voltdrop calculation

[empress]

been to look at a job with a long main supply approx 60m long underground 25mm 3core SWA running off a 100amp fused isolator.
meter position is at drive entrance and house is approx 60meters away.
not done a voltdrop calc for ages so can someone correct me if i,m wrong.
this is a new build from 3 years ago which has changed hands and new owners have built a huge double garage that needs wiring.
first look i thought it was to small for the length of run but my calc seems to put it ok.

1.64 x 60 x100
------------------- giving 9.84 v with 11.5v being max as i recall
1000

 

[Simonslimline]

Out of curiosity what is the sub-main supplying & is the Ib anywhere near 100Amps?

 

[empress]

Out of curiosity what is the sub-main supplying & is the Ib anywhere near 100Amps?

sorry the 25mm 3 core swa in question is the main supply for the house not a sub main .i have been asked to run sub main to the new garage.

 

[Simonslimline]

So it is supplying lighting finals then! Max VD is 3%. Just a thought and happy to be corrected.

 

[empress]

So it is supplying lighting finals then! Max VD is 3%. Just a thought and happy to be corrected.

yes it is good point on 3% would be looking at 6.9 V

 

[telectrix]

work out the current likely to be drawn, then do your vd calc. bet it;s nowhere near 100A.

 

[empress]

work out the current likely to be drawn, then do your vd calc. bet it;s nowhere near 100A.

no your right but is that the right way to determine cable size ?

 

[telectrix]

yes. you size cable according to design load. corrent 1st., the VD, then OCPD rating (Iz>In>Ib)

 

[atm84]

When I do vd calc I usually only allow 0.5% from the intake to main DB then 2.5% max for final lighting circuits.

 

[Knobhead]

When I do vd calc I usually only allow 0.5% from the intake to main DB then 2.5% max for final lighting circuits.

What size cable would that result in for 60m 100A supply?
(Hint, I fell about laughing.)

One thing that has struck me, the OP is suggesting a 100A fused switch. No allowance for discrimination between a standard 100A service.

 

[Besoeker]

been to look at a job with a long main supply approx 60m long underground 25mm 3core SWA running off a 100amp fused isolator.
meter position is at drive entrance and house is approx 60meters away.
not done a voltdrop calc for ages so can someone correct me if i,m wrong.
this is a new build from 3 years ago which has changed hands and new owners have built a huge double garage that needs wiring.
first look i thought it was to small for the length of run but my calc seems to put it ok.

1.64 x 60 x100
------------------- giving 9.84 v with 11.5v being max as i recall
1000

Others have wisely suggested looking at the actual load for the volt drop calculation.
Another thought. The 1.64 is presumably the mV/A/m from BS7671 - the regs.
It's per conductor. If you have a 60m run of single phase you have two current carrying conductors each 60m long.

 

[Spoon]

If you have a 60m run of single phase you have two current carrying conductors each 60m long.

and what is your point, regarding the calculation of VD?

 

[telectrix]

Others have wisely suggested looking at the actual load for the volt drop calculation.
Another thought. The 1.64 is presumably the mV/A/m from BS7671 - the regs.
It's per conductor. If you have a 60m run of single phase you have two current carrying conductors each 60m long.

surely the mV/A/m figure in BS7671 allows for the fact that you have 2 conductors. if you had only 1 conductor, there would be no current flow, hence no volt drop.

 

[Spoon]

It does what it says on the tin.... just pick the column that is relevant to you..

 

[Chr!s]

A good way to estimate VD.

22.5/ csa, now times 2 for single phase and 1.73 for 3 phase.

so

22.5/25 = 0.9 now x 2 = 1.8

60x1.8x100/1000 = 10.8 volts

good for cables upto 50mm, beyond that add 0.08 for reactance.

 

[empress]

What size cable would that result in for 60m 100A supply?
(Hint, I fell about laughing.)

One thing that has struck me, the OP is suggesting a 100A fused switch. No allowance for discrimination between a standard 100A service.

i have not suggested anything and have not done any work on this installation , just described the lay out of the existing installation which includes a switch fused 100amp isolator at the mains position.
the plot thickens.
i'm told there use to be a caravan were the new garage has been built .
found an enclosure with a consumer unit fed in 25mm SWA.
with no sign of the "Caravan supply " SWA at either end the bulder made a few calls and i was told there is an underground " tee joint " in the main SWA before the house !
anyone come across that before.
whipped the cover off the CU in the house for a quick look and has all the hall marks of a "Pub Job" ,just had a copy of the toilet paper /test sheets
box marked reg no reads "12345678 " .
due my inspection next month and jobs like this make you want tell em to shove there fee up the rusty rivet hole.