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Old 05-30-2010, 08:40 AM   #1
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wiring panel for room addition


I have 200 amp service in my house. I have a 100 amp service in my garage that comes from my house panel. My house panel only has enough room to accomodate 2 breakers. I plan to install another 100 amp breaker in my house panel to run to another service box in my added on room and wire that room from that new box. Can anyone tell me if i can do this? Will i be taking a chance on over loading my house panel? Thanks

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Old 05-30-2010, 08:52 AM   #2
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there's no chance of overloading your house. It makes no difference how many 100A sub panels you install, the main is still protected by a 200A main and at no point will you go through more power than that.

It may be a good idea to do a load calc though and see if a 200A service is sufficient for what your using it for as 2 x 100A and 1 200A panel seems like an awfull lot of breakers to need for a standard house.

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Old 05-30-2010, 09:01 AM   #3
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How is a load caculation performed?


In my garage i basically operate lights and occasionaly a welder ( 720 sq ft ). In the new room i will be running lights, 2 celing fans and a PTAC ( motel style heating and air unit ) The new room is 384 sq ft.
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Old 05-30-2010, 09:16 AM   #4
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You do a whole Load Calc
Ther is a zip file of an Excel spread sheet for a load calc in this thread:

NEC- National Electric code 2008

Do you have heavy electric loads currently ?
A lot of electric appliances or heat with electric ?
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Old 05-30-2010, 09:33 AM   #5
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No just the average house hold usage I run a pool pump and a/c in the summer thats about it. I heat with wood stove in the winter and the heat pump hardly ever comes on.
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Old 05-30-2010, 09:46 AM   #6
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I have 200a service & my electric bill was running $250-300 a month
Hot tub, electric dryer, stove, 1.5hp pool pump, TV's & PC's
AC used very little
I have (2) 100a subs & a 60a sub for the pool

Technically you should do a calc
But if you don't have a huge electric bill then you should be fine
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Old 05-30-2010, 06:41 PM   #7
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I looked into the load caculations and that is way above my level of education. The information you provided has been very helpful. My electric bill usually at the very most peaks around 220.00 a month in the summer for aboout 3 months. My house is 1512 sq ft and the addition will add 384 more sq ft. Pulling another 100 amp circuit to the addition will be ok to do so i feel.
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Old 05-30-2010, 06:50 PM   #8
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I'd say so....your electric bill is a lot less the mine
I recently had mine drop with solar heat for my hot tub

I have one of my 100a subs 3' to the left of my 200a panel
Didn't take much wire & kept it in a central location
4 wire feed needed, keep grounds & neutrals on seprate bars at the sub
Neutral bar not grounded to the case
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Old 05-30-2010, 08:16 PM   #9
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wiring panel for room addition


Instead of a subpanel you could get by with a circuit for the HVAC unit and one for the receptacles and lights. I might separate the lights form the receptacles, depends on the use of the room and the number of lights.
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Old 05-31-2010, 06:12 AM   #10
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How would i make this work with out another sub panel? I only have 2 spaces left in my main service panel, which i plan to use to run service to my addition. My PTAC unit is 230v 30 amp. I will also have 2 ceiling fans, up to 4 ceiling / wall lights, 6 receptacles, 1 outside 250 w flood light, 4 regular outside lights ( 2 by entry door-2 by very large window. ) I would assume i need a panel in the addition with at least 8 breaker spaces. Just a thought, should i just put something like a 60 amp sub panel for service?
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Old 05-31-2010, 07:55 AM   #11
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You can do this without a subpanel if your existing 200A panel accepts tandem breakers. You can either use a Quad breaker to give you the necessary 240V plus two 120V circuits, or you can use tandems to open up one or two spaces for you 120V circuits. Depending on what you plan to use this room for, only one 120V circuit may be necessary.

What is the brand and model number of your 200A panel?
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Old 05-31-2010, 07:59 AM   #12
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Use the last 2 spaces to feed the 100a sub
Make sure it is a 20 space panel

2 spaces..........PTAC 240v 30a
1 space...........light circuit
1 space...........outlet run
8 spaces..........kitchen

That leaves 8 spaces free

60a or 100a sub depends upon what you are going to feed from the sub
If you used a 60a then the PTAC is already using 1/2 of the capacity

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