Any Issue with CAFI Breakers in Subpanel?

[Tom in Seattle]

Hey, my 1st post I originally found this site searching for info on garage heaters, but I find the electrical section to be outstanding as well.

I'm going to install a new QO140M200C 40 circuit, 200A load center in my garage to replace the old GE with the mini-sized breakers. With all the remodeling I've done and new home run circuits I've run the panel is already going to be totally filled... and I really could use another circuit for the garage heater.

So, to make things a little more organized and leave room for expansion I've thought about installing a subpanel up in the garage attic (any recommendation on size?). The attic is easily accessible via a pull-down ladder... has lights, flooring, room to stand up, etc. Virtually all the wiring for the house runs through the garage attic now and down to the main load center in the outside garage wall.

Some of the circuits I would like to put in the subpanel feed the upstairs bedrooms, and thus need to be on CAFI breakers (none of them currently are as the house was built in 1984).

Is there any problem with CAFI breakers in a subpanel? I know the neutral and ground buses need to be separate.

Also, I have some leftover 6/3 that was used for the double ovens. Could I use that for the subpanel supply with a 50A breaker in the main load center, or should I install something heavier and be done with it? The subpanel will be 6-8 feet away from the main. For now the only circuits that will be in the subpanel are two circuits for bedroom receptacle and bath lighting, furnace, and living room receptacles, which will only be for lighting as I have a dedicated audio system circuit.

Thanks for any insight.

Tom

 

[pattenp]

I believe a 100A 12 circuit main lug panel will work for what you’re doing and it’s okay to put arc-fault breakers in the sub-panel. There are no issues with the 6/3 on a 50A breaker if it’s copper. If it’s aluminum then you may have to increase to #4 if the cable is going to be within insulation. If it’s not within insulation then #6 aluminum is fine.

You should put a tag in the main panel to say where this sub-panel is located.

Edit: I just realized you want the furnace on this sub-panel. What volt/amps does the furnace use?

 

[Tom in Seattle]

Edit: I just realized you want the furnace on this sub-panel. What volt/amps does the furnace use?

The furnace is 120V, 15A.

I think I'm just going to order the QO120M100C from Lowe's. That will give me 20 additional spaces and include a breaker. It's under $100. If I do that I'll probably install a 90A breaker in the main panel and feed the subpanel with the appropriate wire size (2-2-2-4 SER or equivalent?).

Although I would like some spare slots, the main reason for the subpanel is to allow me to wire four circuits without splicing them to new wire in J-boxes in the attic. Everything except the old 3-wire w/o ground oven and dryer circuits enter the old GE panel through the top and have an aluminum crush-type fitting securing the home runs. I don't see any way to "uncrimp" them without doing damage. If I cut them where they enter the panel I'll have plenty of length to route them to the subpanel above.

All other circuits except these four are new home runs.

If there are easier/better solutions, please chime in.

 

[pattenp]

If you use SER AL 2-2-2-4, be aware if the cable is run within insulation, such as in an insulated wall or ceiling, the cable has to be sized from the 60C degree column which means it can only be protected at 75A. If it's run in an open space, not within insulation, then it can be sized based on the 75C degree column which is 90A.

 

[Tom in Seattle]

Update: I ordered the QO120M100C from Lowe's for $97.

I'm planning to use a 100A breaker in the new main QO140M200C and then run #2 or #3 THHN/THWN-2 copper in conduit. Questions:

1) Would #3 work just as well as #2 since it should be good for 100A at 75C?
2) Should I pair the #2 or #3 with a #4 THHN/THWN-2 ground wire?
3) What size PVC conduit do I need to run three (3) #2 or #3 and one (1) #4?
4) Is 3/0 copper from the meter to the new panel the correct size for 200A service? I'll probably order some new wire as I'm not sure the current wires will be long enough.

Thanks for any info.

Tom

 

[pattenp]

1) Would #3 work just as well as #2 since it should be good for 100A at 75C?
Yes, #3 is ok for 100A at 75C.
2) Should I pair the #2 or #3 with a #4 THHN/THWN-2 ground wire?
I believe you can get by with a #8 CU ground, but I would use a #6. Edit: changed #6 min to #8 min
3) What size PVC conduit do I need to run three (3) #2 or #3 and one (1) #4?
You need at least 1 1/4 inch, but I suggest 1 1/2 inch.
4) Is 3/0 copper from the meter to the new panel the correct size for 200A service? I'll probably order some new wire as I'm not sure the current wires will be long enough.
#3/0 CU is good for 200A at 75C.

 

[Tom in Seattle]

4) Is 3/0 copper from the meter to the new panel the correct size for 200A service? I'll probably order some new wire as I'm not sure the current wires will be long enough.
#3/0 CU is good for 200A at 75C.

I looked at my old panel today and it currently has 4/0 AL wire from the meter. The neutral is smaller, probably 3/0. I can't find any size marking on the exposed section. Is that normal to have a smaller neutral?

And is 4/0 AL a bit undersized for 200A service? NEC Table 310.16 says it's good for 205A at 90C and only 180 at 75C.

Also, the connection to the meter runs through a 2" dia section of metal conduit, which connects to the panel from the top, rear. The knock-out in my new QO panel at this location is only 1-1/2 inch. I assume I can just make this larger with tin snips? Thanks.

 

[pattenp]

It's okay for the neutral to be smaller. As far as the 4/0 AL for 200A service, you are looking at the wrong table. It's the table listed as 310.16(B)(7) it's now 310.15(B)(7) in 2011 NEC. The table is for service feeds and has no temp columns. It just shows service rating and copper and aluminum sizes. 4/0 AL is rated for 200A.

Don't remove the knockouts and use a hole saw to cut the right size.

 

[pattenp]

Correction on this: For the service feed, actually # 2/0 CU is okay for 200A. I was also looking at the wrong table when stating 3/0 for 200A.

4) Is 3/0 copper from the meter to the new panel the correct size for 200A service? I'll probably order some new wire as I'm not sure the current wires will be long enough.
#3/0 CU is good for 200A at 75C.[/QUOTE]

 

[Provincial]

Just consider this about CAFCI breakers: they can be very sensitive, so do you want to pull down the stairs, climb up in the attic, and reset the breakers all the time? Arc fault breakers are about as reliable as early ground fault breakers were 35 years ago, and trip at the slightest excuse, like voltage fluctuations.

I'm not saying to avoid using Arc Fault breakers, just put them in an easily accessable location, since you will need to reset them often.

 

[pattenp]

I think this is a bit exaggerated. It's not something that can be avoided since it is required by code. I have used arc-fault in some remods and have not had any problems with reoccurring nuance trips. If you have to reset them all the time or often, then you have other problems that need to be addressed.

Just consider this about CAFCI breakers: they can be very sensitive, so do you want to pull down the stairs, climb up in the attic, and reset the breakers all the time? Arc fault breakers are about as reliable as early ground fault breakers were 35 years ago, and trip at the slightest excuse, like voltage fluctuations.

I'm not saying to avoid using Arc Fault breakers, just put them in an easily accessable location, since you will need to reset them often.

 

[Tom in Seattle]

The CAFI breakers were only adopted locally for bedrooms, which in my case is two circuits. The main reason to put them in the attic was to avoid splicing to lengthen the wire for the new panel. However, I think I'll put them in the main panel since they need to be tested monthly. I just thought by having them in the subpanel the main would be cleaner with less clutter.

If 2/0 CU is okay for 200A would there be any advantage to running 3/0, since that will still be less bulky than the present 4/0 AL? I suppose I could run 3/0 for the hots and 2/0 for the neutral.

I tried searching for the tables referenced, 310.15 (B)(7) etc. but could not find a link. If someone has the links I'd appreciate it.

Thanks.

 

[pattenp]

You can join NFPA for free and can get on-line access to the 2011 NEC.

Click on NFPA 70. Then click on "View the document online (read only)"

http://www.nfpa.org/aboutthecodes/list_of_codes_and_standards.asp

You can down load the 2005 NEC from here. The wire size table have not changed. See page 469.

http://www.brunswickcountync.gov/Portals/0/bcfiles/Building_Codes/bldinsp_2006_NC_Code_Handbook.pdf