What size subpanel can I use for 150A Main panel

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?
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On Sun, 21 Sep 2014 03:59:36 -0700 (PDT), Mikepier

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

That depends on if you want a 100 amp subpanel or a 50A subpanel.
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On Sun, 21 Sep 2014 10:07:53 -0700 (PDT), Mikepier

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.
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On Sunday, September 21, 2014 6:59:36 AM UTC-4, Mikepier wrote:

*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.
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snipped-for-privacy@aol.com 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.
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On 10/09/2014 10:14 AM, Pete C. wrote:

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
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Percival P. Cassidy wrote:

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

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
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"Percival P. Cassidy" wrote:

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).
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On Thu, 09 Oct 2014 12:24:23 -0400, "Percival P. Cassidy"

Gee whiz info. A branch circuit is the wiring from the last over current device (fuse or breaker) to the load. The wiring between two overcurrent devices like from the service panel and a sub panel, is a feeder. The wires on the line side of the service disconnect are service conductors. Between the disconnect and the "service point" where the power company lines start are service entrance conductors and underground wires are service laterals. Overhead wires are service drops.
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