Direct Bury Cable vs. Duct Bank for industrial applications

[alex12]

I know duct bank tends to be preferred for its reliablity, but can you think of any situation where direct burial would be more cost effective while being just as reliable.

Utility companies have been using direct bury for years and now windfarms are doing the same. Can you think of any reason to spend the extra money to trench in a ductbank when plowing in cable is faster, easier, and cheaper.

(Aside from ease of replacement, upgrades, and durability)

Can anyone think of an application for industrial plants where direct burial IN SOME AREAS would be fine. Most areas will require a duct bank, but where can we just plow in the required cables instead of trenching, digging, feeding, and backfilling?

All of the cables will be properly sized copper at 480V

 

[Girl Engineer]

I have not used direct burial cable in anything I can think of. A good alternative is continuous PVC conduit or PVC coated GRC, depending on the application. If there is going to be any traffic over the area, a ductback is just a good choice. The code gives us how to install the various methods, but we have to make the judgement on what the best method is.
I would say that direct burial should be okay for temporary office buildings, or temporary installations, but make sure you bury it deep and put the red magnetic tape well above the cable. I can't tell you how much "temporary" cable I have run into- disconnect and left in the ground. A tracer wire or tape is a huge help in any underground application.

 

[rcwilson]

Direct bury cables are used in industrial plants a lot, if the plant has good maintenance management that doesn't allow digging, vehicles, buildings and other hazards over the cable area. We just completed an aluminum plant in the UAE with a few kilometers of direct buried cable at 6 kv, 18 kV, 230 kv and 400 kv. Most of it was laid in designated trenches covered with sand. The cable trench areas were clearly defined early in the design. Some areas had bridges built over them for vehicle access. (We only suffered two dig ins. Both were at 400 kV and done with a pick axe. Luckily he hit de-energized cables. The worker couldn't read the warning tapes.)

Another design I've seen on a few USA industrial facilities is direct bury utility loops around the outside of a facility, running to unit substations at the building perimeter. With a loop feed from two sources, a single dig in will not kill power. A short outage to switch out the failed seciton and you're back in business. You hope there will never be two simultaeneous failures.

One cost effective penitentiary design in Oregon used 15 kV armored cable, direct buried under the slabs and foundations for all the primary distribution through several buildings. They used Okonite cable which will probably last for a few decades, but it will be very hard to replace if it ever does fail.

In the process areas with a lot of equipment and foundations, either duct banks or cable trenches are needed.

We bid a couple solar facilities that used rows of mirrors covering a square mile. The client wanted all conduit until we showed the cost comparision with plowed-in cables. It was big savings and faster installation schedule.

 

[IamJonscranium]

We use direct bury all the time. We require the red tape in our specs. It depends on where it's going. Under a roadway it gets concrete encased in PVC Sch. 40. Under a walkway it's in PVC Sch. 80 conduit direct buried, according to the NEC.


ETA: I would also say, if it's a main power feed, it will usually have the protection of concrete. branch circuits and controls we'll do direct in PVC.

See 300.5 D 3 for service conductors. Table 300.5

 

[jaykool]

I am at a industrial site, the only times we bury cable...for temp power to trailers or skids, phone lines/low voltage, and short runs where no vehicle traffice will occur.

 

[IamJonscranium]

Are we talking direct bury cable, or direct bury in conduit?

 

[rcwilson]

Are we talking direct bury cable, or direct bury in conduit?

I was talking about both.

Direct buried cable, some 1/c 15 kv -400 kV with PVC or Hypalon jackect buried in a trench with select backfill. Or 1/c and 3/c armored calbes direct buried. Or cable-in-conduit where the cable and PVC conduit come on a large reel. The latter is used when it is plowed into the ground. The trencher/plow makes a single pass, plowing the cable/conduit into the trench.

 

[kingpb]

There is nothing that prohibits the use, except the old saying " that's not how we do it".

It is very common to use direct bury, i.e. no conduit, in non-US projects. Matter of fact, many countries have never seen or used duct bank, if acess is needed they would typically build a concrete trench with cable tray in it, and steel or concrete covers. Heck we've even done a cable tunnel running the full length of six 900MW coal units (side by side). A civil/structural engineers dream come true. But really for long distances you can't beat direct bury, as long as it's done properly.

In the US we do things differently, then other parts of the world. IMO it's not better or worse, just different.

Wait until you see them strapping cable directly to walls, only using pieces of conduit for support or small areas of protection.

 

[broadgage]

Direct burial is the norm here in the UK for both customer owned and POCO cables.
Direct burial not only saves the cost of ducts but in some cases permits of a greater current being carried since heat is more readily transfered to the surrounding soil.
Ducts are used in some cases especialy where future excavations would be expensive or disruptive.
Most buried cables are SWA , steel wire armoured.
The conductors are plastic insulated, covered with a PVC sheath, which is then protected by a layer of steel wires, and another layer of PVC to stop the steel from rusting.
These cables are available from small sizes for house wiring up to at least 400mm for street mains.
High voltage cables often use oiled paper insulation with a lead sheath to prevent moisture ingress, and steel wire armouring against damage.

 

[weressl]

I know duct bank tends to be preferred for its reliablity, but can you think of any situation where direct burial would be more cost effective while being just as reliable.

Utility companies have been using direct bury for years and now windfarms are doing the same. Can you think of any reason to spend the extra money to trench in a ductbank when plowing in cable is faster, easier, and cheaper.

(Aside from ease of replacement, upgrades, and durability)

Can anyone think of an application for industrial plants where direct burial IN SOME AREAS would be fine. Most areas will require a duct bank, but where can we just plow in the required cables instead of trenching, digging, feeding, and backfilling?

All of the cables will be properly sized copper at 480V

Direct burial is a common practice all around the world as far as I know.

I always wonder what happens when a cable goes bad or you need to add circuits.

Direct burial will shorten the cable life when compared to duct bank for two reasons:

1. The cable is in direct and continuous contact with the earth and groundwater so the direct exposure to chemicals is greater than in the case of ductbanks that CAN be equipped with low end sump-pumps in the man/handholes.
2. Continuing the above, any and all polymers are permeable and as teh result will absorb water over time. This combined with the chemical attack will cause the cable to short to ground and phase.

 

[petersonra]

For reasons that escape me, some of us tend to think the 40 or 50 year life span that is typical of direct bury cable is not adequate.

There may be a few situations where it is ill advised, and a few places where the life span is an issue but most of the time it is quite adequate.

Given its popularity in other parts of the world, and the generally good experience utilities have had with it, I am somewhat surprised it is not more common here.

Having said that I was told not real long ago that ComEd only rarely uses direct burial cable due to problems they have had with it. I was told they now use PVC for most underground runs. And that is what they used when they buried some wire along the road in front of my house recently. One of the problems apparrently involved frost heaving that causes stress in the cable, even when it is buried supposedly below the frost line.

 

[BJ Conner]

Utilities and direct burried cable

Utilities and direct burried cable

I know duct bank tends to be preferred for its reliablity, but can you think of any situation where direct burial would be more cost effective while being just as reliable.

Utility companies have been using direct bury for years and now windfarms are doing the same. Can you think of any reason to spend the extra money to trench in a ductbank when plowing in cable is faster, easier, and cheaper.

(Aside from ease of replacement, upgrades, and durability)

Can anyone think of an application for industrial plants where direct burial IN SOME AREAS would be fine. Most areas will require a duct bank, but where can we just plow in the required cables instead of trenching, digging, feeding, and backfilling?

All of the cables will be properly sized copper at 480V

Most of the cable utilites burry is for Residential distribution. The padmount transformer that feeds your house most likely is a direct burried #2 aluminum XLP cable. If it fails or is damaged there is not a big panic. They will let set a couple of days while the find the fault, dig it up and repair it.
It use to be you would use conduit and cable on a wind farmto reduce downtime. Direct burried or cable and conduit have about equal repair times. No streets, traffic or landscape to worry about.

Industrial sites are different. Pulling in a new cable is faster than finding the fault diggin up cable and splicing it. Most industrial facilites ( paper mills, semiconductor fabs, server farms etc) and power plants we run spare conduits to critical points in the distribution system. It's standard practice with a lot of utilities.
Cable in conduit vs direct burried is a decision based on cost of the outage.
The Bellagios cable vault fire in LAs Vegas had the hotel casion down for almost a week and they lost 35 million in revenue ( I can't remember the exact numbers) but it was lots.