disconnect box when adding subpanel

I am adding a 125 amp subpanel to the far side of my house. I need to add a disconnect breaker near the new subpanel...but...I can not find anything rated at 125 amps at either Home Deport or Lowes. The largest they have for disconnect is 70 amps for a spa setup.?
Do I need to find a electrical supply house for this...or is there something I am missing at Home Depot ?
Also...I am running a feeder line from the main panel to this subpanel...the run is about 100 ft...can someone tell me what the code is regarding the size of wire needed for this run. The retired electrician at Home Depot says a 4awg three connector cable will be fine...it is being run through 1" EMT across a flat roof with rain tight compression connectors ?
Thanks, Tim
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Does the subpanel you're installing not already have a "main" breaker in it? The disconnect doesn't have to be separate.
Use an ordinary house main panel "package", not a main-breaker-less subpanel for the subpanel.
Further, in the CEC, you can use a main panel breaker pair as a disconnect for the subpanel - you don't have to have the disconnect at the sub. This may be true in the NEC also.
[I _prefer_ to be able to kill the subpanel from the main without killing the main. But, if there's a long distance between the two, having a second breaker in the sub is convenient.]
[Mine has breakers at both ends. A Siemans main panel "package" with 100A main in the garage, and a Sylvania Commander 100A feeding it from the main.]
Large breakers (eg: > 60A) are certainly available, but usually not at places like HD except as part of a "panel pack". Large breakers for old panels are getting _very_ expensive. Just paid $80 for a 30A Commander dual. Ick. Fortunately the 100A feed breaker was given to me free from a friend.

4ga sounds right, but you'll actually need a total of four conductors. Ground and neutral have to be separate in the subpanel (remove the bonding jumpers).
--
Chris Lewis,

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Electrical supply can undoubtedly help, but I'd question whether the service feed is sized adequately to handle the total service first.
In conduit 4 isn't large enough, my table says 2, and that's copper. Somebody w/ updated/full code table handy can supply better data, but I'm pretty sure 4 isn't adequate by code.
Methinks this needs more thought and input before you jump in here...
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Something else to consider in addition to what others have said is the temperature of the conduit and wire up on the roof. I think that it is necessary to upgrade the wire size to compensate for the possible extreme heat that the conductors may be subject to. Of course this depends on where you are located. See if you can get wire with insulation rated for 105 degrees Celsius instead of 90 degrees. Regardless, number 4 is not rated for 125 amps.
Personally I am not a big fan of using EMT in wet conditions, although it is permitted. After several years of long term exposure the conduit can get rusty.
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tr wrote:

No one will make you use a 125 ampere feeder merely because your panel can carry that load. The size of the feeder is based on the actual load to be served. Then an overcurrent protective device is selected to protect the feeder. You need to do a load calculation based on the loads to be served and adding any allowance you want for future expansion of the use of electricity.
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Tom Horne

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