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Q&A About Breaker Space Limits and Ratings in Electrical Panels

November 16, 2009

Question David asks:
I have a Square D QO panel in a one year old home – assume its a 200A panel. Currently, 38 of the 40 spaces are taken up with breakers and I need to add one or two breakers for some new work and to split up one room which is tripping when I use a space heater.

I went to Home Depot and was given two VERY different answers on the use of tandem breakers to preserve space in the panel. One employee (a belligerent and condescending fellow whose knowledge and opinion I question) told me the code limits the use of tandems to 4×15 amp tandems or 2×20 amp tandems.

At another Home Depot, an employee there says the above is not accurate and I can use as many tandems as the panel will hold although he concedes some panels restrict where they can be placed and therefore the ultimate number of tandems. He also pointed out that I probably want to limit the number of successive tandems proximity so as not to create a hot spot in the panel.

On this and other sites, I see there may be a 42 breaker limit. Since only 38 spaces are taken in my panel, there should clearly be room for two more, I would think.

Where I am confused are the following:

1) Is there a 42 breaker limit in a 40 space box?

2) Is there a limit on the number of tandem breakers usable provided I remain within the overall limit of breakers for the panel (eg can I use more than four 15A or two 20A breakers in any one panel?

3) If the 2 and 4 limit are correct – aside from wire gauge and specific need issues, why would anyone want to limit themselves to two 20A circuits when four 15A circuits will allow them additional flexibility?

4) Apparently some breakers count as two for the purposes of total max allowable breakers. I have two remaining spots (#39 and #40) unused. If I were to replace four single breakers with two tandems then it seems I am at the max 42 breaker limit but I would then have four available spots on the panel (the original two free spots plus two I just created by substituting tandems). Am I allowed to add additional breakers later? If so, this would presumably exceed the 42 breaker limit I have seen posted.

5) One HD employee suggested a sub panel if I exceeded the number of spaces on the main panel. Why is a sub panel allowed to get around the 42 breaker limit – surely the maximum load at any one time will be the same?

Guidance will be appreciated. Its a minor project I need to do but might as well understand it properly and do things correctly.

First, let’s start with Home Depot; as you have just learned, this is not the place to get advice about electrical wiring. However, Home Depot does appear to be changing that. I’ve noticed that our local store here in Bozeman and the Billings, MT store do have a master electrician on staff. In my opinion, the only people that you should get advice about electrical wiring from is a licensed electrician, an electrical inspector or an electrical engineer. These are the people with the most training in the electrical industry and they are the “experts”.

OK, enough about that, let’s get on with the answers to your home electrical wiring questions.

1.) It depends on which electrical code has been adopted in your area. We are still using the 2005 edition of the National Electrical Code® (NEC®). Several states have not adopted the 2008 NEC® because they believe it is so controversial. Heck, Ohio reverted back to the 2002 edition. So you need to check with your local building codes division to see which edition they are enforcing in your area.

According to the 2005 edition of the NEC®, there is a 42 space limit. However, the 2008 NEC® sets the limit at 60 spaces. With that being said, your breaker box still needs to be rated for the additional spaces.

2.) Yes, there is a limit on the number of tandem breakers which you can install in your breaker box. First, your panel must be rated a Class CTL panelboard and it will be marked to indicate how many circuit breaker poles can be installed on the door of the breaker box. So you may be able to install 2 tandem breakers in your breaker box to get to the 42 spaces if you have a Class CTL panelboard.

3.) I’m not sure where the Home Depot employee got the two 20A circuits when four 15A circuits from, but it is wrong.

4.) I’m guessing that the breaker box which you already have was made before the 2008 code change and it is only rated for either 40 or 42 spaces maximum. You need to determine this by either checking the label on the door of the breaker box or contacting the manufacturer.

5.) The sub-panel allows for more breaker spaces, not more current or amperage. I’m guessing that Underwriters Laboratories set the limit of 42 spaces and the new 60 space limit. I believe that the limit is determined by what the buss is rated at. At any rate, there are no limits to the number of breakers which you may install in the building, the limit is in the breaker box and the amperage rating of your electric service.

When is comes to your electric service, you also need to check with your local building codes department to determine if you can do this work legally. Not all areas allow homeowners to install their own electric service. You may need a licensed electrician to do this work for you. You will need a permit for this project as well. The power company usually will not disconnect without a permit and definitely will not re-energize without an inspection.


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