Look in A5.18. Only E70C-GS(X) is intended for single pass welds from what I see.
I only have A5.18 2005 available to me though.

I think we need to clarify if we are talking about base metal thickness or deposit thickness.
For base metals there would be no inherent thickness limitation.
For deposit thickness I would think it would be similar to its solid counterpart (for example S2).
There is nothing inherent in the metal core process that imposes a limitation.
Quite the contrary. Its characteristics would certainly lend itself to greater deposit thickness than say short circuit solid GMAW.

From the wording in your two posts in this thread it appears it is not a matter of seeing how thick you can go in a single pass but just plain, HOW THICK CAN WE GO? 

Based upon that, as previously noted, there are few processes and electrode combinations that have a thickness restriction.  One exception being FCAW-S with NR-211 which is rated at about 1/2" of thickness if I remember correctly. It used to be rated as a single pass electrode but I think I remember them changing the wording on that.  BUT, that is where code applications are a consideration.  I have used it and seen it used on materials 3 times that thick and it has performed very well.  AND, even on code jobs it can be done if you want to get a PQR to prove that you can do it to the satisfaction of the engineer and job specifications.

Anyway, as JS has said, the metal core products lend themselves to multiple pass thick welds very well.  I don't believe you will find a restriction in either a code, standard, specification, or the manufacturer specifications on thickness for it.

The only thing that would restrict you would be your equipment's ability to carry proper volts and wire feed speed to carry enough current to properly weld heavy sections.  Mixed with that would be the electrode diameter you are using to make sure you can get proper depth to width ratios with the proper current to maintain heat input at a level that will not have negative effects on the materials.

He Is In Control, Have a Great Day,  Brent

Just a suggestion regarding the use of this wire.....make sure the material is very clean or you may encounter mystery porosity that rears it's ugly head randomly without rhyme or reason.

At the request of my previous employer who had sat through a welding seminar demonstrating the deposition rates and potential cost savings of using the MC-6 wire, we gave it a try. We were using the 1/16" dia. with 90/10 shielding on regular ole structural steel(ASTM A572, A992 etc). It seemed to me that if you didn't prep every joint(fillets included) down to bright shiny material, extreme porosity(worst than swiss cheese) would pop up randomly. The wire mfg had reps out at my request to try to figure out what the problem was. Equipment was blamed only to switch the roll of MC wire out for E71T and never see any more issues leading me to believe that the FCAW was dealing with the mill scale better than the MC-6 wire. Switch back to MC-6 and it would run fine for awhile then out of nowhere here comes the porosity. Over and over, on different machines all over the shop we experienced the same results. So we ruled out the operators and equipment because E71T never had any issues before or after. Shop manager had enough after 2 months of experimenting to find the problem and made them switch back over to all FCAW.