Installing a sub panel for a finished attic.

8 years ago

Hi folks, I'm planning to install a sub panel in my finished attic. It has 2 rooms 12'x12', and a 10' hallway. I'll need about 50' of cable to connect the sub to the main. I plan to put a 500W heater, and a window A/C unit in each room. One room is a guest bedroom, and the other is being used as a home office.

So here is what I'm thinking of doing. I'd like your opinions though, before I get started.

- 50A 2-pole breaker at the main panel to feed the sub panel.

- 6 gauge 3-conductor with ground cable.

- A 20A breaker for the two A/C units (5,000 btu each)

- A 20A (2-pole) breaker for the two 500W wall heaters.

- Two 20A breakers, one for each room, for outlets.

- A 15A breaker for lights. (The whole finished attic has four fixtures)

Should the sub panel also have a main shutoff? Is 50A sufficient? 6-gauge cable ok? Other insights or suggestions? Thanks!

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Comments (5)

Ron Natalie
8 years ago
A subpanel does not need its own "main" breaker. You can put one in if you want, but it's not necessary. Counting the load on my fingers (5000 btu on a fairly modern window unit is 500W or so), looks like 50A will be sufficient.

6g wire is fine if it's copper or even 75 or 90C insulation aluminum.

Make sure that your subpanel has the provision for separate ground and neutral bars. Some panels, it's just the matter of removing a strap to separate the two bars, others like square D, you have to buy an additional grounding bar.
Mike thanked Ron Natalie
Mike
Original Author
7 years ago
Thanks, I've passed my cover inspection! Only thing I have to change for the final is to turn it around so it faces a larger room. Seems my little 5x6 storage room is too small to put a sub panel in.
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Given the loads you describe, you should be well within the capacity of a 50-amp sub-panel and still have room (and ampacity) for another circuit or two. This assumes that the electrician wired the panel with a 120/240V feed, which would be the usual practice. (It's a two-pole or "double" 50-amp breaker in your main panel, right? Also, the use of the 12/3 doesn't make any sense unless the panel has a two-pole feed.) That being the case, keep in mind that each of the two legs provides 50 amps of current at 120V. If you have no 240V loads, you can think of that as like having two 50-amp feeds, or 100 amps of available power for 120V circuits. IMHO, you'd be better off running a new circuit for additional receptacles than tapping the warmer drawer circuit because, technically speaking, there's a potential code violation lurking when you don't follow the manufacturer's specifications for a dedicated circuit.
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Ron Natalie
7 years ago
The size of the room is immaterial but certain rooms might be considered inappropriate for panels and there may not be sufficient working room in front of the panel in a small space.

I'm glad things worked out with your inspector. That's the key.
User
7 years ago
Unless the panel is on the wall right next to the door, where you would need walking room to get to the "storage" anyway, it is hard to convince an inspector that a room that size can be used for both the panel and storage. Since both rooms are essentially bedrooms, it is also hard to convince an inspector that it won't end up a clothes closet at that size.

I suppose if you ran home runs of Cat6 and coax into the room too, and called it a mechanical or utility room instead of a storage room, then the small size wouldn't matter. But, it is a judgement call or interpretation of the codes by the inspector. If the room was huge with plenty of 'other' walls to put storage shelves on, they usually don't care much about location.

110.26 Spaces About Electrical Equipment. Sufficient access and working space shall be provided and maintained about all electrical equipment to permit ready and safe operation and maintenance of such equipment.

(30" wide × 36")

(B) Clear Spaces. Working space required by this section shall not be used for storage. When normally enclosed live parts are exposed for inspection or servicing, the working space, if in a passageway or general open space, shall be suitably guarded.

http://freenec.com/T13.html

.

240.24 Location in or on Premises

(D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices shall not be located in the vicinity of easily ignitible material, such as in clothes closets.

http://freenec.com/T66.html
Mike
Original Author
7 years ago
Passed the final inspection today! Thanks, all.

Installing a sub panel for a finished attic.

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