Pulling in swa

rainstorm · May 7, 2021

Good evening all
Got a 200m run of 3 core 10mm swa to install in a trench. Rather than pulling it through a duct, was wondering if anyone has ever pulled the duct over the cable, then dropped in into trench.
Duct is 50mm internal diameter flexible type. Might be a non starter, just weighing up options.
Thanks for any help.

cliffed · May 7, 2021

No need for ducting if laid in properly obviously if required by the client then add another duct.

Pete999 · May 7, 2021

rainstorm said:
Good evening all
Got a 200m run of 3 core 10mm swa to install in a trench. Rather than pulling it through a duct, was wondering if anyone has ever pulled the duct over the cable, then dropped in into trench.
Duct is 50mm internal diameter flexible type. Might be a non starter, just weighing up options.
Thanks for any help.

50mm = 2 inches 10mm SWA 3 Core gonna be tight hope you have a strong pull cord good luck, same as what cliffed said, in complete agreement with him

rainstorm · May 7, 2021

Thanks.
Was wanting duct is to protect against stones etc.
Any thoughts on the chances of pulling the duct over the cable?
Comes in 50m lengths.

Yozza19 · May 7, 2021

4 inch is the minimum I would be using and a few draw pits for 200m

snowhead · May 7, 2021

I'd say non starter, unless you did it with several men / women / others, to lift the weight.
50 mtrs of SWA weighs around 40kg so when you're dragging the 50mtr length of duct you've got 40kg of SWA bearing down on it once you get a full length of duct on the cable.

At best you'll rip the duct, defeating the whole purpose of using it.

MJPD29 · May 7, 2021

That's a very expensive cable for about 13ish amps

MJPD29 · May 7, 2021

But on another note I either wouldn't install a duct unless requested my client or 4 inch duct. Wouldn't bother with 50mm it'll be a nightmare

pc1966 · May 7, 2021

If you already have bought the duct and it is not reusable/returnable then you might as well put it over the SWA before dropping it in. But 200m is a long pull no matter what, though a team of heavies and plenty of cable lube might allow it but really you ought to be looking at something bigger.

If you do decide just to put the cable in the ground directly then make sure you have 5-10cm of sand down first, and the same on top afterwards, to stop stones and other stuff from damaging it as you back-fill.

rainstorm · May 7, 2021

Thanks.
Think sand and no duct is sounding best.
It is an expensive cable Matthew, but necessary.

IAmSparkytus! · May 7, 2021

Just being nebby but what's it feeding?

Dustydazzler · May 8, 2021

Sand , lay cable . More sand , warning tape, back fill trench

job done

forget the ducting , waste of money and time

DefyG · May 8, 2021

rainstorm said:
It is an expensive cable

And the sand wont be cheap! 200m at 200mm thick (100mm below cable and 100mm over the top) and say 200mm wide trench = 8cu m (8 builder bags worth)!

telectrix · May 8, 2021

save all the hard work. hire Mighty Mole for a day.

MJPD29 · May 8, 2021

ToonSparky said:
Just being nebby but what's it feeding?

Single socket maybe?

pc1966 · May 8, 2021

Matthewd29 said:
Single socket maybe?

Well 5% drop and 10mm single phase comes to 12.3A over 200m so not far off it!

DefyG · May 8, 2021

Matthewd29 said:
Single socket maybe?

Doubt it could be that, I just did a quick VD calc and 5% is exceeded at a load of 13A over that distance for 10sqmm

DefyG · May 8, 2021

I calculated 7.54v at 13A over 200m but at 12.3A it could be slightly under 5%

pc1966 said:
Well 5% drop and 10mm single phase comes to 12.3A over 200m so not far off it!

but either way certainly nowhere near 3% for lighting!

pc1966 · May 8, 2021

DefyG said:
I calculated 7.54v at 13A over 200m but at 12.3A it could be slightly under 5%

but either way certainly nowhere near 3% for lighting!

~12A is a lot of LED illumination!

Football park?

Mike Johnson · May 8, 2021

If pulling a 200m length of duct over a 200m length of SWA with a 200m pull cord you need 400m of space in a straight line to do it, and that is only if the draw cord is already in the duct, I did this with an alarm cable, but only 100m, would be easier in 50m sections with draw pits.

pc1966 · May 8, 2021

DefyG said:
And the sand wont be cheap! 200m at 200mm thick (100mm below cable and 100mm over the top) and say 200mm wide trench = 8cu m (8 builder bags worth)!

Yikes!

Just looked at that on some web site. Even at 5cm surround (10cm x 10cm) it is saying 5 bulk bags at ~£50 each. Going to a safer/easier to pour in 20xm x 20cm (i.e. 10cm safe margin) it is 4 times that and almost £1,000

Might be cheaper to look at 90mm or 110mm twinwall duct? 200m of that is around the £400 mark.

DefyG · May 8, 2021

pc1966 said:
12A is a lot of LED illumination!

Agreed but the OP is being a bit coy as to what the cable is being used for. Assuming a mixed load it 'should' still meet 3% guideline if only 1A of lighting!

pc1966 · May 8, 2021

DefyG said:
Agreed but the OP is being a bit coy as to what the cable is being used for. Assuming a mixed load it 'should' still meet 3% guideline if only 1A of lighting!

Well the regs still say that but really it is a hang-over from the days of filament lamps and their significant voltage/illumination sensitivity.

I personally would be OK with 5% drop on "mixed" if it is LED lighting. Which it almost certainly would be.

pc1966 · May 8, 2021

From here it has the light versus voltage law for halogen lamps as more or less cubic (V^3):

Halogen lamp - Wikipedia

en.wikipedia.org

LED lamps are more or less linear output with V if a simple current-limiting resistor is used, even less sensitive if some DC-DC coveter/driver that regulates the output.

So best-case LED cares not as long as in-specification, worst case LED is 3 times less sensitive to small changes in voltage (i.e. if 3% limit VD for halogen lamp, then LED equivalent is around 9%)

DefyG · May 8, 2021

pc1966 said:
Well the regs still say that but really it is a hang-over from the days of filament lamps and their significant voltage/illumination sensitivity.

I personally would be OK with 5% drop on "mixed" if it is LED lighting. Which it almost certainly would be.

I probably would too which is why I put 'should' in ' ' and guideline, and is justifiable if not filament/discharge lights.
I would probably aim for 4% though which would limit the OCD to perhaps 10A for 10sqmm!