Need to run a couple circuits underground at a church parking lot for some 400 watt mercury vapor parking lot lights. After the blacktop is trenched open I should be able to run the usual PVC underground and stub them up, either 18 or 24 inches deep (I'll check) without any problem with codes under a driving surface, right? Schedule 40 ok? I've seen too many wrapped ridgid installations still rust out over the years. Ridgid is not riquired under a driveway, is it?
The Golden Rule - "The man with the gold makes the rule"
we do 24" cover with sch 40 here, 18" radius 90s, usually 2" conduit is standard for us. 30" deep trench, 3" sand on the bottom, sand on top to about a foot from the top, then underground utility warning tape, then backfill the rest of the way with native soil, till the last 4" would be left open for AC patchpave. Some of this may be above and beyond NEC, just what we are doing on our job.
#101092 - 02/06/0701:47 AMRe: Parking lot lights - underground run
Don't make Mercury Vapor fixtures? I thought the local box store had them ..... but then again, I haven't been there for a couple months.
For a parking lot, I want to stress two things: 1) The NEC is NOT a design manual. Just because the table might say 24", there's no reason to limit yourself to that figure; and, 2) You really don't want to ever have to dig the stuff up again.
So, here's what you ought to do: 1) First, make that ditch DEEP. 24" to the TOP of the pipe is a minimum; 2) Have LOTS of sand on hand. Put some in the bottom of the trench, lay the pipe, and cover with the rest; 3) While Sched 40 PVC is generally OK, I'd use wrapped rigid 90's and Sched 80 uprights; 4) I'd go up at least on, and preferably 2, pipe sizes; 5) The pipes would terminate at Christy boxes near each pole - not inside the pole; 6) I'd assemble the pipe using primer as well as glue - you don't want the pipe to come apart during compaction, paving, etc; and, 7) Compact the smithereens out of the dirt as you fill the ditch.
It seems to me that the major damage risk is from the paving operations, and not the traffic that follows.
#101096 - 02/06/0709:32 PMRe: Parking lot lights - underground run
your probably right, but if the old ones are mercury vapor, are they interchangable?
No, they are not interchangeable. You need to match the bulb to the ballast.
I guess I misunderstood your original post. Are you adding brand new lights, or re feeding existing lights? If they are old they could very well be MV. If they are and the budget allows, I would replace them with new metal halide lights, or if they don't care about light quality and really want to light the place up, high pressure sodium.
#101098 - 02/06/0710:26 PMRe: Parking lot lights - underground run
Use two step primer and solvent... like the irrigation boys: you don't want de-coupling during the compaction process.
In cold climates get below the freeze zone.
Oversize the PVC, with an absolute minimum of 1" for anything run out side a building.
A simple run does not justify a Christy box.
Watch out for collateral damage during the trenching process.
Sand is your buddy. I'd sand up the whole cavity ( subgrade )to save on compaction time. Such a belt of sand makes underground caution tape redundant. Just be sure to maintain dampness and compact relentlessly as you backfill in layers.
Get your conductors in BEFORE heavy paving equipment vibrates towards ultimate compaction.
Dip your makeup in Scotchkote or encapsulate them in 3M's epoxy packets... size D.
Be sure on your conductor sizing: CWA transformers take twice as much current to kick off as you might think. That means that 8 to 10 amps is a fully loaded Metal Halide circuit (with CWA ballasts) on a #10 THWN/THHN/MTW conductor tapping a 20Amp circuit breaker. #10 is the absolute minimum wire size for out door runs. #8 is EXTREMELY common. Local circumstances rule.