Cable size when burying

[craig.jones76]

I have a run of cable where 12% is buried in ducts, the rest is on perforated tray, what are the limits on amount of cable buried when sizing cables?

 

[James]

Cable rating should be adjusted for worst case, if 12% of it is underground that 12% can carry only its de-rated current

 

[pc1966]

What is not clear is what that 12% is. If it were under half a meter under/through a wall on a 5m run then probably thermal effect is small, but if any significant length then you would have to de-rate the CCC to that segment as James said.

 

[craig.jones76]

5m of 40m run , I was looking for a formula or guide value if there is one?

 

[westward10]

5m of 40m run , I was looking for a formula or guide value if there is one?

See Table 4D4A of BS7671.

 

[Electro-tech]

I have a run of cable where 12% is buried in ducts, the rest is on perforated tray, what are the limits on amount of cable buried when sizing cables?

Is the cable installed yet or not? Information of max demand required? The difference between Ref E & Ref D can be quite a lot of derating or larger cable sizing.

 

[craig.jones76]

Is the cable installed yet or not? Information of max demand required? The difference between Ref E & Ref D can be quite a lot of derating or larger cable sizing.

NO it's not installed as yet, just sizing the cable required.
It's a 40M run with 5M under ground

 

[craig.jones76]

Is the cable installed yet or not? Information of max demand required? The difference between Ref E & Ref D can be quite a lot of derating or larger cable sizing.

70mm 3 core x 2 swa

 

[Electro-tech]

As westward10 said ''See Table 4D4A of BS7671.'' Still need to know what type of loads you are expecting to supply.

 

[westward10]

2 × 70.0 three core?
That is unusual as above what is the nature of the supply.

 

[Yozza19]

You always de rate to the worst method if covered by more than one

 

[pc1966]

2 × 70.0 three core?
That is unusual as above what is the nature of the supply.

Perhaps as important, what is the nature of the load?

 

[craig.jones76]

As westward10 said ''See Table 4D4A of BS7671.'' Still need to know what type of loads you are expecting to supply.

EV car chargers 22kw each x 6

 

[timhoward]

EV car chargers 22kw each x 6

Is your intention 3 on each cable?
Do you mean exactly what you said - 22kw chargers (implying DC to directly charge the batteries) or did you mean 22kw AC charge points which use the charger in the car?
Most people don't every mean "chargers" but as the cable sizing you quoted isn't immediately making sense to me this could be the beginning of a most interesting thread!

(For what it's worth, without doing any maths, Zs, or volt drop etc... my understanding was that a 22Kw charge point on a balanced 3 phase supply draws approx. 32 amps and the one I've seen was installed with 6 sq mm conductors admittedly very close to the supply. )

 

[Electro-tech]

You really need to know what EV chargers you intend to install & if 3 on each cable. You may need to size the cables to allow for future additions of more charge points. Do you intend to charge all points simultaneously? If not, will you be using a management system to monitor & manage the peak building loads without affecting the main use during the day?

 

[pc1966]

Also check what the OCPD requirements are, it might be each charger needs its own protection so you would be needing either 6 cables from the DB, one per charger, or a sub-DB at the end to distribute power.

I am no expert on EV chargers but I would be very surprised if 22kW units are not 3-phase, and also need neutral, so you would be looking at 4C cable for the feeds.

Also are the two cables you mention going in separate ducts, or shared duct? Then you have to allow a grouping factor as well as the buried-method de-rating.

 

[timhoward]

I am no expert on EV chargers but I would be very surprised if 22kW units are not 3-phase, and also need neutral, so you would be looking at 4C cable for the feeds.

Yes, 22Kw EV charge points are 3 phase. The only one I've seen in the flesh required a neutral.
I almost mentioned the 4th core but thought there was a remote possibility these were DC chargers as 6 of them sounded like a large car park where rapid charging might be required and it would begin to explain the larger cables.

 

[craig.jones76]

Also check what the OCPD requirements are, it might be each charger needs its own protection so you would be needing either 6 cables from the DB, one per charger, or a sub-DB at the end to distribute power.

I am no expert on EV chargers but I would be very surprised if 22kW units are not 3-phase, and also need neutral, so you would be looking at 4C cable for the feeds.

Also are the two cables you mention going in separate ducts, or shared duct? Then you have to allow a grouping factor as well as the buried-method de-rating.

HI

Cables will do into conduit under the ground and be grouped together.
The chargers are 415V, no mention of neutral on data sheet but I'll check with supplier.