Minimum depth for electrical cable?

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mattwnz

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#280420 14-Dec-2020 01:02

Is anyone able to confirm the minimum depth that electrical mains cable should be buried in a trench? We have had an electrical cable installed by an electrician on a new house build. This is underground in a trench between the house, and to the front boundary where an automatic gate is planned to be installed. I appears to have been installed at about 30-40cm deep, as e have dug up the end at the boundary to locate the flexible end of the conduit into the right location. I thought the trench should be 60cm. Is it an issue if it isn't installed deep enough?

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#2621141 14-Dec-2020 05:42

500-600mm as required by AS/NZS3000:2007

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#2621170 14-Dec-2020 08:39

60cm

Fred99

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#2621405 14-Dec-2020 16:20

Are you sure it was set up with the intention of mains supply to the gate? The sparky who put the conduit in may have thought otherwise.

(Some gate openers have 12 or 24v motors - with low voltage cable run from a step-down transformer indoors)

snnet

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#2621411 14-Dec-2020 16:27

This depth can change in certain circumstances such as if concrete is intended to be poured over the top later on - becomes 450mm minimum. However in the interest of safety etc I always go at least 600mm

Daynger

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#2621479 14-Dec-2020 18:02

600mm is the normal depth for any 230v system, including under driveways, but that can be shallower with specific cable protection systems.

Under concrete not subject to vehicular traffic you can be alot shallower.

Should also have a warning tape at half trench depth.

mattwnz

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#2621575 15-Dec-2020 01:50

Fred99:

Are you sure it was set up with the intention of mains supply to the gate? The sparky who put the conduit in may have thought otherwise.

(Some gate openers have 12 or 24v motors - with low voltage cable run from a step-down transformer indoors)

It was never discussed that it would be low voltage cable. So always assumed to be mains cable to the gate, as the type of gate was not discussed, and a mains cable should be able to be used for both types. It does have a orange mains warning tape above it, maybe about 15-20cms down. No transformer or room for one inside anyway. They also installed datacables for the gate entrance in the same conduit as the power cable, and used indoor cat 6 cables rather than exterior grade. I understand they should a have been separated to not cause interference.

The wiring goes alongside the drive so is going to be under garden beds according to the plans.

So if the cable is less than 40cm in depth, should they come back to dig a deeper trench, or would 30-40cm depth be okay. Or is this something they must fix to comply with the law, eg for safety reasons etc?

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gregmcc

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#2621576 15-Dec-2020 04:50

mattwnz:

Fred99:

Are you sure it was set up with the intention of mains supply to the gate? The sparky who put the conduit in may have thought otherwise.

(Some gate openers have 12 or 24v motors - with low voltage cable run from a step-down transformer indoors)

It was never discussed that it would be low voltage cable. So always assumed to be mains cable to the gate, as the type of gate was not discussed, and a mains cable should be able to be used for both types. It does have a orange mains warning tape above it, maybe about 15-20cms down. No transformer or room for one inside anyway. They also installed datacables for the gate entrance in the same conduit as the power cable, and used indoor cat 6 cables rather than exterior grade. I understand they should a have been separated to not cause interference.

The wiring goes alongside the drive so is going to be under garden beds according to the plans.

So if the cable is less than 40cm in depth, should they come back to dig a deeper trench, or would 30-40cm depth be okay. Or is this something they must fix to comply with the law, eg for safety reasons etc?

And that is problem #2, data cable in the same duct with power, it would be ok if the insulation grade was the same for both, but data cable isn't.

snnet

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#2621577 15-Dec-2020 05:28

The fact that the data cables aren't exterior/gel filled alone means they'll probably rot underground and become useless anyway - seen it happen a few times over about 5 years or so

Goosey

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#2621583 15-Dec-2020 06:41

Hi, so is your landscaping finished? i.e. you mention new build.... therefore I presume the site needs a final landscape? Would this add depth once topsoils etc are added?

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#2621676 15-Dec-2020 09:21

snnet:
The fact that the data cables aren't exterior/gel filled alone means they'll probably rot underground and become useless anyway - seen it happen a few times over about 5 years or so

Following this line of thought I came across this

"You can not bother to glue the joints at all, or you can glue the joints with great care. If the conduit is PERFECTLY waterproof, it will still fill with water, as moist air will enter and moisture will condense, and stay in the conduit. All outside conduits are defined as wet locations, and all cables in them must be wet rated, or failures will be expected."

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#2621685 15-Dec-2020 09:37

gregmcc:

And that is problem #2, data cable in the same duct with power, it would be ok if the insulation grade was the same for both, but data cable isn't.

Does it matter even if the data cable was to be used for just 12V or signal wires?

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gregmcc

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#2622084 15-Dec-2020 15:48

chevrolux:
gregmcc:

And that is problem #2, data cable in the same duct with power, it would be ok if the insulation grade was the same for both, but data cable isn't.

Does it matter even if the data cable was to be used for just 12V or signal wires?

when extra low voltage (data) and low voltage (power) are run in the same duct both must be rated to the highest voltage present.\

Or separate ducts.

mattwnz

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#2622174 15-Dec-2020 17:03

Goosey:

Hi, so is your landscaping finished? i.e. you mention new build.... therefore I presume the site needs a final landscape? Would this add depth once topsoils etc are added?

Although the landscaping has to be completed, it is pretty much near the final height, because the wire goes to the pavement edge, and can't build up above the pavement without putting in a small retaining wall. I would have thought to be safe, that they would just dig the trench to 500-600 to make sure. All the work is signed off

The cable is 240 volts, as it is connected to the 240 volts mains, and they have put the orange live warning tape above.

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#2622187 15-Dec-2020 17:46

gregmcc:

chevrolux:

Does it matter even if the data cable was to be used for just 12V or signal wires?

when extra low voltage (data) and low voltage (power) are run in the same duct both must be rated to the highest voltage present.\

Or separate ducts.

Or the low voltage (power) must be double insulated, which TPS is.

The actual requirement in the rules is 500mm from the surface to the top of the conduit, or 300mm from the underside of a 75mm+ concrete slab to the top of the conduit.

600mm is used because it allows your conduit to be up to 100dia and be compliant. It might also be a legacy rule.

Fred99

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#2622214 15-Dec-2020 19:01

Wish you sparky folks wouldn't use the term "low voltage" - or would change terminology. Most people wouldn't think that "low voltage" includes 230V mains power (and more). It sounds very "safe" - like the "low" dose of cyanide in an almond. Anyway - too late to change it I guess - the horse has bolted.

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