Adding new subpanel

[Sam19]

Hi All,
My Main service panel is ratting 125A currently it had one 100 A breaker feed to the exit subpanel in the garage.
I check at my Main service panel it have an extra slot can I add a 30A or 40 A breaker to this open slot to feed into a new subpanel I want to add.
I have the pictures attached for this question.
Thanks for helping
Sam

 

[Stuff]

As long as the calculated load is under the rating of the panel you should be fine. https://www.mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls
The hidden issue is that when adding a breaker to this service the feed to the the garage panel will no longer supply power for the entire structure. So instead of 2 gauge aluminum wires you need to have 1 or even 1/0 gauge to cover a 100 amp feeder. Many inspectors will miss this but others will nail you.

Other issue is that current breaker is upside down. If you add a circuit you might be asked to correct this. I don't know about GE but others made a different breaker for this.

 

[Sponsor]

 

[Sam19]

As long as the calculated load is under the rating of the panel you should be fine. https://www.mikeholt.com/documents/calculations/formulas/ResidentialLoadCalculations.xls
The hidden issue is that when adding a breaker to this service the feed to the the garage panel will no longer supply power for the entire structure. So instead of 2 gauge aluminum wires you need to have 1 or even 1/0 gauge to cover a 100 amp feeder. Many inspectors will miss this but others will nail you.

Other issue is that current breaker is upside down. If you add a circuit you might be asked to correct this. I don't know about GE but others made a different breaker for this.

Thanks!

 

[hj]

quote; The hidden issue is that when adding a breaker to this service the feed to the the garage panel will no longer supply power for the entire structure.

You lost me. How would that happen if all they were doing is adding a second breaker below the existing one? The dynamics fo the box would ENSURE that one breaker was ALWAYS "upside down".

 

[Stuff]

Right now the load for the entire dwelling goes through the wires on the top DP breaker. Adding a second breaker means that only part of the load would go through those top wires. That changes things so you no longer get to use the special rules for a dwelling service conductors and instead need to treat the top wires as general feeder conductors which have lower ampacity rules. The rules don't make sense in this application but still could be enforced.

As mentioned other brands used to make special breakers that mounted upside down but the lever worked normally. From what I recall they were used for 100+ amp mains and were called reverse handle.