# Substation Entry- Short section of cable in duct

Hi I am in the middle of a design for a new substation and one of the supplies is circa 150m long and will run for 146m on a ladder in free air. The final 4m will be routed underground into the substation through a duct then the substation trench into the LV switchboard.

when calculating for this if I say the cable is just on the ladder then I need a single 240mm2 cable, however if I say it’s buried in ducts then the cable requirement becomes 3 x 300mm2 cables in parallel which seems extreme in practical sense.

does anyone have any thoughts on this or experience of a similar situation and how to comply with BS7671 whilst still being practical.

Parents
• what are you assuming about cooling and ambient temps in the ducted section, vs assumptions in the open air run

I think I can see what's going on here.

240 sq mm 4-core in free air has a current-carrying capacity of 516 A for three-phase operation (70 deg C conductor temp - Table 4D4A). When you look at ducts, this drops to 280 A. This is simple physics.

However, if this cable really is running near 500 A for 150 m, isn't this a problem for volt-drop ?

When going up a conductor size, this doesn't help much in the duct, and when you parallel you end up with a grouping factor issue ... which, as we've seen before, can really mess things up.

Overall, the simple laws of physics apply and if you enclose cables they will get hotter, but as   says there are other models to use; however, I still think there will be a big difference between "in ducts" and "free air", made worse with grouping for parallel cables.

• 516 A for three-phase operation (70 deg C conductor temp - Table 4D4A). When you look at ducts, this drops to 280 A.

I wondered that, but even then I struggled to get to needing 3 lots of 300mm in parallel even at the full 500A unless they are tied together and the bundle thermally insulated ;-) (oh and assume the soil type is desert sand..)

My suggestion to the OP is that the ERA papers that consider  bigger ducts and cables on brackets where cables are more thermally independent are probably worth a browse to see how that affects things.
And then there are tall ducts with vented tops (or even a grid top like a boot scraper) so that hot air is not trapped and that is more like cable on a wall. The nos in the regs are similar to to the solid/ unvented case where really it is a cable in a pipe.

I agree though that if it is the full 500A   then volt drop over 150m, won't be stellar with 240mm2  (~ 0.2V per amp per km call it 100V per km  at 500A >>  15V on your 400V over 150m ) though I suppose depending on what is fed by it, maybe OK - but I agree it leaves precious little slack for voltage drop in whatever final wiring is at the far end and may not clear the transformer end protection especially promptly in a fault. Of course all this is guesswork on my part as we don't know the design current or duty cycle or even what LV fuses protect the cable leaving the Tx ....
Mike

• 240 sq mm 4-core in free air has a current-carrying capacity of 516 A

Hi Graham, Think you've read the wrong column 516A is for a two core cable. A 4 core CCC is 445Amps.