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Old 08-27-2012, 12:12 AM   #1
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sub panel hookup to main panel using main panel power lugs


Main panel is full and no more space for breakers nor can I change the breakers to make room.

My question is can I hook a sub panel directly to the main panel power lugs. I am talking about the lugs on the main panel that power the main breaker. These are the lugs on the main panel that the power company bring the power from the meter to the house.

Is there an extension that can be added to the power buss bar to make room for the additional wire going to the sub panel?

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Old 08-27-2012, 05:19 AM   #2
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sub panel hookup to main panel using main panel power lugs


Remove 2 circuits from your panel, install a 2 pole breaker to feed your sub panel. Rewire the 2 removed circuits to the sub panel. Add new circuits to the sub panel.

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Old 08-27-2012, 07:00 AM   #3
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sub panel hookup to main panel using main panel power lugs


Those lugs are for one wire only. Follow RJs advice.
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Old 08-27-2012, 03:09 PM   #4
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sub panel hookup to main panel using main panel power lugs


Thank you for the instructions and advise. Please advise on how best to extend the two circuits that are being moved to the sub panel:
1. Should I add a junction box to splice the circuits I am moving or do I just spice at the main panel and tuck the wires back into the main box?

2. Do I need to spice all three wires to extend the circuit to the sub panel or do I just splice the hot wire and leave the common and the ground in the original slot on the main panel?

3. The main panel is an ITE 150 AMP box. The left side of the bus bar is full with 2 pole breakers. If I add the total number of amps these breakers are rated, it equal 140 amps. The right side of the bus bar consist of individual twin breakers each take up one slot. If I add the total number of amp these breakers are rated, it equals 190 amps. Therefore, how do I calculate the largest double pole breaker I can use for the sub panel feed.

4. The main panel door read the following:
"SUM OF QT BREAKER RATING NOT TO EXCEED 110 AMPS PER BRANCH CIRCUIT BUS SLAB"
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Old 08-27-2012, 03:22 PM   #5
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sub panel hookup to main panel using main panel power lugs


Just add up the breaker ratings of any two pole with any breakers on the opposite side across. This needs to be 110 amps or less.

All the conductors of the circuit need to be kept together. you can splice in your panel unless you are in Canada.
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Old 08-27-2012, 03:52 PM   #6
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Thank you Jim:
Sorry, but I am not understanding "Just add up the breaker ratings of any two pole with any breakers on the opposite side across."

When you say the opposite side are you referring to the sub panel and all the breakers that will be added plus the 2 pole breaker feeding the sub panel?

Or are you referring to sum total of the breaker on the same branch of the main where the 2 pole breaker is being installed to feed the sub panel?

If the left branch of the main panel has a total of 140 amps in breakers and the right branch a total of 190 amps in breakers does this mean that each branch of the main panel is over loaded?

Which is the best way to check each branch current draw to make sure no one branch is actually drawing more that 110 amps, eventhough the total amp rating of all the breakers per side is greater.
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Old 08-27-2012, 04:41 PM   #7
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For example add the breaker ratings of the breakers in spots 1 and 3 with the breaker in spots 2 and 4. If you are less than 110 amps you can add the breaker across from 1 and 3.

The breaker values on the left and right when added down are meaningless as far as a load calculation.
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Old 08-27-2012, 06:47 PM   #8
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sub panel hookup to main panel using main panel power lugs


Thanks, I understand you make a good teacher at a tech school.

So If slot 1 & 3 have a 2 pole 50 amp breaker I can remove the the 15 amp and 20 amp breaker on slot 2 & 4 and install a 2 pole 60 amp breaker to feed the sub panel and be within the requirement of the main panel.

S0 how do I check the total amperage being used on each leg to make sure that when I install the 60 amp breaker to feed the sub panel I do not exceed the main panel 150 amp rating?
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Old 08-27-2012, 09:04 PM   #9
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sub panel hookup to main panel using main panel power lugs


You have grasped the bus stab limit.

To see if your main breaker will support a new load you would perform a demand load calculation. This involves the square footage and the draws of specific appliances.
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Old 08-27-2012, 09:34 PM   #10
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sub panel hookup to main panel using main panel power lugs


Quote:
Originally Posted by Jim Port View Post
For example add the breaker ratings of the breakers in spots 1 and 3 with the breaker in spots 2 and 4. If you are less than 110 amps you can add the breaker across from 1 and 3.

The breaker values on the left and right when added down are meaningless as far as a load calculation.
Do we have to take two rows of breakers (slots 1-2 and slots 3-4) into account at the same time?

Or does the 110 amp or whatever limit get applied to just one row at a time, for example just slots 1 and 2 (left/right) and if the two breakers in any one row add up to more than the limit then there is an overload?
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Old 08-27-2012, 09:42 PM   #11
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sub panel hookup to main panel using main panel power lugs


I would say 1,3 and 2,4 since the largest single pole breakers would never come close to overloading a bus stab. Suppose a two pole 100 was added, you could now have 100 amps on one bus stab. The breaker across from both poles would need to be 10 amps or less which isn't going to happen. In that example the other side would need to remain unused.
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Old 08-27-2012, 10:35 PM   #12
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sub panel hookup to main panel using main panel power lugs


Jim:
Just a quick question on calculating the demand load for the main breaker. You said:

"This involves the square footage and the draws of specific appliances."

My question is in regards to the square footage. I am not sure I understand what square footage you are referring to.

Taking an amp reading on each wire feeding each leg of the bus is that not sufficient to see how much current demand is being applied to main breaker?
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Old 08-27-2012, 10:51 PM   #13
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sub panel hookup to main panel using main panel power lugs


The square footage in the house of any finished area or area that could be finished later. Do not count porches, garages, attics etc.

Using an ampmeter would only tell you how much is being used at that moment.
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Old 08-27-2012, 11:08 PM   #14
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sub panel hookup to main panel using main panel power lugs


ok understood you are referring to the square footage of finished areas.

What roll does the square footage play in terms of the current demand on the main breaker? In other words, is there a formula to calculate amps per square foot?

I guess what I am getting at is if the size of the main breaker depends on the finished square footage of a house? Please elaborate because I am not grasping the roll of square footage and main breaker demand.

Lastly, if using an amp meter only give me the current demand at the time of the reading, how else do you calculate the total current demand for a circuit? Do you have to physically read the current draw of each aappliance or item being used on that cirucut?
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Old 08-27-2012, 11:13 PM   #15
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sub panel hookup to main panel using main panel power lugs


This might help you. There are also apps for this for smart phones.

http://www.naperville.il.us/loadcalculator.aspx

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