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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. If so what size wire to go with?

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

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

Rohit Gupta
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tracyrobert
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4 Answers4

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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.

nobody
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Retired Master Electrician
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11

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.

DO NOT JUST HIRE THE CHEAPEST BID

Tester101
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4

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.

Harper - Reinstate Monica
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4

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.

ArchonOSX
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