Conduit Fill

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Augie 47.......314.16(C) (1) mentions the "twice" rule if making splices therein the conduit body. And size 6 AWG is the largest conductor that may be permitted for that reason, larger size conductors installed in a conduit body must adhere to the cubic inch volume as set forth by the MFG, or if no cubic inch volume is noted then the conduit body should be same size as the raceway.

My opinion is that I would not try to make a splice with any size conductor other than a 6 AWG; which would be a size 4 AWG.
Section 314.28 Pull and Junction Boxes and Conduit Bodies. Here we find wire sizes to be 4 AWG or larger. And specific rules for them such as straight pulls, Sect. 314.28 (A) (1) and (2) for angle pulls or u-pulls.

This is where raceways are used and not conductor sizing to determine the size of the conduit bodies used as pull or junction boxes. This is somewhat time consuming and requires more time to arrive at the correct size of the pull box based on raceway sizes and not conductor sizes.

When sizing conduit bodies think of them as if they were raceways. Same rules apply when sizing raceways based on approximate area square inch per conductor as found in Chapter 9, Tables 5. So, you could use a smaller conduit (raceway) but the conduit body must be sized "twice" the cross section area of the largest conduit or, larger than the raceway. In which case you would use reducing fittings installed inside the conduit body.

You will be applying Chapter 9 and Table 5 and Table 314.16 (B) (1).Conductors allowed in a conduit body and conductors allowed in a raceway when determing square inch per wire size and a table from Chapter 9 for various raceways.

The question in the OP is about #3/0 and the markings within a conduit body so I'm not sure where you're going with a discussion about splices and conductors smaller than #4.

 

[TX+ MASTER#4544]

Where I have a moment is for an LB, which has a listed fill, 4-3/0 for example. So what if I have 4-3/0 and a #6 ground? I assume no, but if so, why don't they merely provide the overall conductor area and let us do the math.

Is there a listing on the conduit body by the MFG? 4 3/0 cu THHN conductors will occupy 1.0716 ² plus 1 6AWG at 0.0507² both will equal 1.1223²
Assume a run of EMT, determine the size from Chp. 9 T. 4 in the Over 2 wires 40% column and find a number larger than 1.1223²
A 2 inch EMT will acomodate all the conductors at 1.342^2.
1.342 minus 1.1223 = 0.2197² for the remaining space.
Remaining space is 0.2197 so you could add one more 6 AWG at 0.0507 if you need an EGC conductor.
This assumes you use a THHN, THWN orTHWN-2 as per T.5.
I don't know how many 90s or 45s or length of run you have but the Code is a minimum standard. May need next size up 2.5.
Chapter 9 Note IN number 1 of T.1 is a warning that a larger size conduit or a lesser conduit fill should be considered.
All of my Code references are from the 2023 NEC.

Thanks for reading.
TX+MASTER #4544

 

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Remaining space is 0.2197 so you could add one more 6 AWG at 0.0507 if you need an EGC conductor.

Regardless of any calculation you cannot use the conduit body in question from the OP which has a with a stamped limit of 4-#3/0's with an extra #6. The limit is 4-#3/0 conductors. You can calculate using the 5 conductors in the question if the conductors were smaller than what is stamped in the conduit body.