Detached building sub panel replacement

[Dsg319]

Replacing a Main lug only sub panel in a garage.

#2awg aluminum is existing. Hoping the feeder is on a 90amp breaker as it’s already existing.

Customer wants a main breaker in garage for ease of adding stuff and working in panel.

I plan to just use a sub panel with a 100amp main as that is a standard size in the big box stores with the amount of spaces I need.

To keep from a fused or knife switch disconnect or other methods is anybody else seen #2awg aluminum feeding a 100amp subpanel if the feeders protected at 90amps.

I know the conductors are properly protected with the feeder breaker but just wondering if this is the norm for this scenario.

 

[LarryFine]

To keep from a fused or knife switch disconnect or other methods is anybody else seen #2awg aluminum feeding a 100amp subpanel if the feeders protected at 90amps.

You can feed a 600a MB sub-panel with a properly-protected feeder of any size adequate for the load. So, yes, it's okay.

The sub-panel's oversized main breaker is effectively rendered a switch (but could theoretically trip first with a bolted fault)

 

[qcroanoke]

Perfectly ok

 

[Dsg319]

I am just wondering if anyone sees this done very much. Seems about the cheapest route to go.

 

[LarryFine]

I see your plan as ideal. Don't forget the usual grounding and (non-)bonding rules.

 

[Ohm2]

I am just wondering if anyone sees this done very much. Seems about the cheapest route to go.

Done all the time.

 

[qcroanoke]

I am just wondering if anyone sees this done very much. Seems about the cheapest route to go.

If you want to feed a 100 amp panel with a 100 breaker my understanding is your feeder would have to go to 1/0 Aluminum.
There is nothing wrong with feeding a 100 amp breaker with 2 Aluminum fed by a 90 amp breaker. Wire is protected to its rating.

 

[Little Bill]

2-2-2-4 Al mobile home feeder is advertised as 100A. It is if it's for the main panel in a dwelling. But if going to a subpanel, you have to use the table, which says #2 is 90A. With that said, I do it all the time for a sub. It's the cheapest way to feed a sub. I use a 90A breaker. Most won't even come close to pulling 90A so it is fine. I also use a main breaker panel since that's the easiest way to comply with the remote building needing a disconnect.

 

[BarryO]

I am just wondering if anyone sees this done very much. Seems about the cheapest route to go.

Seems to be the case more often than not in detached building subs around here. The 100A sub in my detached garage is fed by an 85A feeder.

 

[Little Bill]

Seems to be the case more often than not in detached building subs around here. The 100A sub in my detached garage is fed by an 85A feeder.

You sure it isn't an 80A breaker? 85A isn't a standard size OCPD.

 

[BarryO]

You sure it isn't an 80A breaker? 85A isn't a standard size OCPD.

You're right; brain fart on my part.

 

[Fred B]

If you want to feed a 100 amp panel with a 100 breaker my understanding is your feeder would have to go to 1/0 Aluminum.
There is nothing wrong with feeding a 100 amp breaker with 2 Aluminum fed by a 90 amp breaker. Wire is protected to its rating.

I am just wondering if anyone sees this done very much. Seems about the cheapest route to go.

I don't think I've seen a single instance (other than mine) where the wire size increased or breaker reduced for a 100A sub feed panel. Almost to a 100 % will find #2 and 100A feeder breaker.
Most work in this area is being done by non-electricians, and for a subpanel will almost never get an inspection, and they simply go to big box or supply house ask for 100A wire and given a #2 AL. Around here even supply house if you want a a minimum size #1AL or even 1/0 it's not available or would be a 6-8 week order time. 2/0 is a little more available but not what the store will cut them when they ask for a 100A wire.

 

[infinity]

Around here even supply house if you want a a minimum size #1AL or even 1/0 it's not available or would be a 6-8 week order time.

Really that long, WOW. I put a sub-panel in my garage and ran #1/0 SER which I picked up the same day at my local supply house. Around here if you want to waste money and skip the supply house Home Depot will deliver it in a few days.

Why would your local supply even stock #2 SER and not #1?