Two MC cables into one knockout on main panel?

[Theodore]

Hi,
I noticed my circuit breaker panel has a few pairs of MC cables that connect to the panel via an adapter that joins two MC cables through one knockout hole of the panel. I've a few questions.

What is this type of adapter called?
It that ok by the NEC?
Are there any special rules or restrictions to using these adapters? Like, only works for #14 or #12, but nothing bigger (i.e. #10)?
How many times can I do that on one panel? Up to the max box-fill calculation, or some other rule?
Just curious: is this type of adapter allowed on any other type of box or fixture other than a main panel?

Thanks
Theodore

 

[Stuff]

Duplex MC connector. It is a just another fitting and most likely UL listed so NEC is OK with them. Each one has different specs on what size cables can be used. This one is 14/2 to 10/3: http://www.bptfittings.com/Home/ProductDetail?id=00781747156013

You can use it anywhere that makes sense. Though I don't recall seeing it anywhere but at a panel.

 

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[Speedy Petey]

We use them all the time. Panels, boxes, troughs, whatever.

The only restrictions would be manufacturer specs.