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
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.
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"
Even if town didnt mandate it buyers get scared off by nearly
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
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")
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.
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
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.
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
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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