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Our home has a Square D breaker box Cat. No. QO 30HW Series E6 (based on the sticker inside the panel).

It has 30 physical spaces, and the bottom 4 (27-30) have tandem breakers on them. I am looking to add another tandem breaker which as I understand it is acceptable based on info provided in this article provided the box will accept it.

What I do not know is whether my box will accept another tandem switch. Based on this article, the diagram should indicate whether it will accept tandem switches.

That article has this diagram as an example of a box that will not accept any tandem switches: enter image description here

Looking at the diagram inside my box, it looks pretty similar to me: enter image description here

However as mentioned, 27-30 already have tandem switches on them, and they work (work = I can independently turn off each switch and only impact that switch).

I cannot seem to find any literature on the box (probably because I don't know what to actually search for). Also, if it will accept another tandem switch, I don't know if it has to be the kind with clips on top and bottom, or if it will take the hook style. Again this might be answered with some literature about the box.

Does the diagram indicate where tandems are acceptable?

EDIT: This is another picture of the diagram in my box. It's about as good as I can get without pulling out the breakers on that side.

enter image description here

  • Can you get a better photo of the box label? It seems that your box predates CTL but I can't be 100% on that w/o a better photo... – ThreePhaseEel Sep 10 '16 at 17:39
  • Added another picture of the diagram. It's about as good as I can get for the picture without pulling out the breakers. – Fuzz Evans Sep 11 '16 at 4:42
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Your box predates the UL requirements for a feature known as Circuit Total Limitation (CTL). This feature restricts where double-stuff/tandem breakers can be installed in the panel to prevent it from exceeding its designed circuit capacity, which was restricted to a maximum of 42 circuits by the NEC (i.e. nobody could make a panelboard for more than 42 circuits). However, the 2008 NEC lifted the 42 circuit restriction on panelboards.

Practically speaking, I would go ahead and install the new double-stuff/tandem breaker provided that there is sufficient remaining wiring space in the panelboard gutters to accommodate the extra wire. You'll need a QO2020 (not a QOT2020, which won't fit) breaker for this.

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