What determines size of main breaker?

On branch circuits the size of the wire and type of circuit determines the breaker size but what determines the size of the main or combination of mains? Can it be as large as the panel is rated for or must it match the size of the utility's drop?
We have an old panel with two mains, one is a double 50A feeding the rest of the panel. The other is a double 30A feeding the dryer outlet directly. The town now mandates 100A and I think homes get inspected when sold. Rather than replace the whole panel I could replace main #1 with a double 70A (if available) OR maybe better would be to replace main #2 with a double 50A (or higher) and have it feed a small subpanel in which I would put a double 30A for the dryer and maybe throw in a few others as well. That's within the rating of the main panel but not sure about the utility drop.
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On May 7, 12:29�pm, Big Giant Head

Main breaker must match main service drop and meter can and panel.
With such low ratings how is the physical condition of your drop?
I have a friend epoxying his drop to keep the outer jacket together
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go with 200 amps its well worth just a few bucks extra
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As another poster noted, the main panel breaker is sized to the drop from the power company. IF the drop supports 200 amps then, yes, you might as well spring for the larger panel. You never know when circumstances might cause your unit to draw over 100 amps.
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How would you know if the drop was sized for 200A?
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Ask the power company. But an electrician can probably give a reliable answer.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
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On Tue, 8 May 2007 13:09:55 -0700, "Eigenvector"

Typically a 200a drop is 2ga aluminum "triplex". From the service point where it connects to the SE in the mast you will need 2/0 copper or 4/0 aluminum. The difference is the code the utility uses vs the code mortals use and the fact that their wire is in "free air"
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On Mon, 07 May 2007 16:29:22 +0000, Big Giant Head wrote:

cha ching
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Even if town didnt mandate it buyers get scared off by nearly everything today.
The DIY people here wouldnt agree, heck I like fixing stuff
90% of home buyers today want a move in condition no work, if you have issues all thats left is the 10% of buyers looking for a bargain: ( worse most home buyers spend every dime they can having no money for even necessary repairs
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Big Giant Head wrote:

The maximum size mains are determined by the box rating, or the size of the service conductors, or the rating of the meter, whichever of these is smallest (very rarely is it the meter.)
The 100A minimum requirement shouldn't apply to you *if* an electrician does a load analysis and your service is large enough (look up the terms "ex post facto law" and "grandfathered")
Bob
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In most areas minimum upgrade standards exist. You can live with a 30 amp servicve but change ANYTHING and you must upgrade.
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I'll have to check around to see what is actually being enforced. Just going by what I saw in the paper. No move is in the works yet; just want get my ducks in a row. For now I'll concentrate on getting rid of other code violations such as the A/C condensor's cutoff box is connected by a short conduit run over to the meter box. That's gotta be a major one, running the wires carrying post-breaker power past the pre-breaker power.
Also our town of maybe 24,000 believes it knows more about these things than the NEC so it adopts the NEC minus the chapter on NM so better replace or conceal any Romex or UF.
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directly.
It sounds as though you have an old split bus panel. No main was needed for the entire panel, but only for the lower section due to some archaic six circuit code exception which saved the original installer the cost of a main breaker.
It's possible that your service already is 100 amp. You can determine this by the wire size of the service. Assuming other things are in good condition you may be able to get away with just replacing the panel. The grounding electrode system will need to be updated as well.
Do not replace the existing breakers with a larger size. Most likely the wire will not be rated for the larger size.
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what condition is the existing drop and amp capacity of meter can?
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The size of your service is determined by the service entrance conductors, and if you have a single service disconnect, it would correspond to the size of the entrance conductors as well. The "drop" is generally owned by the utility company and doesn't conform to NEC standards. When you have multiple service disconnects, the amperage of the total of the disconnects is not required to equal the ampacity of the entrance conductors. As John Grabowsky said, you probably have a 100 amp service already, and may be able to just replace the panel and update the grounding electrode system

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On Mon, 07 May 2007 16:29:22 GMT, Big Giant Head

If it is working, leave it alone. This does sound like a 100a split bus panel. Is there a place for another breaker on the "hot" side (where the dryer is) Since you don't mention other 240v appliances it sounds like you have oil or gas so you don't use that much power.
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