Square has made tandem breakers since QO was invented. They make two types. The QOTxxxx which is not a cheater breaker, it can only be installed in a proper CTL (circuit limiting)location. And, the QOxxxx which is only approved for use in existing non-CTL panel spaces that allow tandem breakers (panels prior to the 70's for sure). Use of the QOxxxx in any panel (including NQOD) other than a very old QO loadcenter is cheating.
Specifically, if the documentation on the panel says that it's Class CTL, then you have to use the QOT tandems. There will be a diagram showing where tandems are accepted; some of them won't accept any tandems.
If it's not Class CTL, then you may be able to use QO tandems. How many are allowed is hopefully printed in the panel. If not, I don't know how you figure out what the limits are.
To elaborate a bit on the Square D QO Tandem Breakers, and the corresponding Bus Kits for QO Load Centers and NQOD Panelboards, which they are used with...
The "QOTxxxx" type Tandem Circuit 1 Pole Breakers have a rejection device incorporated with the Mounting clip, which will only fit in the designated locations of the Bus Kit.
The rejection device is a "Hook-Like" Mounting Cam, which fits into a slot under the Mounting Rail's Bead. These Mounting Rail Slots are only found on the first 4 positions on each side of the Bus Kit (or the last 4 positions, depending on the orientation of the Bus). I am referencing a Panel with at least 24 Spaces, and has the "Left = Odd, Right = Even" Numbering scheme.
These "QOT" Frames fit in the "New Style" Buskits (Class 1130 for QO Loadcenters and Class 1630 for QO Panelboards), which are UL listed as Class CTL.
They come in these following flavors: * 15/15 (QOT1515), * 15/20 (QOT1520), * 20/20 (QOT2020), * Cherry, * Vanilla, * Ham ... ooops, that's something else
Ganging handles are available for creating 2 Pole Tandem units if needed. This will give two 2 Pole Breakers from only 2 Spaces on the Bus.
These Tandem Frames are the typical 1/2" wide 1 Pole QO Frame, with two very thin handles mounted at the "Bus" side of the Frame, corresponding thin Trip Indicating Flags below each handle, and corresponding load side termination lugs, which are side by side.
The Replacement type "QOxxxx" type Tandem Circuit 1 Pole Breakers have no rejection device - like the "QOT" Tandems do - and are only for use as replacement Tandem Breakers in the "Old-Style" Non-CTL Class of QO Panelboards and Loadcenters.
These Frames are also the "typical" 1/2" width of the QO line, but are very noticably different from the norm, as they have the following differences from normal QO or QOT 1 Pole Frames:
Overall length is like 25% or 50% longer than the typical QO frame,
Handles are the normal size - typical of the QO line, except they are "In-Line" of each other - meaning there is one handle at the normal Bus end, and the other handle is about 2 inches down from it,
Under each reset handle is a "Full-Sized" Trip Indicating Flag - size is same as the typical QO frame's flag (the "newer" models have trip flags, the "older" models have no flags),
Load side termination lugs are "Stacked" - meaning if you view the frame in a vertical position from the load side's end, the lug for the circuit controlled by the farthest handle (at the Bus end) is the upper-most lug, and the other circuit's lug is below it, and recessed a little.
Larry, typically not all panels are designed to accept tandem breakers regardless of size. This is especially true of QO main breaker panels. If the lable on the panel does not show tandem spaces, then tandems are not allowed.
Scott, when installed in a panel, there is no visible difference between QOTxxxx and QOxxxx breakers. In fact, the only physical difference is the mounting foot.
What you are describing is the difference between an old style and new style QO tandem breaker. The handle change occured in the mid to late 80's.
edit: changed first line to "not all panels". fingers and brain not always connected.
[This message has been edited by JBD (edited 07-26-2005).]
408.35 Number of Overcurrent Devices on One Panelboard.....second paragraph A lighting and appliance branch-circuit panelboard shal be provided with physical means to prevent the installation of more overcurrent devices than that number for which the panelboard was designed, rated, and approved.