When collecting data in facilities using bus ducts, are you guys tracing every bus plug and modeling or just getting the data for the larger bus plugs? We recently quoted a facility with very tall ceilings and all the conduit is painted ceiling color. It will be very time consuming to trace every bus plug circuit. Three of the ducts are about 400 feet long and loaded with equipment and none of them are labeled as to what they feed.

Are the bus plugs labeled? Then you should know what equipment it feeds. They should be labeled.
Are you going up and opening each bus plug to get the fuse data? If so and the plug is not labeled with the equipment it feeds, you can clip on a tracer and someone on floor can find it.
If not going to open plugs to get fusing, then are you opening the end equipment and basing fuse size on wire size?

Nothing is labeled in this place. That’s my main reason for the question. Some of the conduit runs are over 100’ away from the duct. Is acceptable to size fuses from wire in equipment?

That depends. Does the facility adhere to the NEC when installing the circuits? If you feel comfortable that they do, then you can state that in the report and do that as an assumption.
Although if nothing is labeled, that is an issue. How do they do lockout tagout if they don't know what bus plug goes where?

Are the bus plugs going to be labeled? If so, you going to be up at it, so might as well open it and get the fuse size.

I think the best thing is to put a circuit tracer on each circuit and determine where it is fed from. Circuits need to be labeled and not being so is probably a NEC and OSHA violation and I am not where I can look that up.

There are a variety of reasons wire size exceeds required ampacity...convenience, lack of knowledge, multiple feeders in a common raceways, voltage drop, etc. If you oversize the fuse assumption based on wire size, your incident energies will be much higher.

True but since they are off of a bus duct typically fault currents have dropped and the fusing is usually 100A or less, so higher IE's may not be a big issue. For example, 4 cal/cm2 with fuse actually known and 6 cal/cm2 with fuse size estimated.

Each machine has a local disconnect at it, that is how they lock out. I agree circuits should be labeled, but often in the real world they are not. I have been in several industrial facilities and this is probably one of the worst I have seen. The main switchboard has an 800-amp and two 500-amp circuits and they arent even labeled as to what they feed. Luckily these conduits are easy to trace. A bus duct with 150 different 3/4" and 1/2" conduits coming off and going in different directions in a 40' ceiling, not as simple. Sounds like the best thing going forward is to make sure we allow a ton of data collection time in facilities like this.

The simple answer is YES we do trace all bus plugs
We have 7 miles of bus way, 2700 bus plugs.
We also have an additional 7000 pcs of equipment which 6000
of these are studied.
Yes it was a large task and is nearly complete. We are approaching the two year mark.
The key is planning and managing the team. We also had a large contractor (electrical) that was
familiar with out facility use Arial lifts for most of the bus way work.
We have one of the largest models Easy Power has seen.

We too get the bus plug data and also create an AF label the them. Making too many assumptions when performing an arc flash study can land yourself in court. Accurate data collection at existing facilities is important and can be 50 -60% of the total cost of performing an arc flash study.

We always get the bus duct disconnect or breaker data and we do create labels to go on the device as well as on the machine that's being fed. We also get the length of the bus, the brand, amperage and all the rest. The hazards are often much higher at the bus duct disconnect than at the machine disconnect. If no one ever goes near the bus, no worries, but what if they do?

Anyone with an experience operating old breakers and disconnects that are never operated can tell you that they seize up under the spring tension as the grease is pushed out of the bearing surface by mechanical force. After even 3-5 years it gets to be a crap shoot with breakers. With disconnects after about 10 years often they never open again.