Hello,

I have a 150 amp main and a full 150 amp circuit breaker box with a number of doubled 20 amp breakers. Can I still add a sub panel? If so, how does one calculate the maximum amount in amps of such a sub panel?

-Terry

The adding of a subpanel is not something that is calculated. You can have 6 circuits in a home, or you can have 84 circuits. All depends on the size of home and the load calculations of the service.

I need the square feet area of all livable space, no basement, unless finished as livable, garage, attic, etc. Add need a list of all appliances, and type of heat.

What is the subpanel for? What type of added load is planned? Are you adding to the livable sq ft?

I was thinking of increasing to 200 amp service,
But thought after reading up on sub-panels that this may work too


Current living space 2100
400 main living (master bedroom and addition to existing bathroom)
400 storage/workshop (no big electrical demand tools, only 110)


Gas hot water
Gas heat


Currently have

Ac
Dishwasher
Dryer
Stove/range
Microwave
Garbage disposal
Hot tub 220 on 50 amp coming directly off 150 main breaker by existing meter

I have add everything running and did not pop the 150 main :)


In new space plan on adding jetted tub
And add small steam unit in shower


I will be moving the AC unit so will open space on 150 amp circuit box
If I add the AC unit to the sub



-T

So total livable space will be 2500SF? Assuming this, and using average KW loads for the appliances, such as hot tub and AC, I calculate 121 amps must be available from the 150 amp service by using Optional method with AC load larger than heating load as allowed by the NEC.

Operating at 100%, this leaves you with 29 amps per leg. If I were hired to add the proposed load, I would only allow it if the service was upgraded to 200 amp.

I am intrigued by the hot tub statement "coming directly off 150 main breaker by existing meter. What exactly does that mean? Is there a 50 amp breaker next to the 150 amp main breaker? There better be.

Not sure what you added how, but that is not how the calcs are done. Won't go through the details here, if you like you can review the method in a National Electric Code book , usually available at most libraries in the reference section.

The new tub and steam generator can need 30-40 amps alone.

What does "I was thinking of increasing to 200 amp service,
but thought after reading up on sub-panels that this may work too" mean? Whatever you read, adding a subpanel cannot increase the ampacity at the main breaker.

Your project should include a budget of $1K-$2K for a new 200 amp service, otherwise, you take a large chance of tripping the main CB, often.

If this project is permitted, the inspector may look at the size service,( he can tell quickly, he has done this often) and ask for the load calcs be done to determine if the service can handle the added load.

You then would only be issued a permit contingent on upgrading the service.

Hope this helps, not sure this is what you wanted to hear, but , not here to be popular. Have I done a good job?

Yep----done an excellent job---just thought I would investigate not uping to 200 amps... and yes there is a 50 amp breaker in place... again thanks for the info... just got to bite the $1k-$2k bullet... crap... :) have a great weekend...

-T