I currently have a 100amp main service box but it is all full and some are even doubled up.I'm wondering if i can hook up a 100amp subpanel off of it? If so what size wire to go with?

It would be on same wall next to it within 12 inches. Do I have to use conduit between the two, or does it have to exit the bottom of main to top if subpanel?

Also all I'm seeing at local store is 100 amp main boxes. Is that allowable by code to use as a sub? I know sub can't have neutral bonded by that screw either way.

My local town in Illinois uses the 2011 NEC handbook. Sorry for so many questions, but i just want to do it right. Thanks.

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    Hey, you may knows this, but just a heads up: The mains (the big wires from outside) coming into your main panel are (probably) always hot, even if you turn the main breaker off at the top of the panel. The main breaker turns off power to everything downstream, but the wires before it are still hot. Usually, there's not much exposed, but the screws to the lugs are always exposed so don't touch those, don't even get close. Both screws are live and want to kill you and your tools. – Zach Mierzejewski Jan 16 '18 at 15:29
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    As someone who had there panel upgraded and it almost full, again, get an electrician and replace the panel. Get 300A,and get one with 60-80 breaker slot, so you don't run out. Honestly I would buy one with 100 breakers if I could. – cybernard Jan 16 '18 at 22:45
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    300A and 100 breakers - what the heck are you running @cybernard?? – cr0 Jan 17 '18 at 16:38
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    @cr0 Honestly nothing special. We remodeled, and its like this, oh by the way 201# code book requires separate circuits for <a long list of items> We have 40 full, and only 1 working outlet upstairs. Appliance outlet has to be dedicated. Fridge,dishwasher,microwave, lights, 2x regular outlets and that is just the kitchen. 2x exterior outlets separate for mower,snowblower. Exterior lighting a couple more. Each room has smoke detectors at least 1 circuit. 3 for computer room. I think bedrooms have 2ea. Plus many for outlets and lights in the basement. Many of them separate for power tools. – cybernard Jan 17 '18 at 22:25
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    @cr0 My only comfort is I had 2 panels install, one for upstairs and one for down, and upstairs is still almost empty. drive.google.com/open?id=1ZpLp53PKExojWXlNZSU3ynH1m9mBxW1A6w There are number of things I wish I had room for, and most has to be eventually duplicated for upstairs. – cybernard Jan 18 '18 at 3:29

Let me try and answer some of your questions. First the panels you are looking at that are rated 100A simply means you can use them for any application up to 100A. You can for example add a 60A breaker to your existing panel and protect the new subpanel with a 100A rating. It is not so much what the panel is rated as what the protection is rated.

You can use other methods other than conduit but first you need to select what type of method you prefer to use and that must be permitted as outlined in Chapter 300 of the NEC "Wiring Methods". So you need to select a method then follow the installation directions as outlined in that particular selection. As an example, let's say you want to use type NM (romex) to connect the new sub panel. That would be Article 334 in the NEC. It will have uses permitted with exceptions and uses not permitted and give installation rules you must follow depending on where these panels are and the type of environment they are in.

I would like to suggest that it may be a better idea to replace and upgrade the existing panel. I believe this would be a better method and give you a better value added rather than several subfed panels.

Hope this helps

Oops caught use overlapping

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  • Tester101 and myself were answering the same question at about the same time. – Retired Master Electrician Jan 16 '18 at 14:08
  • great info for me to take in. I should have been more specefic on what was going to be hooked up to subpanel. I was going to hook up my basement lights,bathroom,and receptacles. I have them all on separate runs and wanted them to have tgeir own box.So maybe the 100 amp subpanel may be to much.I agree a sercice upgrade would be better if i can afford it.Would they have to change the service cable that runs from pole to meter that runs underground?my environment would be in a basement and current panel is secured to the cement wall – tracyrobert Jan 16 '18 at 15:04
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    @tracyrobert Even if you put a double pole 60 ampere breaker in the main panel to protect the new panel feeder, you can still install a 100 ampere panel as the second panel. As Retired Master Electrician points out, the panel rating is the maximum the panel can be used for, not what it has to be used for. As for the service upgrade... If the existing service conductors are not large enough, they would have to be replaced if the service was upgraded beyond their capacity. You'd have to check with the local utility, to determine who pays for that. – Tester101 Jan 16 '18 at 15:14

If your panel is full, you'll likely want to have the service evaluated, to determine if it's still large enough to meet your needs. It's possible that you may want to upgrade to a larger service, which will likely require new service conductors and a new service panel. If this is the case, you'll simply have a larger panel installed.

If you don't have/want to upgrade the service, you could still have a larger panel installed. Just make sure the new panel is larger enough for any future expansion.

If you do want to install a second panel, you'll have to determine what size feeder you want to supply it. This will dictate the wire size, and the size of the breaker supplying the new panel. To figure out what size feeder you'll want, you have to decide which circuits you'll move over to the new panel.

Once you've got that worked out, you'll have to reroute all the selected circuits to the new panel location. This could require making junctions in the old panel, to extend the length of the circuit.

All in all, I'd say this is not a project for a novice DIYer. There are a lot of subtle details that will likely be overlooked by a non-professional (and maybe even some "professionals"). My advice, would be to get quotes from a few local licensed electricians. Make sure to get pricing for the options I described above, and then decide how to proceed.


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What you want in the subpanel is enough spaces.

Get a subpanel that is large enough (in amps) to give you enough spaces to future-proof your house. Keeping in mind a few things:

  • the way you are using double-stuff breakers right now, is not a viable long-term strategy. New work now requires AFCI or GFCI breakers, and those are not available in double-stuff, each circuit will need a full space.
  • having spaces lets you do stuff you wouldn't otherwise do.
  • Plug-in electrics (including engine-too types) are exploding in the auto industry (just go to The North American Auto Show or view their app) and we foresee that mixing with the smart grid too (your car backfeeds to help the grid stay up, and you get paid for that) - that could well be coming to your garage. So at least 2 more breaker spaces.
  • On-demand hot water is making more and more sense, 2-8 spaces depending on setup.
  • it may be a good long-term strategy to eventually make this panel the main panel.
  • Some people want generators, off-grid switchable solar, or grid-tied solar. More spaces.

So it isn't wrong to buy a 42-space 225A subpanel. You can feed that from any breaker <=225A, including 150A, 100A, 60A, 30A and even 15A. The key is to think ahead to give yourself freedom to maneuver, rather than painting yourself into a corner.

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I currently have a 100amp main service box but it is all full and some are even doubled up.I'm wondering if i can hook up a 100amp subpanel off of it?

Certainly, that is what sub-panels are for.

If so what size wire to go with?

The wire should be #3 copper for 100 amps and a 100 amp two pole breaker to feed it.

Do I have to use conduit between the two, or does it have to exit the bottom of main to top if subpanel?

No, and no. You choose where to install it and what wiring method to employ. It just cannot be higher than 6 1/2' to the top of the highest breaker.

Also all I'm seeing at local store is 100 amp main boxes. Is that allowable by code to use as a sub?

Yes, just don't use the bonding screw or strap.

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This is were most people give wrong advise about the electrical calculation. If you have 100 amp service, this means your house was calculated at a 100 amp consumption, subpanels are made to provide easy access to circuits instead or running multiple circuits for the main panel. For example is I need 5 or 7 circuits in my garage and my main panel is in my basement, It make sense to install a subpanel in the garage, so this way i run just one circuit to my subpanel and then install the 7 circuit from that subpanel to the garage. Also if you are max out in your main, you have to see how many amps are you currently pulling at your highest consumption time of the day, this will determine if you need to upgrade your service or just reorganize your circuits in your panel. Sometimes some circuits are not been used or could be bond with another circuit and make it one to free space. Electrical breakers and service sizing is an important calculation needed to keep your home/business safe. My advice is contact an electrician to evaluate your service before increasing your breaker capacity without increasing your service. Also ask him questions and any concerns about electrical safety. Eventhough it seems simple to do any electrical connection you have to remember that is a very dangerous trade, and is not worth it to risk your safety just for a couple of bucks, specially if you don't know the whys of things.

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    Hello, and welcome to Stack Exchange. This is interesting, but it's hard to read, and doesn't seem to answer the original question. – Daniel Griscom Nov 19 '18 at 23:01

You don't want more panels, but need to feed more loads. You gave no info on how many more loads, and how big those loads are. Also not about the next expansions that can be expected after this one.

When you get this planning, you might want to consider replacing the already crammed panel by a bigger one, instead of adding more and more panels and getting an even more messy installation.

Then you should discuss this with a qualified electrician, and compare the total installation price of adding more panels or replacing with one big panel, instead of trying to do this yourself.

An electrician will also consider factors as: is this a home, workplace, moist environment, outdoor, etc etc, all for the sake of usability and safety, with the help of NEC.

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  • This doesn't really answer the question. – mmathis Jan 17 '18 at 13:51
  • @mmathis True, but this might better solve his problem :-) – Roland Jan 17 '18 at 16:24
  • Ok..after talking with the inspector, he suggested i should just go with a 60 amp breaker in main to run the subpanel for the small circuits i want to move. as 60 amps is more then enough he said. Basement lights, and outlets. I think he told me #6 awg wire would be sufficient for this.I notived that it only comes in black. I got #6 copper for ground but what do i do for neutral? Can you use black and mark it white on outside from end to end? – tracyrobert Jan 21 '18 at 1:28
  • @tracyrobert About using or marking with colors: the questions are: does it work, and: is it safe? Some people are done when it works, but I think it is much more important that it is safe. Electricity can be dangerous, just suppose someone gets a shock and gets hurt, or even dies. How much will you regret that you considered the job done when it worked? Thats why we can endlessly lament to you to get this job done by someone who knows what (s)he is doing, who is trained and knows the safety code. NEC is not so much the bible on how to fix it, but much more on how to make and keep it safe. – Roland Jan 23 '18 at 10:43

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