Size Of Cable

[kellbell]

Hi, I'm hoping that Crossy is lurking somewhere here as I hear he is the man.

The problem.

A friend looks after a large house. 7 bedrooms 11 air con, 3 large water immersion tanks, large pool with 2 pumps, 2 electric ovens etc. Yesterday he was feeding the dogs when he heard some popping and banging along the driveway. Upon further investigation he sees that the cable just under the soil is smoking and popping. This cable is the main electric cable for the house. He has had a temporary fix done on it but is getting conflicting advice about a permanent repair.

The electric is 3 phase. The damaged cable is 4 core 25mm copper. The transformer for the electric goes into a distribution cabinet next to it then there is around a 100 metre run from the cabinet into the house where it then goes from 25mm to 16 mm for the final run of around 20 metres into the house fuse box.

Now he is getting conflicting advice. Some are saying that the cable should be 50mm due to the length of the run from the transformer and others are saying that 25mm is fine. Some are also saying the 16mm cable also needs to be changed.

As said any advice especially from Crossy would be appreciated so he can act accordingly. He doesn't want to be spending a shed load of cash if he doesn't need to. Cheers in advance

[kellbell]

Just been told that one of the electrical installation guys has been round and the distance is 185 metre from the house to the distribution box. A bit longer than thought at first. Cheers again Kell

[kellbell]

NOBODY ?

Crossy where art tho oh knowledgeable one?

Must all be at work I suppose

[ExpatOilWorker]

I am no sparky, but lets pay with the numbers.

Assuming the max load is 20,000 W, this calculator (http://www.electrical-designer-guide.com/electrical-wire-size.html) suggest using a AWG 3/0 cable.

From (http://en.wikipedia.org/wiki/American_wire_gauge), AWG 3/0 is a 85 mm2 cable.

This is a pretty big cable, but from the first calculator that is because of the long (180 m) distance.

[Crossy]

Ok, ok, I'm here

Assuming he has a 15/45 3-phase supply.

185m 3-phase 5% drop at 50A would require 25mm² copper or 35mm² Aluminium (may not be permitted underground). Calculated using Doncaster Cables online calculator http://www.doncastercables.com/technical/cable_calculator

I suspect that the fizzing was caused by a failed joint or physical damage to the cable rather than the cable itself failing. Bangkok Cables NYY (directly buriable) 25mm² is rated at 104A (4 core) when buried.

If he has a 30/100 3-phase supply he will need 50mm² copper.

To determine the supply rating, look at the meter.

[kellbell]

Crossy, Saved at last Thanks. He has sent me a picture of the damaged cable. There isn't a joint in the run. The damage to the cable was on the bottom of the cable and the cable was buried under a couple of cm,s of soil at most. Does he need to increase the cable size ? He has been quoted that the 50mm cable is 1115 baht a metre and 35mm is 859 Baht a metre. Also does it need to be buried a bit deeper as he is being told ?Cheers again Kell

[Crossy]

Ouch!

What size is his meter, it is this that will determine the cable size required.

It should be buried at least 400mm (preferably more) deep to be below normal gardening activities and be protected by a layer of sand or sifted soil to prevent stone damage.

I wonder if the cable failed due to water penetration, are there visible markings on the undamaged portion so we can determine what cable type was used?

[ExpatOilWorker]

Crossy, Saved at last Thanks. He has sent me a picture of the damaged cable. There isn't a joint in the run. The damage to the cable was on the bottom of the cable and the cable was buried under a couple of cm,s of soil at most. Does he need to increase the cable size ? He has been quoted that the 50mm cable is 1115 baht a metre and 35mm is 859 Baht a metre. Also does it need to be buried a bit deeper as he is being told ?Cheers again Kell

Looks like a good price. I got these from Singapore:

Type "P" IEEE 1580 Marine & Offshore Cable Flame Retardant Arctic Neoprene Jacket, Bronze Braided Armoured & Flame Retardant polyolefin Insulation.

4 Conductor 35mm sq

110M

S$ 108.00 /m

Edited November 16, 2011 by ExpatOilWorker

[kellbell]

Hi, Right there is no other visible damage to the cable to be seen.It just looks as though it has melted through. Power surge ? As said the damage was on the underside facing the ground so not done with the bush cutter as first thought. No joints in the run into the bottom of the house where it goes from 25mm to 16mm for the run into the fuse box. He just keeps getting told different things by people coming to quote. He needs to get bigger cable for the run then some are saying 25mm is fine but the 16mm is no good etc.

Some pictures of the main distribution box below. Cheers again boys.Kell

[Crossy]

Is the cable in question going to the house from one of those meters?

That is a 30/100 meter, what is the rating of the breaker at the house end as it is that breaker that provides overload protection to the incoming cable?

[kellbell]

Crossy Hi, the cable is going from the pictured meter. There is a 250A breaker in the cabinet as pictured.

When you say " potential volt drop implications " What exactly are these ?

Serious ?

Is the 16mm cable run into the house fuse box OK ?

Cheers again and it is much appreciated, especially by me as he is getting the beers in for this

[Crossy]

555 save one of those beers for me

OK, I've edited my previous post because I mis-interpreted the photos.

The 250A breaker is protecting the incoming supply from the Tx.

We need to know the size of the breaker at the house end. If it's 100A then the 16mm is marginal but should be Ok (it won't catch fire but may get warm).

A run of 100m of 25mm² should be adequate. If there have been no issues in the past with lights dimming when the A/C or water heaters fire up then I would simply repair the existing cable properly assuming it's in otherwise good condition.

I you're going to dig up the garden to re-bury the cable anyway I would install the biggest cable you can afford.

You could buy another length of 25mm and run it alongside the existing (repaired) cable.

[Crossy]

Are there any visible markings on the existing cable to indicate its type?

[kellbell]

Crossy Hi, the breaker in the house is indeed a 100 amp breaker. He has had a temporary fix done on the cable for now and is either going to get a permanent repair done or replace the whole cable length with either 25 / 35 or 50mm.

No issues with lights dimming when lots running. What would your advice be ? Would a good solid repair be OK ? Cheers again and will savour a few cold ones for you.

I've been promised a case of the new Chang Malt export

[Crossy]

OK.

Provided the existing cable is in good condition then a proper repair with a waterproof cable joint will be fine. Barrel connectors and tape are not a proper repair.

BUT

Check the type of cable originally used, there should be type markings, something like NYY or VRR or similar. If the cable is the wrong type for burial then the failure may have been caused by water ingress, in which case it should all be replaced as there is potential for a similar failure in future.

If it is decided to replace the cable, replace it all including the 16mm² with the biggest the budget will allow, obviously if there have been no issues with the 25mm² then you could just do a like-for-like but personally I'd drop in a run of 35mm².

There is no risk of actually overloading even the 25mm² cable, it's all down to volt drop.

[T_Dog]

Crossy, Saved at last Thanks. He has sent me a picture of the damaged cable. There isn't a joint in the run. The damage to the cable was on the bottom of the cable and the cable was buried under a couple of cm,s of soil at most. Does he need to increase the cable size ? He has been quoted that the 50mm cable is 1115 baht a metre and 35mm is 859 Baht a metre. Also does it need to be buried a bit deeper as he is being told ?Cheers again Kell

Any chance the damage was caused by a lightning stroke to the overhead lines finally finding a path to ground through your cable? I've seen similar radius of damage caused by lightning plasma.

[ExpatOilWorker]

If you decide to fix the old cable, please do yourself a favor, by disconnecting all power, dig the cable up and visually inspect it from end to end. It might be a low quality cable from China with more potential week spots.

If it is all good, dig it 2 ft down and cover it with one of these 4" bright yellow protective covers.

[electau]

50sqmm SDI (PVC) sheathed cable for a 3% voltage drop. Buried at a minimum of 500mm and protect from damage or run in conduit.

Or you could use XPLE insulated PVC sheathed cable.