On Fri, 25 May 2018 20:07:36 -0400, Clare Snyder
wrote:

On Fri, 25 May 2018 20:35:01 -0000 (UTC), Wayne Boatwright
wrote:

On Fri 25 May 2018 10:44:02a, Mistert told us...

replying to PipeDown, Mistert wrote:
I have a small 1800 sq foot home 2 full bath and 2 old fogies live
here retired and i want to go to the tankless Eco 36 but have 200
not 300 amp svce. and we never have 2 showers at once just a load
of clothes. Do I have to add more elect'l service?


More could have been mentioned about what you have in your home. It
would be better to take a complete invenory of your home's contents
that use electricity than to guess at it.

When we bought our condo (only 1500 sq, ft.)it had a 200 amp panel
(it had already been upgraded from a 100 amp panelsome years before
that). Knowing all of the electrically dependant items in our condo,
we added a second 200 amp panel to handle any possible combination of
electric usage. We have the following:

HE washer and dryer
2 dishwashers
1 electrically operated hot tub
2 Jacuzzi bathtubs
electric range
warming drawer
induction cooktop
2 1300watt microwave oven
numerous countertop appliances
central a/c
central electric furnace
a tankless water heater dedicated to serving our washing machine
The rest of our hot water supplied from a central boiler.
a vast number electronic devices, many of which opeate 24/7
ceiling fans in every room that operate 24/7

an elaborate installation of outdoor lighting
at least 5-6 light fixtures or lamps in each room

That probably doesn't cover everthing, but we never wanted the risk
of tripping a breaker, and never had except for a shorted out
appliance. We had several brown-outs, but that was external to the
panels in our home.

In our case it wa easier and less costly to add a second panel than
to upgrade the existing panel.

We have replaced most of our incandescent light bulbs with LEDs
wherever possible. However, out bathroom light fixtures use
proprietary halogen light bulbs.

Sorry for the long discourse!
So you have 2 meters too?
If not you have a 400 amp service.
To do that here I would need to have another service feed brought in
from the underground transformer vault at a cost of some $8000 -
Whether I went for a 400 amp oor 2 200s - or even to put in a single
200, as my underground feed is only good for 150. (So I upgraded to a
150 amp panel when I replaced my old 100amp fused service entrance
panel)

In the US you are going to have one service with a few very specific
exceptions like a fire pump, different voltage characteristics or a
building so big it is not practical to feed it with one.
That still does not mean you can't have up to 6 service panels tho but
it will all be on one drop or service lateral. They are usually going
to be metered together too unless it is permitted for multiple
dwellings.
You are right that you are limited by the service lateral for buried
services (the "service point" is at the street) but generally a
utility owns an overhead drop so they will replace that for free. That
service point is where the crimps are at the house. You still own the
"service entrance conductors" from there to the meter and from the
meter to the disconnect.
You can see, buried services are far more expensive to upgrade, even
if the pipe is big enough for larger conductors.
Usually, if you have a 2" service lateral conduit, you can go up to
320a without changing the pipe but good luck pulling in three 350
Kcmil copper wires in there if it is a long ways or has more than 2
bends in it. This will also have to be copper. If it dead ends into
the transformer vault in a straight line it might not be horrible.
Start at the house end and feed it through the 90 at the bottom,
preferably before you mount your new panel.