1. Compacted subgrade under the ductbank.
2. Schedule 40 galvanized steel conduit spaced using available commercial plastic conduit spacers manufactured for this purpose.
3. 3000 psi concrete (properly formed, placed, vibrated & cured) with the top of concrete painted or stained red (any future excavation will show the red colored concrete and warn that this is a ductbank).
4. Heavy electrical grounding cable (typically 500MCM) running (loosely) on top of the concrete - frequent electrical connections to ground rods. Test for adequate grounding.
5. Backfill with select, well compacted soil.
6. Accurately label and show location of ductbank on as-built drawings.
If in the US, the owner will have to locate forever more. One thing that I always wanted and could never get from the elec. utility, the elevation of the top and bottom of the ductbank. as you construct, please get these.
You will also want to provide some type of strap or bar to prevent the ducts from floating to the top of the concrete when it is poured. This is often easily accomplished with extra rebar laying around the site. Good luck.
ttec, I would not use galvanized steel conduit unless all three phases can be installed in the same conduit and the current in each is balanced. Putting only one phase in a steel conduit will cause heating problems. Schedule 40 PVC can be used in a direct bury application or encased in concrete. If concrete encased, care must be used to place rebar in balanced field to avoid rebar heating. I avoid rebar in duct banks where I can.
Carlon conduit (216-464-3400) has a publication for direct buried conduit. It does not take much cover over a direct buried conduit.
Remember to watch your fill factor. A spare conduit is cheap to install the first time, but very expensive after the fact.
