another voltage drop question

grousedogtom

Hi
I need to run an swa to a pump house 55mts from consumer unit buried underground , the submersible pump in well will be 1.5 hp or 1.1kw, now i have checked several online VD calculators and the results show 1.5 cable , this seems a little small , are these online calculators not accurate and is there a basic VD formula to calculate this?

pieface_ie

Yeah, a very quick calculation going by the information you supplied a single phase pump @ powerfactor of 1 (as not specified)

1.5 mm = 7.3 v drop over 55 meters

2.5 mm = 4.5 v drop over 55 meters

so a 1.5 mm is fine.

sparcocars

grousedogtom wrote: »

Hi
I need to run an swa to a pump house 55mts from consumer unit buried underground , the submersible pump in well will be 1.5 hp or 1.1kw, now i have checked several online VD calculators and the results show 1.5 cable , this seems a little small , are these online calculators not accurate and is there a basic VD formula to calculate this?

By my calculations you would only have a 1.3% volt drop which is 3 volts on that cable if it was a 1.5 square. Well below the 4% regulation. Maybe a 2.5 square cable would be better for the simple reason that the pump might be uprated at a later date and this would accommodate that.

Formula : Resistivity of cable x Length of cable
CSA of cable

Resistivity of annealed copper : .0172 micro ohms per mm

.0172 x 55
1.5 = .630 ohms


1100 watts
230 volts = 4.78 amps


.630 ohm x 4.78 amps = 3 volts

pieface_ie

By my calculations you would only have a 1.3% volt drop which is 3 volts on that cable if it was a 1.5 square. Well below the 4% regulation. Maybe a 2.5 square cable would be better for the simple reason that the pump might be uprated at a later date and this would accommodate that.

I think you made a mistake somewhere in your calculation mate, that cant be correct.

Randyleprechaun

I'd say you should be using 2.5mm2:

You are allowed 2.5% volt drop for 230V which is 5.75V

FOR 1.5mm2 29(VD)X55(mtr)X4.8(A)/1000=7.65V

For 2.5mm2 18(VD)X55(mtr)X4.8(A)/1000=4.75V

Figures are approximate, but even at that, in my opinion it has to be 2.5mm2

M cebee

i think your FLA for a 1.5hp is up around 10amps inc. efficiency and power factor

3*4 swa by my reckoning to be safe

that's about 6.6v running and about 30-40 at startup

grousedogtom

the overload fitted in a 1.5 hp control box is a 10 amp so probably runs a couple of amps less than this ,I was thinking at least 2.5 myself so i put in 4 sq today to allow for future changes etc , its amazing the difference in replys from 1.5 to 4 sq is the full formula for VD that complex or is there a simplified formula that would give you a good indication?Many thanks

M cebee

1.5HP at 230v 10amp FLA

according to most charts
http://www.elec-toolbox.com/usefulinfo/flamtrcharts.htm

55 *12mV*10 is approx 6.6 from memory

being 12mv for 4sq

you have to allow at bit extra with motors for startup

sometimes 125% of FLA is used or you can check terminal voltage for LRA or startup

1.5 and 2.5 are undersized imo

M cebee

during startup you might be allowing 15% or so

or 85% of nominal voltage

pieface_ie

The are numerous factors to be taken into consideration when sizing a cable.

Ambient temperature
Cable type
voltdrop per meter of the cable
No of other circuits run with it
Installation method
...etc

When sizing a cable for a given application there are many different installation scenarios. They all do not fall under the one bracket/calculation.

Bruthal

grousedogtom wrote: »

its amazing the difference in replys from 1.5 to 4 sq is the full formula for VD that complex or is there a simplified formula that would give you a good indication?Many thanks

2.5 would cover it. A 1.1kw resistive load and 1.5 would possibly cover it. But with a motor 4 square will give you better coverage and is better for the starting current, and would likely improve starting performance. A 1.1kw load would take 5 amps for the 1.1kw output, but with a small single phase motor it could be double this current with efficiency and power factor as said. So for the 55 meter run and close to 10 amp current a 4 square cable would be more than enough and probably allow a bigger pump also if ever required.