Originally Posted By: Dennis Bozek
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I believe what Alan is referring to, is that the main panel has a 100 amp breaker in it and also another 100 amp breaker to control what may very well be a subpanel. Remember that you do not add up the breaker sizes to determine the load on a panel. The addition, which is more than likely smaller that the house itself seems to have a subpanel rated for 100 amps. If you look at the addition and it has base board heaters, hot tubs, radiant heaters, or any thing that will typically create a lot of current, then yes something is wrong.
Chances are though all you are going to see in the addition are some outlets and some lighting circuits. Therefore is a 100 amp subpanel to much protection? Yes it is. If you see nothing there but a few outlets and a light or two, or maybe even a ceiling fan, I would recommend that the subpanel feed and breaker be inspected by a certified electrician. If you want...and this is totally up to you, recommend that the breaker in the main panel be decreased to a 40 or a 60 amp breaker. Be sure that the wire size feeding the subpanel is sufficeint first though for those breaker sizes. In a residential atmosphere, 4 gauge is typically good for 100 amps although us sparkies would run 3 gauge. 6 gauge is good for 60 and 8 is good for 50 but typically protected with a 40 amp breaker. 10 gauge is good for 30 amps.
You would have to determine why there is a 100 amp feed to the addition subpanel though. The previous owners may have been doing something in there that needed that 100 amps. Either way though, the load of the house based on the square footage and appliances used should be calculated. If as a HI you do not wish to do such calculations, recommend a certified electrician to do the load calcs. It does seem odd to see a 100 amp service and main breaker and then an additional 100 amp breaker to feed a addition. If the addition is in need of 100 amps....the main panel best at least be a 200 amp then or 100 amps for the addition and a remaining 100 amps for the rest of the house.
The subpanel may not be bonded to the neutral. The neutral must be floating from the enclosure itself and a separate ground bar, which is bonded to the subpanel enclosure must be installed. A EGC from the main panel must be connected to the ground bar....NOT the neutral.
All neutrals in the subpanel must go to the neutral bus and all grounds must go to the ground bus. They cannot be combined in any way. Although one is not likely to experience any problems from having such combined or by improperly grounding the subpanel, it is the code to do it as I previously explained.
Ideally, you would want the wire coming from a breaker to be rated for more than the breaker. This insures that the breaker trips before the wire burns. Typically in a residential environment, you do not see that often. In a industrial atmosphere, such would be more of the case than not. You want the protection device (breaker or fuse) to trip or open before the wire burns due to an overcurrent situation such as in a overload or short circuit. If the addition is nothing more than a few outlets and a lighting circuit, and something shorts...it could be possible to draw like 80 amps and never trip the breaker for the subpanel. However, the breaker for the branch circuit should trip in this case thus protecting the house from fire. However, if anything is double tapped off the main lugs for the subpanel....and it's wire gauge is smaller in comparison to the subpanel feed....one would be asking for a fire for you are then protecting wire not rated for 100 amps with a 100 amp breaker.
I know this all sounds confusing to those not electrically minded...but then it should answer your questions Alan.
This information has been edited and reviewed for errors by your favorite resident sparky.