I have a large attic (on 3rd floor), 38'x28', which is unfinished space. The 2nd floor central
air-condition is located there and used to be fed by a 60 feet of 12/3 cable (I changed the cable to 10/3 to reduce the chance of voltage drop) and it's fed by a dual 15A breaker
(240v breaker) in the basement.

Here is what I like to do: In the near future, I like to finish this space and have one or two rooms, and a bathroom with shower in this attic space. Note that, due to the nature
of the roof rafters, and therfore headroom, the room(s) will not be as big as the full floor dimension I listed above.

Now back to the electrical details. Due to no free room in the main breaker (which is in the basement), I like to replace the dual 15A 240V breaker (which was feeding the
central air unit in the attic) with a dual 60A 240V breaker. Then use COPPER #6/3 romax
cable from the basement to the attic (4 floor up), and install a subpanel to feed the
central air, and feed the light & outlets in the attic.

Questions:
1. Should I be using a Main breaker load center or a Main Lug load center?
I read in a forum about situation like this that, if a sub-panel is not in the same
floor as the main breaker load center, then the sub-panel must have a cut-off capability.
Is this correct?
2. If question 1 is correct, does this mean that I have to buy and use a main breaker
in the attaic?
3. If question 1 is correct, can I just buy a switch box which the fed from
basement goes into, and then connect the output of the switch to a main lug?
Of course, locate that switch box right next to the main lug.
4. Should I be using a thicker cable, say COPPER #4?
5. Home Depot only sells #6 Copper romex cable. Is there romex #4 copper offered
by any manufacturer/retailer? I like romex cable better due to it's convenience
during installation, and not having to use conduit all the way to attic.
6. Another forum concerns I read similar to my question, is the voltage drop
on the circuits that will live next to the central air breaker. I was planning on using
SquareD QO612L100DF main lug Load Center. I will have two slots used for the
central air, and the other 4 for the lights and switches in the attaic.
7. Anything else you may think of for installation and planning for this work?
-Thanks
-Ab

2 #6, 1 #8 and 1 #10 for ground to your new main lug panel. 60 Amp Breaker in main panel, In new panel, 2 pole 240 volt/ 15 amp for your air, and 2 single pole 20 amp for the 120 stuff. You can never use to big a wire(usually)cost is a factor.
Central Air on a 15 amp doesn't sound? Right.
What size service do you have, and what electrical do you have Instaheat/, Gas dryer/stove? Water heater, welders, other sub panels?
Don't forget to run TV, Tel, Computer, Alarm, Speakers.

Don't forget smoke detectors, one per level, one per bedroom. And Arc fault breakers for bedrooms and living rooms.

Thanks for the reply.
Stratmando, the central air is just the blower unit, which is on the 2pole 15A 240V breaker.
It's second half is the condenser unit outside, which runs off a 2pole 40A 240V breaker.
I felt the wires when these units were on, and it's cool, not even warm, so the wire
And breakers must be correct. Thank you for your comments.
Regarding the # of sub-panels, I have a QO612L100DF main lug in the basement,
Feeding the basement room, hallway, switches, lights. And I was going to install
Another one of those QO612L100DF main lug in the attic. My service is 200A, and
With previous tkrussell calculations he kindly done for me, I have like 87A load.

tkrussell, arc fault must be new code since our house was built 20+ yrs ago without it.
Or the breaker that I thought was a GFCI in the main load center, it's a arc fault breaker.
I have to double check it. Thank you for the important safety notes.

Any suggestions for the question #1:
Should I be using a Main breaker load center or a Main Lug load center in the attic?
This question came up when I heard we need a way to shut the power to sub-panel
When sub-panel is on a different floor than the main load center. Imagine in some
Emergency situation, needing to shut the power to the sub-panel while one is
In the attic. Now one has to rush to the basement to shut the power to the sub-panel. Is this a code requirements or just a convenience issues to locate
A shutoff to a sub-panel on the same floor as the main?

I wonder, Is it legal to use a main load center than a Main Lug as a sub-panel?
If so, why use a main lug at all?


Thanks again,
-Ab

Don't forget floor joist size. If you have 2x4 or 2x6 ceiling joist that will not suffice as floor joist. If you a truss system you can't go cutting bracing. If 28' is the length of joist with a load bearing wall in the center you joist span will be 14'. Probably require a 2x10 floor joist.

I mention Arc fault in case there will be a bedroom or living area in the attic. Correct, the existing home will not need Arc Fault, but any new renovated space will.

You may have a main breaker in the subpanel if desired for convenience, but it is not required by code. The feeder and panel will be protected by the breaker you install in the Main panel to feed this subpanel.