is rated for 100a so no issue there.

If you do end up using #2 AL you will need to breaker it at 90a in the disconnect.

 

[theoldwizard1]

... I plan on running 2-2-2-4 AL MHF in 2" PVC conduit from the 100 amp distribution box (not sure if that's the correct terminology) on the outside of the house ...

If the box on the outside of the house has a fuse block or a breaker it would probably be called a "disconnect". If not, then it would a junction box. Common for A/C units.

 

[pattenp]

Ok [B]wyliesdiesels[/B], I stand corrected #6 is correct for their ground rod connection (#4 is for a 200 Amp service), but my 2017 code book Table 310.15(B)(16) for 90* AL RHW2 shows 100 amps. What do you see that I do not.

Even though the wire has a max 90 deg c rating, NEC sets the max amps at 75 deg c because breakers are rated at a max of 75 deg c.

 

[75gmck25]

Simple answer is that in most jurisdictions you need a 90 amp breaker to protect the 2-2-2-4 AL MHF where it connects to a main panel or a disconnect with a breaker. The breaker at the sub-panel end could be 100 amp, since the 90 amp breaker at the house is what is protecting the line from overload.

A few folks on this board have said that their AHJ allows a 100 amp breaker for 2-2-2-4 AL MHF, since 90 amp breakers are relatively uncommon. YMMV