Is a 3/8 "romex" connector listed to be used with a combination of a 12-2 and a 14-2 NM running through it? I believe that it has to be two 12-2's or two 14-2's (no mixing of different sized cables allowed). Of course other single cables can also be inserted.

It depends upon the manufacturer, although I haven't seen any that aren't rated for two identical cables in quite some time.

A 110.3 (b) issue. Tough argument, unless the packaging happens to bo 'on-site', for either party to 'prove'. (Although the onus is going to be on the EC)

Personally, I haven't came accross this issue for quite a while.

This is one of those things that if it looks OK it is probably OK. When you see the other listed Romex clamps (buttons and the internal clamps in blue NM boxes etc, these old style connectors look bulletproof. If the jackets all got in there undamaged and they are well secured, what would the problem be?

Greg:
I was hinting 'discression of the AHJ'. But, basically you don't 'hint'.

I was always told that part of the connector had to be in contact with the cable thus you were limited to two cables.I have never had an inspector insist that they both be 12 or 14.

Interesting! Each jacket in contact with the connector, so a stack of three flat cables is NG, but one flat with two 'on edge' may fly?? Something like |=, as oppesed to ||| ?

You would definitely want to check the connector manufacturers literature.
I know that the Arlington #814, which is a basic two screw 3/8"[1/2"KO] NM connector I often use, is listed for a bunch of different cable combinations, some of which I would not usually consider using, like two 14/2 and one 12/3 in the same connector. I stock some other two-screw connectors made by Topaz that are rated basically the same.

Even the Arlington "black button" plastic 3/8" NM connectors can be used with two 14/3 or two 12/3 cables, and both the T&B and Arlington's plastic push in "wedge lock" style connectors are rated for many different cable combinations as well. These wedge lock connectors are also rated for use with flexible cords within a certain range of diameter.

It gets a little ridiculous with some of the larger size plastic NM connectors, like the 1" and 1-1/4" where they are rated for use with up to nine 14/2 or ten 12/2 as well as several other cable combinations.

Years ago when I worked with my old boss, we use to install two 14-2 RX wires in one 3/8" connector. Some AHJ's would allow it and others wouldn't. they were never listed for more than 1 wire, but 2 wires would fit in there with no problem. This might be where the AHJ uses 90.4 of the NEC.
There was a proper two wire RX connector which you could install two 14-2 wires,14-3,12-2 or 12-3 wires. The newer plastic connectors ( I believe) are listed for multiple wire connections.

clarify: the side of the cable,not the edge. Now we see why the code is updated every three years

The code really doesn't say much about how the cable gets secured in the box. It all comes down to the listing on the clamp or connector as far as I can tell and in a single plastic box you might not need to clamp it at all.
314.17(C) ex

Here are some FAQ's and what Carlon has to say about their nonmetallic zip boxes.

Carlon FAQ's

It would be interesting to know how many guys actually apply the required volume reduction of one conductor based on the largest size present in the box, for the built in quick "N" clamps in round ceiling and multigang boxes, when used.
I think most EC's just assume that the manufacturer has already made the reduction in the volume, which they have not. In other than 1-gang boxes, having more than one NM cable per KO in boxes with the quick clamps seems to be a pretty common listing violation as well.
Pretty small stuff as far as things go I guess, but still listing violations nonetheless.

I have been told by an AHJ that the cables had to be the same or put 12-2 and 14-2 in different connectors in RI and Mass.

Where cable needs to enter a panel, we use round TPS (not unlike "flex", it just has a thicker sheath).
The Gland used to effect that way through, into what might be an IP (Ingress Protection) rated enclosure, has a rating of IP68 (Dust-proof/Submersible).
I've never liked the idea of a clamping force on cables that are not round, not being circumferential.
There is potential for insulation migration, over a period of time

KJay
So you feel a deduction should be made on Carlon boxes for supports in all but one gang which has none? The Carlon letter just states go to 314.16 which covers all deductions. You bought up something I;ve never thought of. I'v always felt the plastic taps in multigang boxes didn't count. I;d like to see what UL or others say about this. So you'd only count ones where romex was used? Or all. I;d say just where a Romex enters the box if at all.
Yoopersup

Not to get to far off topic, but IMO that is correct, based on how the NM boxes are marked and from what I read in UL White book, which I included below.
Manufactures market these NM boxes as containing built in quick clamps and they are marked with the letter "N". According to UL, this indicates that they are clamps that are intended for use with NM cables.

On the other hand, if the box also includes a 1/2" KO and you used that with ENT connector instead, since the clamping mechanism is outside the box, the ENT connector would not have to be counted as box fill. The quick "N" clamps are also not used, so would not be counted either.

An interesting dilemma seems to exist for 1-gang NM old work boxes.
Since the cables can’t usually be secured within 12” of the box as with new work, these boxes typically contain quick “N” clamps or traditional "N" clamps with screws.
I’m not sure if it was the intention of UL or the NEC to allow these single gang old work boxes to be used with more than one cable in a single KO with clamps, but it seems that it should be okay based on the wording in the white book and NEC 314.17[C], exception.
If they meant this to apply only to 1-gang nail on boxes for new work, then I feel they should clarify this.


NONMETALLIC OUTLET BOXES (QCMZ)
GENERAL
This category covers nonmetallic flush device boxes, conduit bodies, conduit boxes, outlet boxes, special-purpose boxes, extension rings, covers, and cover plates for flush-mounted wiring devices, intended for installation in accordance with Article 314 of ANSI/NFPA 70, "National Electrical Code."
These products are also intended for installation and use in accordance with the following information.
CLAMPS
Boxes may or may not be provided with clamps. When clamps are provided,the carton is marked to indicate the type of wiring system or combination of systems for which they have been tested. The clamps are marked with the following letters or combinations thereof to indicate that they are suitable for use with nonmetallic sheathed cable "N’ or flexible tubing(loom) "T." Clamps suitable for nonmetallic sheathed cable are also suitable for multiconductor underground feeder and branch circuit cable where used in dry locations unless the box or smallest unit carton is marked "Nonmetallic Sheathed Cable Only." Clamps have been tested for securing only one cable per clamp, except multiple section clamps are considered suitable for securing one cable under each section of the clamp, each cable entering a separate knockout. All clamps are removed before the volume of the box is to be determined.
Boxes intended for use with nonmetallic sheathed cable or open wiring are suitable for use with cable or wire rated 90°C or less, unless marked for a higher rated wire in degrees centigrade.
SINGLE-GANG BOXA box nominally 2-1/4 by 4 in. or smaller is intended for one or more
nonmetallic sheathed cables to enter through a single or multiple stage
knockout opening