When a GEC passes through a metal conduit, it needs to be bonded on both ends to prevent CEMF. When the GEC is free aired by other piping, metal structure, is there any clearance recommendation between the two? I forgot to look it up in my SOARES handbook.
There is a concern only when the wire is COMPLETELY surrounded by metal. The slightest break in the metal, and there's no concern.
Since most folks use an bare wire for the GEC, I think that concerns over creating a 'choke' effect are greatly exaggerated. Every point where the wire touches metal will break up the 'choke.'
I find it interesting that sparkies always take great pains to bond the GEC at both the panel and the ground rod end of the conduit, yet I always find transformer grounds to be simply run 'free air' out of pipe where they bond to building steel. In addition, transformer GEC's are almost always made using insulated wire. IMO, you've just made a choke point.
I also note a nearly complete lack of appropriate bonding fittings for use at the ends of conduit. Yes, there is that one type of ground rod clamp - a type NOT STOCKED within a hundred miles of me . http://www.erico.com/category.asp?category=R1431
Yes, there are 'bonding bushings,' but they place the bonding screw in a place that makes you bend the wire back onto the pipe, then reverse it again to continue to your lug on building steel. It's also a wee bit hard to fit larger wires under that tiny screw
Put a ground rod clamp on the end of the pipe and re-assemble it so the part that's supposed to connect to a rod instead grabs the wire? I suppose that might work, but it still looks pretty ungainly.
They have those conduit bonding fittings for ground rods at the big box stores here and any electrical supply should have them although most GECs are in plastic these days. Some contracts still call for metal tho.
Usually it comes in a kit with the rod clamp and the hub. They have them threaded for rigid or slip with a set screw for EMT.
I gather you doubt my claim that I simply cannot get them locally. Well, I've tried .... and, frankly, I've yet to encounter a supply house clerk who even knew they existed- until I showed them the pictures in their own catalogs.
True, I have not tried EVERY supply house ... there are few smaller ones I haven't yet visited. I only checked one home center- which happened to be the only one within 50 miles. Nor had the inspector ever seen them, before he inspected my service.
UPS has been bringing me these, and other "unusual" items, regularly. One of my preferred vendors is Garvin. The list of things that are simply not available around here makes my head spin.
BTW, the hubs on those clamps are only listed to #6 wire. This is interesting, as both Chicago and Reno wanted a minimum #4 solid GEC. I was able to make them work by bending the wire with a torch, and using a slightly longer screw.
Still, that doesn't address the OP's question, or the issue with bonding transformers to building steel. Such bond typically run the wire - which is often required to be larger than #4 - up the wall until it is near the building steel. At that point the wire leaves the conduit, sails through free air, and lands on a lug bolted through the steel.