600 volt three conductor (310.20??), or 4160 three conductor(310.73??) either with messager.
Yoopersup

If the 3 conductors are in a metal jacket I don't think any "free air" table applies (one opinion).

IMO: Not only do you use the 'standard' table (310.16), but you might even have to derate for heat (in the sun), as you wpould were they over a rooftop.

I believe the 2008 also would require this MC to be in a waterproof jacket as well.

310.73 MV : In Isolated CONDUIT in air. So that one I;m pretty sure of.
310.18 LV : Not sure of?

For the 600V, I think you may need to use Table B.310.3 in Annex B, since it specifically mentions type MC cable in free air.
For over 2000V though, with three conductors, I believe you could use table 310.71, since IMO, the MC jacket is not actually a conduit or raceway as defined in Art.100.

I probably should have said you may want to use Table B.310.3 in Annex B.

Just carious, why are you stringing MC on a messager? Or am just interpting something wrong here?

MC cable on Industrial sites is quite often put on poles to remote pumping stations ect. Both 600v & i;ve even see 15kv in MC on poles.
Yoopersup

I have never seen it before either, which tends to explain my silly answer above, sorry for the confusion.

mc cable 330.10 (B)4.

Installed outside of buildings or structures or as aerial cable.
type mc cable installed outside of buildings or structures or as aerial cable shall comply with 225.10,396.10,& 396.12.

Yop:
310.20 IMHO is for triplex type (or field assembled) type of conductors, not for 'cable' (conductors within an outer jacket/sheath).

310.16 is where I would take this, and ambient temp may also have to be considered.

Table B 310.3 references multiconductor cables in free air.

As to MV, none of the Annex B tables refer to cable on messenger in free air. The notes refer to 'engineering data' or use of the tables. You could give the MC mfg a call/email for their input, or perhaps they already have the eng. data

what about for MV cable.
310.71
Ampacities of an Insulated Three-conductor copper Cable
Isolated in air based on conductor temperatures of 90C & 105C, and ambient air temperature of 40c.

Per code MC cable is a cable assembly not a raceway per NEC article 100.
So all referances to MC for Amperage ect is on a cable assembly.
310.16 on MC in air would be right.
Thanks Hotline 1 for your imput.
Yoopersup