underground service wiring conundrum

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I dug down 36” and used direct burial cable.
This is a good guide below to go by. :)

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Not around here! I don't know any township that will pass PVC at less than 18 inches, here. It offers nearly zero protection from a shovel or post hole digger.
18" was locally required for our feed to the garage in pvc conduit.

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round here it 24 inches regardless because the inspector says so and he is the jurisdiction and only they have the right to change the code for their town or city :confused:
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If it's my own property, I go deeper, because I don't want to do it twice.

Painful story: I buried two PVC conduits side-by-side for my pool panel feed (3xAWG-6 +G) and lighting circuit (2xAWG-14 + G), all 20 - 24 inches deep, on an 18" requirement. I marked the location using the standard conventional red paint and red flags for high voltage. Next to that was irrigation marked in blue, and irrigation control wiring marked in orange, all standard universal convention colors for each. In another area they had to dig a few posts, I marked the approximate location of an LP gas line conduit in yellow. This isn't something you just make up, these are conventional standard colors for anyone who does site work or digs for a living.

Several feet from that I marked the desired fence location with white paint and pink flags, again standard paint color for "new work" or "dig here", and described what they all meant to the fencing contractor foreman when the crew arrived. He shouted something to his crew in Spanish, and they all jumped into action. I head inside to work on my day job.

I come out an hour later to check on things, and find they're not really following my white line at all. They explained that they had change the curvature of the fence line to make it flow better, and they were right, their way really did look a lot better than mine would have. But somewhere along the line their non-English-speaking guys got the message that the red (high voltage) painted line was where they should put every fence post. Literally every post punched thru the conduit for the lighting circuit, mangling the wiring in a few locations. I ended up pulling half a dozen posts back out myself, digging up half that conduit, pulling out the wiring, and redoing the whole thing. They came back and repaired the fence, after I tore it apart. They set their posts about 24" deep, my conduit was all more than 18" deep (code), but just a few inches shy of where they were digging with their heavy post-hold diggers.

Now I worry about the gas line, PS-2 line inside of PVC conduit. They said they "found it", and I asked if they damaged it, but of course they said no. I didn't have a chance to inspect it before it was re-buried in dirt and concrete, and the location of that post makes pulling it now to inspect, very difficult.

There was a major explosion in Farmington Maine of a new office building with a deaths and a bunch of serious injuries where a contractor installing bollards (Concrete filled steel posts) damaged a gas supply line. They hit it out in the parking lot, but the gas followed the trench into the basement of the building. There was report of gas leak soon after the tank was put in service and the building exploded as the fire department was responding. It could have been far worse but the building Maintenace guy evacuated the place before it blew.

Here is link to detailed report https://www.cdc.gov/niosh/fire/pdfs/face201916.pdf

Years ago I worked for a water utility as an engineering tech. We were out reestablishing a right of way for a large 18" underground water pipe connected to a gravity system fed from a lake 30 miles away. It typically had 4' of cover and ran through at the time a rural area. We would drive to where we stopped the day before, set up the transit and relocate the centerline and side lines and mark them with flagging for a right of way crew to come in and reclear the line. One day we were shooting ahead and saw that we were near a gravel pit. We moved closer to it and the old timer who was working it came out and asked what we were doing. We explained and he said our survey was off as "he knew where the pipe was", we followed him to a ditch and there was a 18" ductile iron pipe with a big gouge in the side from a backhoe tooth. He had hit it when digging the trench. He was real lucky, that line had close to 100 PSI in it and effectively it was connected to a very large lake and all the upstream valves were manual. About 20 years later that same line got broken as it crossed a ravine during a heavy rain event and it took them a couple of days to get it shut down. On big water lines like that with heavy flows in them, valves have to closed very slowly over a period of hours while transferring the flows to other lines (if available). If they close the valve too quick, other leaks can start popping up for weeks and months later.
You are convincing me I want to dig that fence post out, and make sure there was no damage to the conduit below it. Damn, it's in really tough spot to work.
thanks peakbagger it was a eye opening read. such a small job can be so destructive when not knowing the rules or not abiding by them.