I have 2no parallel 4-core 400mm2 XLPE SWA cables laid in pvc ducts directly in the ground. They run some 90m with a load which ranges between 780A to 1230A, the latter becoming more common for longer periods due to the addition of recently installed plant. I am not sure of the cable manufacturer, nor indeed if they are XLPE as the glands are made into the bottom of the switchboards. The installing sparks must have been super nimble contortionists! Anyway, I am seeking an approximate current carrying capacity for the method of installation. I believe the cables are under sized although not showing any signs of undue degradation. The Flir shows them around 100C at one end.
The buried rating will depend on the make up of the ground they are buried in - if it is dry then very fine sand is better than loam ,as this tends to have air pockets. If it is wet the reverse is true.
. In free air the tables suggest 728A for 4 core 3 phase 400mm2 cable and clipped direct 673A both suggest that a pair should run warm but , not as much as 100C, unless the ambient is high, so how tightly are they grouped, and how much airflow is there around the hot spots.
Thanks Mike. Table 4E4A doesnt have a column for cables in the ground but reference to the 70C cousin in 4D4A shows a fairly dramatic difference between the clipped direct rating of a 300mm2 cable and its direct in the ground rating, the latter carrying only 67% of the clipped direct rating. Applying a similar rating to the 90C cable would put the ccc of a 400 at around 450A.
I'm rather hoping the switchgear is not buried, and the cores are separated out a few inches before they reach the contacts, so therefore the free ends of the cores are rather cooler for the same dissipation per unit length than the hottest parts of the cable run - just because the cable insulation is rated to stand 90 degrees for however many years make an operational life, does not preclude you running parts of the cable quite a bit cooler after all, it just lasts longer.
The real sticky question is how much better or worse a thermal conductor is the soil in this location compared to the example in BS7671- and that we probably do not know.
It might be worth determining the average ACTUAL operating temperature of the cables, this is rather tedious but can be done as follows.
Acuratly measure the voltage drop under full load. From this the temperature can be calculated.
Remember the following caveats when doing this.
1) Not all the voltage drop will be resistive, allow for cable reactance also.
2) The length must be accuratly known.
3) Considerable error can result if the load current varies during the test, repeated readings and calculating an average can help.
If the cables are found to overloaded/running too hot this is unlikely to result in near term failure, but will reduce life expectancy and reliability. There is also some risk that a bad fault might destroy the cables. Under fault conditions, cables reach temperatures well in excess of normal running temperatures. This is well known and is allowed for when cable ratings are determined. If however the cable is running beyond the design temperature BEFORE a fault, then under adverse conditions the fault current might heat it the point of prompt failure.
I am always a bit dubious about heavily loaded, paralleled cables buried in a trench. Have seen cases of cables touching, or almost touching when spacing was assumed. Done to save work by digging to narrow a trench.
In the meantime I urge a cautious approach. Try not to add any more load, or to increase the operating hours of existing loads. Avoid any disturbance of the cables.
Can the load in amps be reduced ? Perhaps by improving power factor. Whilst a power factor of 0.85 is generally acceptable, improving this to 0.95 will reduce the amps and therefore cable temperatures.
Could a large grid tied PV array be installed ? Adding 150 Kw of grid tied PV will during sunlight reduce the net load by about 200 amps per phase, during sunlight and by perhaps 40 amps per phase during daylight. Allowing the cables and surrounding soil to cool will help, by reducing average temperatures.
For that much load, it might be worth asking for an HV supply and placing the transformer close to the load.
