I'm redoing a section in my house in which I am adding a new kitchen and bathroom. Since the main panel is getting pretty full, I wanted to put in a sub-panel. My house has 150A main panel. House has gas appliances. The only 220V device is the CAC.

My question is are there any rules as to what size sub-panel you can get with regard to what main is feeding it? I wanted to get a 100A/ 20 ckt main lug as the sub-panel. Any issues with this? Or should it be a smaller sub-panel?

There is no problem with a 100a sub panel on a 150a main. You would

feed that with #2 copper and off you go. You can protect the feeder

with a 100a breaker in the main panel and use an MLO sub panel as long

as you are in the same building.



The real limit is on the service as a whole. To know for sure whether

150a is enough you need to do a load calculation.

There are several load calculation templates on the internet you can

try.

If you have Excel the one from the city of Naperville seems pretty

good. There are 2 methods and they usually do not come out with the

same answer but you can use either one.,

Load calcs are something you measure with a micrometer, mark with

chalk and cut with an axe. Residential load calcs are just a wild

guess anyway. A frugal person would never get close to the calculated

load and a house full of people who never turn anything off will

easily exceed it.

Do I need to run #2 wire and use a 100A breaker? Or can I just run #6 to the sub-panel and use a 50A or 60A breaker in main?

On Sunday, September 21, 2014 1:07:53 PM UTC-4, Mikepier wrote:


There is no problem with a 100a sub panel on a 150a main. You would



feed that with #2 copper and off you go. You can protect the feeder



with a 100a breaker in the main panel and use an MLO sub panel as long



as you are in the same building.







The real limit is on the service as a whole. To know for sure whether



150a is enough you need to do a load calculation.



There are several load calculation templates on the internet you can



try.



If you have Excel the one from the city of Naperville seems pretty



good. There are 2 methods and they usually do not come out with the



same answer but you can use either one.,



Load calcs are something you measure with a micrometer, mark with



chalk and cut with an axe. Residential load calcs are just a wild



guess anyway. A frugal person would never get close to the calculated



load and a house full of people who never turn anything off will



easily exceed it.



Do I need to run #2 wire and use a 100A breaker? Or can I just run #6 to the sub-panel and use a 50A or 60A breaker in main?

That depends on if you want a 100 amp subpanel or a 50A subpanel.

On Sunday, September 21, 2014 6:59:36 AM UTC-4, Mikepier wrote:
I'm redoing a section in my house in which I am adding a new kitchen and bathroom. Since the main panel is getting pretty full, I wanted to put in a sub-panel. My house has 150A main panel. House has gas appliances. The only 220V device is the CAC.



My question is are there any rules as to what size sub-panel you can get with regard to what main is feeding it? I wanted to get a 100A/ 20 ckt main lug as the sub-panel. Any issues with this? Or should it be a smaller sub-panel?


*The label in the 150 amp main panel should tell you the maximum size branch circuit breaker that is permitted to be installed in it. Get the largest sub-panel you can fit in the space available.

wrote:

On Sun, 21 Sep 2014 10:07:53 -0700 (PDT), Mikepier
wrote:


There is no problem with a 100a sub panel on a 150a main. You would

feed that with #2 copper and off you go. You can protect the feeder

with a 100a breaker in the main panel and use an MLO sub panel as long

as you are in the same building.



The real limit is on the service as a whole. To know for sure whether

150a is enough you need to do a load calculation.

There are several load calculation templates on the internet you can

try.

If you have Excel the one from the city of Naperville seems pretty

good. There are 2 methods and they usually do not come out with the

same answer but you can use either one.,

Load calcs are something you measure with a micrometer, mark with

chalk and cut with an axe. Residential load calcs are just a wild

guess anyway. A frugal person would never get close to the calculated

load and a house full of people who never turn anything off will

easily exceed it.

Do I need to run #2 wire and use a 100A breaker? Or can I just run #6 to the sub-panel and use a 50A or 60A breaker in main?

Sure, the 100a is the max the sub panel can be used for but you could
use less if you want.
In residential, there are a lot of required circuits that might have
non coincidental loads. A lot depends on how many people are in the
house at any given time.

125A would be the maximum size you can use (with appropriately rated
wiring), that is the max branch circuit size allowed by code for
residential services. You'll have to special order 125A breakers though
as they are not common.

On 10/09/2014 10:14 AM, Pete C. wrote:

125A would be the maximum size you can use (with appropriately rated
wiring), that is the max branch circuit size allowed by code for
residential services. You'll have to special order 125A breakers though
as they are not common.

What is the definition of "branch circuit"? We have a small panel in the
garage back-to-back with the meter. That has two 100Amp breakers
(ganged) from which the "main" panel in the basement is fed, and a 15Amp
breaker feeding the lights and outlets in the garage. Is everything fed
from that panel in the basement part of a "branch circuit" and limited
to 125A? E.g., IF* we got our service upgraded (I have no idea what
gauge wire is used to the meter and from the meter to the panel in the
garage) and ran heavier-gauge conductors from the panel in the garage to
the one in the basement, would we still be limited to 125A from the
panel in the basement?

* We've never tripped the 100Amp breakers, so the question is academic.

Perce

Percival P. Cassidy wrote:
On 10/09/2014 10:14 AM, Pete C. wrote:

125A would be the maximum size you can use (with appropriately rated
wiring), that is the max branch circuit size allowed by code for
residential services. You'll have to special order 125A breakers though
as they are not common.

What is the definition of "branch circuit"? We have a small panel in the
garage back-to-back with the meter. That has two 100Amp breakers
(ganged) from which the "main" panel in the basement is fed, and a 15Amp
breaker feeding the lights and outlets in the garage. Is everything fed
from that panel in the basement part of a "branch circuit" and limited
to 125A? E.g., IF* we got our service upgraded (I have no idea what
gauge wire is used to the meter and from the meter to the panel in the
garage) and ran heavier-gauge conductors from the panel in the garage to
the one in the basement, would we still be limited to 125A from the
panel in the basement?

* We've never tripped the 100Amp breakers, so the question is academic.

Perce

Hi,
You have breaker box behind the meter, main panel is in the basement,
To protect wiring
between garage and basement. I don't think you have sub panel. Isn't sub
panel branching
out from main panel? No?

On 10/09/2014 12:37 PM, Tony Hwang wrote:

125A would be the maximum size you can use (with appropriately rated
wiring), that is the max branch circuit size allowed by code for
residential services. You'll have to special order 125A breakers though
as they are not common.

What is the definition of "branch circuit"? We have a small panel in the
garage back-to-back with the meter. That has two 100Amp breakers
(ganged) from which the "main" panel in the basement is fed, and a 15Amp
breaker feeding the lights and outlets in the garage. Is everything fed
from that panel in the basement part of a "branch circuit" and limited
to 125A? E.g., IF* we got our service upgraded (I have no idea what
gauge wire is used to the meter and from the meter to the panel in the
garage) and ran heavier-gauge conductors from the panel in the garage to
the one in the basement, would we still be limited to 125A from the
panel in the basement?

* We've never tripped the 100Amp breakers, so the question is academic.

You have breaker box behind the meter, main panel is in the basement,
To protect wiring
between garage and basement. I don't think you have sub panel. Isn't sub
panel branching
out from main panel? No?

So that panel in the garage doesn't count as the "main panel"? I would
have thought that the "main panel" would be the first one after the
meter. No? (That's why I put "main" in quotation marks referring to the
one in the basement -- because I thought that technically it might not
count as the "real" main panel.)

Perce

"Percival P. Cassidy" wrote:

On 10/09/2014 12:37 PM, Tony Hwang wrote:

125A would be the maximum size you can use (with appropriately rated
wiring), that is the max branch circuit size allowed by code for
residential services. You'll have to special order 125A breakers though
as they are not common.

What is the definition of "branch circuit"? We have a small panel in the
garage back-to-back with the meter. That has two 100Amp breakers
(ganged) from which the "main" panel in the basement is fed, and a 15Amp
breaker feeding the lights and outlets in the garage. Is everything fed
from that panel in the basement part of a "branch circuit" and limited
to 125A? E.g., IF* we got our service upgraded (I have no idea what
gauge wire is used to the meter and from the meter to the panel in the
garage) and ran heavier-gauge conductors from the panel in the garage to
the one in the basement, would we still be limited to 125A from the
panel in the basement?

* We've never tripped the 100Amp breakers, so the question is academic.

You have breaker box behind the meter, main panel is in the basement,
To protect wiring
between garage and basement. I don't think you have sub panel. Isn't sub
panel branching
out from main panel? No?

So that panel in the garage doesn't count as the "main panel"? I would
have thought that the "main panel" would be the first one after the
meter. No? (That's why I put "main" in quotation marks referring to the
one in the basement -- because I thought that technically it might not
count as the "real" main panel.)

Perce

I believe it counts as the service disconnect and thus also the bonding
location for ground and neutral. There should be a four wire connection
to the load center in the basement. The branch circuit would be the
breakers that snap into the load center and those are 125A max. The
service disconnect isn't a branch circuit so those breakers could
readily be 150A, 200A, etc. as appropriate for the load center. I've
seen 400A service split feeding two 200A load centers in residential
service before (big f'n house).