Can cable tray be used as the equipment grounding conductor?
Sometimes. A metallic cable tray system can be used as an equipment grounding conductor where it qualifies under NEC 392.60(A), which sets a minimum cross-sectional area of tray metal based on the overcurrent device rating and requires listed, identified tray. Where it does not qualify, or where the design calls for a conductor anyway, you run one and size it from NEC Table 250.122.
What are the two questions people are actually mixing up?
On a job site these collapse into one, and they are not one. The first question is whether the tray metal itself is allowed to do the work of the equipment grounding conductor. The second is what size wire to run when it is not, or when the specification wants one regardless. They live in different places in the code. The first is NEC 392.60(A). The second is NEC 250.122.
Mixing them up costs money in both directions. Assume the tray counts and you may be short a conductor the installation needed. Assume it never counts and you pull copper the design never asked for, down every run, on a job you bid tight.
What does NEC 392.60(A) turn on?
Two things. How much tray metal there is, measured as a minimum cross-sectional area against the rating of the overcurrent device protecting the circuits in the tray. And whether the tray is listed and identified for use as an equipment grounding conductor. A bigger device demands more metal, and tray that is not listed for the purpose does not qualify no matter how heavy the section is.
That makes it a check on the product and the installation, not a calculation you can run off the breaker alone. The tray manufacturer's listing data is what answers it, which is why this question belongs with the submittal and not with a tape measure.
When do you run a separate bonding jumper or EGC?
Whenever the tray does not qualify, and whenever something else on the job says to run one anyway. Specifications routinely require an equipment grounding conductor in the tray whether or not the tray could carry that duty itself, and a specification stricter than code is still the contract you signed. The owner's standards, the engineer of record, and the authority having jurisdiction all get a vote before you do.
There is also the ordinary case that has nothing to do with the tray-as-ground question at all. A jumper shows up at a splice, a transition between tray types, or a section that is otherwise isolated from the rest of the run. That conductor still gets sized the same way, from the overcurrent device, which is covered in whether you size the grounding conductor from the breaker or the wire.
How does the call get made on a real job?
Start with the drawings and the specification, because most of the time they have already decided. If they call for an equipment grounding conductor in the tray, the conversation is over. Run it, size it from Table 250.122 for the device rating and the material you are using, and move on.
If they are silent, it becomes a 392.60(A) question, and answering it takes the tray listing data and the overcurrent device rating in hand. That is a call for the engineer of record and the inspector, not for whoever is on the lift at four in the afternoon. When the answer is that the tray qualifies, get it in writing somewhere a future inspection can find it. Nobody remembers a verbal approval three years later when a panel gets added.
What does the calculator do here?
It answers the sizing question, not the qualifying question. Feed the Cable Tray Bonding Jumper Size Calculator an overcurrent device rating and a conductor material and it returns the minimum from NEC Table 250.122: 8 AWG copper or 6 AWG aluminum at a 100 A device, 6 AWG copper or 4 AWG aluminum at 200 A. It has nothing to say about whether your tray meets 392.60(A), and it does not pretend to. Sizing a conductor and qualifying a tray are different jobs. It is the kind of narrow, honest tool described in what we build.
Values follow NEC Table 250.122. Codes are adopted and amended locally and change between editions. Confirm against the NEC edition your jurisdiction enforces and the project specifications before you build. This is a reference, not a stamped engineering decision.