My homebuilder says upgrading my electric service to 300 or even 400 Amps
will result in more "efficient" use of electricity by my appliances. Can one
of you experts out there please explain how this is so?
Only if you are planning to use up to 300 or 400 amps, otherwise he is full
of BS, sounds like a sales pitch to get you to pay a premium for the extra
amps. More important is to get a panel with as large a capacity of circuit
breakers as you can, which may take more than one panel to do the job, so
that there are a lot of lightly loaded circuits instead of loading each
circuit to the max. This will give you more "head room" in each circuit so
the ones that end up with a load are not overloaded. Also to provide a
separate circuit for each large load or critical load such as a separate
line for a freezer, etc.
I agree with you.
But you remind me of a long time question I've had.
When I lived in a 1930 apartment building, the fuse box in my apt. had
2 slo-blo 15 amp fuses, and in the basement the fuse that served my
apt. was 1 slo-blo 20 amp fuse.
I was told this was bad, because 2 x 15 = 30 which was more than 20.
I was told the sum of the fuses downstream should never be more than
the upstream fuse. Any truth to this????**
I sort of figured first, that the landlord would not rewire the whole
building and I needed my 30 amps, so I should keep my mouth shut.
second, the building had been working this way since
1930 and there were no electrical fires, and that's still true today,
75 years after construction.
and third, the 20 amp fuse in the basement would protect
the 12 gauge wire from the basement to the 5th floor. That was its
job. But there were sometimes I would use 14 amps through one fuse
and 5 through the other, and that would be under 15, and under 20
total so what's wrong with that. Anything higher would blow one fuse
or the other.
**If there is any truth to the idea that two 15 amp fuses can't feed a
20 amp fuse, why is it all right for the sum of all my circuit
breakers to be greater than the main serving my house. I have cb's
totally 200 or more amps now, and room in the box for in the box for
120 amps more. Maybe more. I'll never use anywhere near all of it
at once, but if this is ok, why was my apartment setup not ok??
BTW, slo-blo is only 5 or 10 seconds, right? It's more than 1/2
second like non-slo-blo, but no where near 30 seconds, is all of this
Nope. The current is limited to 20A max, no matter what the downstream fuses
are. Fuses are sized to the wiring they protect. Period.
You weren't getting 30A, only 20.
No surprise -- nothing inherently unsafe in what you've described. Silly,
perhaps, but not unsafe.
Ahh, now that's a different story. Two 15s feeding one 20 is not the same as
one 20 feeding two 15s.
It's just not a problem.
It is ok, and that's why -- you won't ever use anywhere near all of it at
once. For example, the probability is near zero that you'll ever use your air
conditioner and furnace simultaneously. It's also quite unlikely that all of
the lights in your house will be on at once, or that you'll simultaneously be
using all of your appliances.
It was ok, at least as you describe.
I never timed one, but yeah, it takes a little while, but only a little.
Doug Miller (alphageek at milmac dot com)
OK. I'm happy. It's been a long time but I thought one or two people
said it was bad at the time, and that 2 or 3 others have said it was
bad since then. Whoever did complain to me, it seemed like an
absolutist rule that didn't make sense. I'm glad it's not a rule
Thanks. Several more reply lines below, but this was the essence of
Right. My mistake. I really needed my 20 amps. 4 people for a few
years, and later 2 people and one room air conditioner. I also worked
things out so I could go to the basement to change the fuse in the
middle of the night without bothering the super. I think I replaced a
blown fuse about 10 times in 10 years. Somewhere between 8 and 18. I
vaguely remember a short period where I blew a whole bunch, but I
can't remember why. 18 in ten years doesn't seem like too many.
much by having only 20. Washing machine, no dryer or course, gas
stove. Plenty of lights, radios, I might have had two tv's on at once
once in a while (one in the kitchen), but iirc no roommate (I had
about 20) ever had even one tv. I wonder why not.
I think this is typical for 100's or thousands of buldings in NY.
Immigration was high all the time, but especially the 1880's to 1914,
I think it was, so there were plenty of buildings nicer than tenements
being built by the '30's. The war may have prevented much building in
until '45, and after that there was probably more electricity provided
for each apartment.
In the garbage room on our floor (where there was a chute that led to
the basement, where the garbage was first burned and then later
compacted, there was a sticker on the wall: "Save your cans. Defeat
the Axis." They only painted this tiny room every 20 years or so --
it didn't seem to get dirty -- and when they painted in 1980, they
didn't paint over this.
I meant to say it the other way. Or I was looking at "feeding" in a
different way. But regardless, I didn't mean anything different from
It better not be, because probably most people would be in violation
of it. The sum of the ampacities of branch breakers in a main panel
is _usually_ higher than the main feed breaker.
[IIRC, the breaker sum in our panel is > 3 times the size of the main
Slo-blos are _specifically_ designed to ride out the startup current surge
of a motor - typically 3-5 times higher than the steady state amp rating
of the motor. If that takes more than 5-10 seconds (eg: the motor is stalled),
the fuse better blow or the wiring might do something real bad (eg: melt).
So, no, you don't want a slo-blow to take much longer than that.
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
Only in the sense that if larger service entrance conductors were installed
you would have less of a voltage drop and losses from resistive heating that
you would otherwise under very heavy loads. A lot depends on the length of
the drop. If all the facts are known, it can be calculated with reasonable
accuracy (the resistance of the SE conductors and the resulting voltage drop
as a fuinction of current used).
You would have to be using most of that 300A most of the time to see a
difference though. It probably would reduce the incidence of lamp dimming
under heavy load conditions.
I don't think it would make any diffenence in your bill whatsoever since
metering is done after the SE conductors make it to your house. Any energy
lost in the SE conductors as heat (P=I*V or I^2*R) would not be metered.
It depends on your needs and the size of the home. I know a couple with
a 7000 SQ foot home they have two 250 amp main panels. I was told 250
is the largest main home panels available but dont know if this is
Frequently main entrance cable around here is alunimum, I would spend
the extra for copper from the meter can to the main breaker.
sounds like a big home have you included a standby generator for power
failures? thats $ well spent
Actually Al is better for heavy gague service entrance conductors. The
metal oxidizes less, it weighs less and it stands up to the elements better
than Cu if the jacket is damaged (due to the thin patina of oxide). You
only need slightly more metal to get the same resistivity as Cu but these
things are over engineered anyway. Al also expands less than Cu which is an
advantage for long overhead lines to reduce droop in the summer. Other than
that, in a poll, 4 out of 5 electrons don't care what metal they are in. :)
Rule of thumb, more house, more lights, plugs and loads. A smaller house
(~100SF) could justify so much current but it would need a big load like a
Kiln or an electrically heated pool.
Just a techinical issue, I am unaware of any manufacture that makes a 300
Get a service for the load. Bigger is not better in this case.
I will provide my address if you feel the need to give away money
He is right. While you are at it, make sure you get the extra thick
paint, the more dense drywall, the bigger element light bulbs, the
heavier gauge metal appliances, the dryer lumber, the triple sanded
floors and cabinets with the extra thick polyurethane, quicker curing
concrete and 5 dimensional shingles. All this will help make your home
more energy star and lighter because after your builder removes all
that money from your wallet your home will be much lighter than before
since you wont be carrying all that cash around.
The quicker curing concrete isnt as strong as regular stuff:(
The advantage to a copper conductor from the meter can to breaker box
is minimizing resistance losses. Sure its not much but its still
What the power company loses is their trouble as long as my service
I dont know of anyone who makes a 300 amp service
Ah, no. When using aluminum, you're switching to a higher gauge wire,
so the resistance will be about the same.
The issue with aluminum is poorly done/improper terminations. It's
less forgiving of sloppy workmanship than copper. Properly installed,
there are no issues with aluminum feeds.
Individual circuits with aluminum multiply the risk of poor workmanship,
so many/most juridictions no longer permit aluminum for branch circuits.
But with main feeds or subpanel feeds, they're still quite common.
But 300A and 400A service drops _are_ common. So while they may not
make panels that big, there's nothing to prevent you from splitting
a (say) 400A drop to two 200A panels. If done to code...
Chris Lewis, Una confibula non set est
It\'s not just anyone who gets a Starship Cruiser class named after them.
A three hundred ampere service is a four hundred ampere enclosure with a
three hundred ampere breaker or fuses installed, two 150 ampere main
breaker panels installed in parallel from a single set of service entry
conductors, up to six main disconnecting means supplied from a single
set of service entry conductors that are sized to carry 300 amperes, and
the list goes on. The size of an electric service is based on the
calculated load with an allowance, if wanted, for future expansion. The
resultant amperage is the figure that governs the size of the service
entry conductors. The only time that the service Over Current
Protective Device (OCPD) has to match that ampacity is when it consist
of only one OCPD. If it is two or more OCPDs it can exceed the size of
the calculated load by a large amount as long as the calculated load is
within the ampacity of the service entry conductors that are installed.
This is just one of the things that is easy for untrained and
unqualified persons to get wrong when doing electrical installations.
If the eventual connected load exceeds the ampacity of the service entry
conductors the overload can cause a fault in the service entry
conductors that can lead to a burn down and may kindle a structure fire.
Service Entry Conductors do not have short circuit or ground fault
protection. In many cases the only protection they have against
overload is the conscientious work of the electrician in properly
executing the service calculations. This is why some localities do not
permit the use of multiple service disconnecting means that total to
more than the ampacity of the service in individual residential
240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard ampere ratings
for fuses and inverse time circuit breakers shall be considered 15, 20,
25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200,
225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600,
2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard
ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses
and inverse time circuit breakers with nonstandard ampere ratings shall
"This alternating current stuff is just a fad. It is much too dangerous
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