Wiring questions

It seems this is a not uncommon desire, to overfill a too small breaker box. There are two different styles of SqD breakers, industrial and residential. One form will fit the other box, but costs a lot more. I will admit, the residential grade is new (to me) and I don't know it well.

The style/form of the existing breakers is "QO-120" and "QO-230", as an example. The "QO" is the designator for the breaker form, the "120" means single pole, 20 amp. The "230" means a two pole, 30 amp. Single pole being 120 volt and two pole being 240 volt. I won't go into that theory, see:
for more details. It's actually a small book, being about 50 pages PDF.

To take Capt Jas' original question, the following is how I would set it up. I am in a different state and used to working with older wire tables, so cannot be taken as the most appropriate way to do it. Just how I would do things.

The far right is a QO-230, that remains as is. The far right could be set up with QO-3030's, providing yet another 30 amp circuit.
The center two slots both get QO-1515 (or QO-2020). That is to carry the four 120 volt circuits that are currently in the box.
The far left gets two QO-3030. This provides 2 ea 30 amp 230 volt circuits. Watch how the circuits are connected, one leg to each breaker.

Very important: The breaker that feeds this "submain" must be sized for the wire used. There was mention of an AWG-8 wire. Per the "old" wire tables (ca.1990) I am familiar with, that is 40 amp wire. AWG-10 is 30 amp, AWG-12 is 20 amp. AWG-14 is 15 amp. This must be observed, that the breaker is rated at or lower than the wire.

Assuming the AWG-8 is the feed for this box, it is the limiting factor for all circuits combined. It cannot be exceeded, ever. The 30 amp 240 volt circuits should be used "one at a time". There was mention of a two to three phase converter being installed. Such a device is "tuned" to the motor connected and should not be shared. If two such devices are used, they should be on separate circuits. That is what the described panel is configured for.

There are other considerations involved, many more. Some are covered in the web link, others are available in the NEC code book. All in all, you would theoretically want an electrician experienced in the local codes to at least oversee the work. Re my reference about fires in the book... ... The bottom line, this entry must be considered in the context of its' presentation, an unknown source on a widely distributed web site. I am not liable for any damages from this text. However, I do release any and all information into the public domain.

I'm not sure I covered all the points asked about, but I did get the gist of it. I would advise that if the originator walks away from this configuration, that it be returned to its' original configuration.
Bill Hudson​
 
Hi Cap Jas,

I do not know what your technical back ground is, but...

I just saw your post and Bill Hudson comments. I am an electrical engineer and while I am not a licensed electrician I have wired several houses. Bill is correct, that the key to all of this is the size of the wire coming to your garage sub-panel and the size of the breaker feeding it from your main panel. It is not simply how many breakers that you can shove into the garage sub-panel. Code is important, but safety is even more important. In concept Code should cover safety, but depending upon where you are and the age of your systems and wires there can be serious errors. What I have found is that sometimes what has been installed is not safe. So just because it is currently in your home/garage, especially if it is older, it may not be safe...but is working. Especially, after you make changes things maybe unsafe. Also, there is the possibility of an old breaker not actually working properly. They do age and they do fail. So safety says make sure the wires are bigger than actually needed so that you never have to test the breaker. Bill mentioned the AWG-8 as possibly feeding your box, but when I looked at your first post you appear to be hoping to add the AWG-8 for your new equipment, not that it was the current feed. So be careful. check out the wire coming to your sub-panel and the breaker at feeding it at your main panel. Too much load in the garage can cause a fire in the house! .... So while you are busy working in the garage with a noisy milling machine, the house is burning!

Your local code will tell you what the max number and size of breakers are that you can put in a box, based upon its ratings. Normally via code, one can fill a box with a current capacity of breakers that is considerably larger than the capacity of the input FEED wire coming to the box.... provides to the various services, ... because an assumption is made that not all loads will be on at the same time. However, the total load should never exceed the capacity of the FEED wire. However, when one only has one or two large services like your mill or lathe along with outlets and lights this assumption may not be going to work. Having said all of this, the safe route is to make sure that for what ever maximum current draw you are EVER going to cause to flow through the wire FEEDING your sub-panel that current never exceeds the rating for the wire supplying it. I have even found that sometimes the FEED wire coming out of the main is not the same kind of wire coming into the sub-panel. This is scary as it means somewhere in your line from one to the other there is a splice or worst yet, it is servicing another sub-panel .... So check this. You might have to run a new wire to the garage sub-panel.

There are some introductory books on electrical house writing rules... if you go looking for them. The very first one I purchased, before I knew a lot more, was sold by Sears..... long ago. I am sure it is no longer in print, but it was called "Simplified Electrical Wiring A Handbook for ...." and was pretty good and only about 50 pages long with lots of pictures. https://www.worthpoint.com/worthopedia/sears-simplified-electrical-wiring-1734783623 Anyway, it is worth the cost if you can find it. Reading a code book is like trying to read ..... way too much.

Lastly, I should mention that just because a wire is an 8 AWG size does not mean that it will carry a lot of current. The capacity of a wire is largely dependent upon the insulation around the wire. If the wires are new they are much more likely to carry the current you need. But you can look up the insulation ratings as well. If you have ever hooked up the wires going to an oven you will find that they are quite small in diameter, but carry very large currents before they will cause the insulation to catch fire. Special materials and in a location where the heat can escape.

So, if you do not have a technical background, Bill is right again. Get an electrician to look your work over.

Be Safe and have fun with your new tools.

Dave
 
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Thanks for all the replies. I am going to have an electrician do all of the work. I just like to know what is going to happen and my options.
I have 400 amp service to my house and I have a 60 amp double pole breaker in the house that feeds the sub in the garage. I'm not positive what the line feeding the garage is ( will find out in a couple of days when I get a few estimates) but I think its 2-2-2-4 underground.
 
Wow! Sounds like you have plenty of power to do what you are up to ... plus. A 400 Amp Main is much larger than you can get in most residential areas. If the FEED wire is copper (not Al) then your feed should be good for over 100 Amp and may be good for as much as 175Amp. Just need to check the insulation type and see if it is Cu vs Al. Even Al will have a high rating, but I am just not very familar with these large wire sizes rated for underground. Even without this, and keeping your current panel box you can probably do all that you want. As suggested, you maybe be able to get rid of all of the breakers and put in 1/2 width breakers, then you will have room for 12, 1/2th width breakers in your current 6 slot box. You may be able to make 6 of these slots into the 3 double pole 220 circuits and still have your original 4, for 110 volt circuits, plus you would have an extra full sized slot. You would be done with the breakers and just need to run the new wires to your tools. (If you do not want to directly wires them you may want to also put in shut off switches boxes at the tools rather than outlets with pig tails.) You never mentioned what other large loads you would have in your garage, but if you know you are not going to exceed the main breaker or FEED wire than you should be fine. This is your least expensive option. An electrician will cost more than the breakers by a lot. Usually, every other breaker slot is connected to the alternating sides of the 220 lines. So exactly how these are hooked up on the panel that you have may determine how many 1/2 sized breakers you can get in. If you take the cover off you can see how the breakers connect to the two power bars. You may also need to expand your ground or neutral bar. Sometimes just getting big wires into small holes is problematic.

If you have the money and inclination you could replace the Main breaker with a larger one (100 amp double pole) and probably replace the Sub-Panel with one with a lot more breaker slots. When I rewired my current 1930's house I removed most of the old wiring and all of the knob and tube wires and put a new main panel and 3 new 200 amp large sub-panels. It is a large two story house and running one large wire to each sub-panel was a lot easier than running a lot of 20 and 15 amp wiring long distances. Then I put in 125Amp breakers and 200Amp Al sub-panel feed wires to each of the 200Amp sub-panels. While my feed wire was larger than my breakers the 125Amp breakers were the largest capacity breakers I could find which only occupied 2 slots in the main panel box. I found the I could get the parts I needed from Lowes and Home Depot if I stayed with GE panels, which are probably not as rugged as the Square-D, but I have found to be reliable and a lot cheaper. If you have room in your garage wall you could probably use something like, $85: https://www.lowes.com/pd/GE-PowerMa...p-Main-Breaker-Load-Center-Value-Pack/1091463 and get all of your smaller breakers you would need with the panel. But this is over kill for your application.

For the large breakers, 125Amp, I found that I could get them cheaper on line and ordered from https://www.superbreakers.net/ . I got the GE
THQL21125 - Plug-In Circuit Breaker for $55 but that was in 2011. Amazon would probably be cheaper now: https://www.amazon.com/General-Elec...rds=thql21125&qid=1554435613&s=gateway&sr=8-1

Larger breakers can wind up costing more than the box you are putting them in!

Anyway, not all breakers mechanically fit in all boxes. I doubt that the GE would fit in the Square-D box. Look at the type name currently in your box. I could not read the breaker labels in your photo.

Have fun.

Dave
 
You could always make a "tap" and nipple another circuit breaker box to the existing and then have all the breakers you need plus spare if you choose to expand. Ask your electrician what are your tap options.
 
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