Adding 20A tandem breaker to QO panel


  #1  
Old 01-27-02, 04:53 PM
hotarc
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Adding 20A tandem breaker to QO panel

I have a Square D QO Load Center with 16 breaker positions. Slots 1 & 3 are occupied by a 30 AMP double for the dryer circuit; 2 & 4 is a 60 AMP double pole marked MAIN on the circuit list; 5 & 7 are empty, 6 & 8 is 40 AMP double pole Range circuit. There are stickers that were applied to the metal cover inbetween the left and right banks of breakers reading: MAIN DISCONNECT. These stickers are applied next to each slot 1-8. Below these there are (7) 15 AMP branch circuits on the right (3 tandems and 1 single), and (5) 20 AMP branch circuits on the left (2 tandems and 1 single). My questions are:

(1) Can I add a 20 AMP tandem breaker (two 20 amp circuits) to slot 9 on the left side without overloading the panel?

(2) Are the range and dryer circuits really main disconnects or were those labels incorrectly applied?

(3) Is there any way I can calculate how many amps my service is? The map on the door says the panel is rated up to 150 amps, so I assume it's not any more than that.

Any help is greatly appreciated.
 
  #2  
Old 01-27-02, 06:30 PM
Jxofaltrds
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Hotarc you should be OK. What overloads the service is everything on at the same time.
Why not use 5 and 7?
 
  #3  
Old 01-27-02, 06:40 PM
hotarc
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I wanted to leave them open in case I install air conditioning in the future. Also, I'm unclear about the main disconnect and if 5 & 7 are part of it in some way.
 
  #4  
Old 01-27-02, 06:52 PM
Jxofaltrds
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Hotarc I am going to bow out of this one. My "gut" says that I am going to steer you wrong . Too much on my mind.
 
  #5  
Old 01-28-02, 09:06 AM
T
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Sounds like you may have a split-bus panel. Where are the wires connected in the service panel from the electrical utility meter? You didn't mention any loads for space heating and a water heater. Are they natural gas?
 
  #6  
Old 01-28-02, 09:27 AM
hotarc
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If my understanding of a split bus is correct, then I have one. I'm assuming a split bus mean that two adjacent slots, such as 1 and 3 are on different phases. Moreover, 1 and 2 would be on the same phase. There is a red conductor connected to one side of the panel and a black connected to the other. My water heater and furnace are both natural gas and the furnace blower is on a dedicated 15 A breaker.
 
  #7  
Old 01-28-02, 10:04 AM
T
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(1) Can I add a 20 AMP tandem breaker (two 20 amp circuits) to slot 9 on the left side without overloading the panel?

How much load are you adding to the panel? What will the new 20 amp circuit supply?

(2) Are the range and dryer circuits really main disconnects or were those labels incorrectly applied?

If this is in fact a split-bus the answer is yes. Circuit breakers 1-8 are the main disconnects. The 60 amp circuit breaker turns off circuit breakers 9-16.

3) Is there any way I can calculate how many amps my service is? The map on the door says the panel is rated up to 150 amps, so I assume it's not any more than that.

You need to find out the wire size (gauge) of the hot (black & red) service-entrance conductors. This will designate their ampacity. Are the black and red service-entrance conductors connected to the main lugs in the service panel?
 
  #8  
Old 01-28-02, 11:41 AM
joelp
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You need to be careful when using a tandem breaker. Do not use 12/3 as you will be sharing the neutral and could overload it.
 
  #9  
Old 01-28-02, 12:55 PM
hotarc
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One 20A circuit will first connect to a 120V smoke detector in the basement, next a GFCI protected duplex receptacle and from that I was planning to connect two more duplex receptacles, all located in an attached garage.

These three receptacles will be primarily for a circular saw, miter box, electric drill, and will be used rather infrequently and almost never simultaneously.

After the three convenience receptacles I would like to install two or possibly three outdoor incandescent ighting outlets with 75-100W lamps.


The other 20A circuit will power the basement and supply 4 shoplights, each with two 4' 40W tubes, and 4 duplex receptacles which will power the miter box and circular saw, when I'm not using them in the garage, and occasionally a small tv or radio. The lights will normally not all be used simultaneously.

I will have to remove the panel cover to determine the service entrance conductor size. It is an underground feed and there is not enough exposed cable to see the markings.

I will be running two separate 12/2 NM cables. I don't want to share the neutral. Too many potential drawbacks.
 
 

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