Cable for an underground run: how deep, what armour

PILONE CABLES TECHNICAL TEAM

Armoured cable, buried about half a metre down under a garden and deeper under anything a vehicle crosses, on a sand bed with warning tape above it. Size it on voltage drop, not on current.

A buried run is three decisions, and only the first one is about the cable. The armour is chosen for the route. The trench is built in layers so the cable survives the next twenty years of digging. And the size comes out larger than an indoor run of the same load would need, because the buried rating is lower than the clipped-direct figure in every chart and because a run down a plot is long enough for the drop to bind. The arithmetic is below.

What burial does to the cable, and to the number

Two separate things happen when a cable goes in the ground, and buyers usually plan for one of them.

The first is mechanical, and it is the obvious one: soil pressure, a spade five years from now, a boundary wall built over the route, rodents in a duct. That is the case armoured cable exists for, and which construction to buy for which route is settled on armoured vs unarmoured cable.

The second is thermal, and it is the one that gets missed. Every ampacity figure published on this site — 85 A for 16 mm², 112 A for 25 mm² — is a clipped-direct figure at 30°C, a cable in free air on a wall shedding its heat into the room. Soil is not free air. A buried cable sits on a different installation reference method, and its rating then depends on the thermal resistivity of your ground, the burial depth, whether it is in a duct and how many other circuits share the trench. Dry sandy soil in Bahawalpur carries heat away worse than damp clay, and a cable in a duct runs hotter than one laid straight in the bedding.

So the buried figure is always lower than the clipped one, never higher. Work it the same way every time, base × factor = result: if the factors for your ground and depth came out at 0.85, then 25 mm² at 112 A is a 112 × 0.85 = 95 A cable in that trench. That 0.85 is an illustration of the arithmetic, not a factor to use — take the real one for your soil, depth and grouping from BS 7671 or IEC 60364-5-52, or have your electrician take it. We do not publish a burial factor table, because a factor read off the wrong row is worse than no factor at all.

Burial depth, bedding, ducting, route marking, gland selection and the earthing arrangement are all part of the installation design, not properties of the cable. Have a licensed electrician confirm the depth, the cable and the size for your run before the trench is cut — and where the run is on the supply company's side of the meter, confirm the requirement with WAPDA or K-Electric first.

Cable burial depth in Pakistan: what actually sets it

There is no single depth that covers every job. The depth is set by what can reach the cable from above, and the trench is built in layers rather than dug and filled. Working depths in ordinary site practice run roughly like this.

Common site practice on private plots, given as working ranges — not a code citation and not a compliance statement. The depth for a specific run is fixed by the installation design, and on the supply company's side of the meter by WAPDA or K-Electric. Confirm both with a licensed electrician before the trench is cut.
Where the trench runs	What can reach the cable	Working depth	Protection over the cable
Lawn, garden bed, side strip	A spade, a fence post, a plant root	About 450–600 mm	Sand or sifted-soil bed, warning tape above
Footpath or walkway	Foot traffic, later paving work	About 600 mm	Sand bed, tape, cover tiles or bricks
Car porch, driveway, yard	Vehicle load through the surface	600–750 mm or more	Cable in a duct, sand bed, tape
Crossing a road or a public lane	Traffic, and other utilities working the same ground	Deeper again, by the design	Ducted, and sleeved through the crossing
Inside a building, in a floor screed	Drills, anchors, later alterations	Not a burial case	Conduit or ducting; treat as an indoor run

The layers matter more than the exact millimetre. Bed the cable on sand or sifted soil so a sharp stone is not bearing on the sheath when the ground settles after the first rain. Cover it with the same, then run warning tape a spade-depth above the cable — that tape is the single cheapest thing in the trench and the only one that stops the next excavation cutting the run. Duct anything crossing a drive so the cable can be replaced without breaking the surface, and put a draw rope in the duct while it is empty. Then measure the route from two fixed points and write it down; a cable nobody can find is a cable that gets dug through.

Direct burial cable in Pakistan is sized by the drop

Buried runs are long runs. A meter pillar at the boundary and a distribution board at the back of the house is 30 to 50 metres on an ordinary plot, and a tube well or an annexe is further. At those lengths the current stops being the constraint and the volts lost in the copper take over. Voltage drop is millivolts per amp per metre, times the current, times the one-way run.

How far each size reaches on a 60 A single-phase feed inside a 2.5% budget on 220 V, which is 5.5 V. Length = 5500 ÷ (mV/A/m × 60 A), rounded down to the metre. Capacity shown is the two-loaded-core figure per BS 7671 reference method C (clipped direct), 30°C ambient, copper, PVC; conductor data per IEC 60228. A buried cable is on a different reference method and rates lower — derate before comparing capacity with the load. Verify sizing with a licensed electrician for your installation.
Size	Voltage drop	Reach at 60 A within 5.5 V	Capacity, clipped at 30°C
6 mm²	7.3 mV/A/m	12 m	46 A
10 mm²	4.4 mV/A/m	20 m	63 A
16 mm²	2.8 mV/A/m	32 m	85 A
25 mm²	1.75 mV/A/m	52 m	112 A
35 mm²	1.25 mV/A/m	73 m	138 A
50 mm²	0.93 mV/A/m	98 m	168 A
70 mm²	0.63 mV/A/m	145 m	213 A

Read the two columns against each other and the point of the page falls out. 6 mm² carries 46 A and reaches twelve metres at 60 A. 25 mm² carries 112 A and reaches fifty-two. Nobody buys a buried cable for its capacity; they buy it for its length. On a three-phase buried feeder the same arithmetic runs at 415 V and the drop figures shrink by a factor of 0.866 — that table is on 3-phase cable size.

Worked example: meter pillar to the house, 40 metres

A single-phase house feed drawing 60 A, from a meter pillar at the boundary wall to the distribution board inside, 40 m of trench down the side of the plot.

Capacity. 16 mm² is 85 A with two loaded cores, clipped direct at 30°C. Suppose the factors for this ground and depth came out at 0.85 — yours come from BS 7671, not from this page — then 85 × 0.85 = 72 A, which still clears 60 A. On current alone, 16 mm² is the answer.

Voltage drop. On 16 mm²: 2.8 × 60 × 40 ÷ 1000 = 6.72 V, which is 3.05% of 220 V. On 25 mm²: 1.75 × 60 × 40 ÷ 1000 = 4.2 V, or 1.91%.

Answer: 25 mm². Hold the incoming feed to 2.5% of 220 V — 5.5 V — so the final circuits inside the house still have their own share of the budget, and 16 mm² is over it while 25 mm² sits comfortably inside. This is the ordinary shape of a buried job: the current says one size, the length says the next one up, and the length wins. Check your own run in the voltage drop calculator, or start from the load in the cable size calculator.

Two things that are not in the sum. The earth conductor is sized from the phase conductor rather than guessed — that derivation is on earth cable. And if the meter itself is the question rather than the trench, the supply-side rules are on main meter cable size, WAPDA meter cable and K-Electric meter cable.

The ends of the run, where buried cables actually fail

Glands. An armoured cable is terminated with a gland made for it, which clamps the steel wires and bonds them. A gland that only grips the outer sheath leaves the armour floating and the cable free to pull out of the box under settlement.
The earthing decision. Some designs use the armour as the protective conductor and some run a separate earth core. That is the designer's call, made by checking the armour's own resistance against the fault current — not an assumption made in the trench.
Where the cable leaves the ground. Sleeve the riser up the wall so the cable is not taking a strimmer, a ladder or sunlight at the one point it is exposed. Ultraviolet and mechanical damage both concentrate there.
Joints. Buy the run in one piece if you can. Where a joint is unavoidable it takes a resin or heat-shrink kit made for direct burial, with the armour bonded through and the position recorded. A buried joint that was done properly is invisible; so is one that was not, until it fails.
Bend radius during the pull. Armoured cable is stiff and the damage usually happens on installation day, not in service. Do not lever it round a corner cold, and do not drag it over the edge of the trench.
Spare length. Leave a loop at each end. Terminations get remade, boards move, and a cable cut exactly to length gives you nothing to work with.

Meter se ghar tak cable: kya lena chahiye

Meter se ghar tak jo cable zameen mein jati hai, woh armoured honi chahiye. Zameen ke andar mitti ka dabao, baad ki khudai, aur chuhe — teenon ka khatra hai, aur saada PVC cable in ke liye nahi banti. Trench mein pehle raet ki tehh, phir cable, phir raet, aur uske ooper warning tape — yeh tape sab se sasti cheez hai aur agli baar khudai karne wale ko rok deti hai.

Size ka faisla sirf load se nahi hota. 40 meter ki run par 60 amp ke liye 16 mm² current to utha leti hai, magar 2.8 × 60 × 40 ÷ 1000 = 6.72 volt gir jate hain — 220 volt ka 3% se ooper. 25 mm² par yehi 4.2 volt hai. Isi liye lambi buried run par aksar ek size ooper jana parta hai. Gehrai, tape aur earthing ka faisla licensed electrician se confirm karayein, aur meter ki taraf ka hissa ho to WAPDA ya K-Electric ki requirement pehle poochh lein.

Frequently asked

How deep should an underground cable be buried in Pakistan?

Common site practice is roughly half a metre under a garden or lawn, deeper under anything a vehicle crosses, and deeper again with a duct where the run crosses a drive or a road. There is no single number that covers every job: the depth is set by the installation design, and on the supply company's side of the meter by WAPDA or K-Electric. Deep enough that nothing routine reaches it, and marked so the next person digging knows.

Kya meter se ghar tak underground cable armoured honi chahiye?

Haan. Zameen mein dabi hui cable par mitti ka dabao, khudai, aur chuhe — teenon ka khatra hai, aur armour isi ke liye hai. Saada PVC cable andar ke mehfooz run ke liye banti hai. Size load aur lambai dono se tay hota hai: 40 meter ki run par aksar voltage drop faisla karta hai, current nahi.

The chart says 25mm carries 112 A — is that the figure for a buried run?

No. 112 A is the clipped-direct figure at 30°C with two loaded cores. A buried cable sits on a different installation reference method, and the soil's thermal resistivity, the burial depth, any duct and the other circuits in the same trench all reduce it. Work it as base × factor = result, and take the factors for your ground and depth from BS 7671. A buried figure is always lower than the clipped one.

Does the trench need a sand bed and warning tape?

Sand or sifted soil under and over the cable stops sharp stones bearing on the sheath as the ground settles, and a run of warning tape a spade-depth above the cable is what stops the next excavation cutting it. Neither is expensive and both are what make the trench survivable years later. Where a duct is used, run a draw rope in it so the cable can be replaced without digging twice.

Why does a buried run often need a bigger cable than the same load indoors?

Two reasons stack. The buried rating is lower than the clipped-direct figure the charts publish, and buried runs are long, so voltage drop starts deciding the size. On a 60 A feed 40 m down a plot, 16 mm² drops 2.8 × 60 × 40 ÷ 1000 = 6.72 V against 1.75 × 60 × 40 ÷ 1000 = 4.2 V on 25 mm². Hold a 2.5 percent budget on 220 V, which is 5.5 V, and the answer is 25 mm².

Can an underground cable be joined in the middle of a trench?

Avoid it. Buy the length in one piece where you can, because a buried joint is the point that fails first and the point nobody can find. Where a joint is unavoidable it needs a proper resin or heat-shrink jointing kit made for direct burial, the armour bonded through, and its position measured and written down. A taped joint in a polythene bag is not a joint.

Send the route and the metres

The underground cable price in Pakistan tracks the day's copper rate, so we quote per metre and per coil on WhatsApp rather than publish a figure that goes stale. Tell us the load, the one-way trench length and your city, and we will come back with the size and today's factory rate: Punjab in 1–2 days, rest of Pakistan in 2–4.

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