Voltage Drop

[Golden_Boy]

Why all the complicated formulea for working out voltage drop?
Why not just connect the load and measure the voltage with a meter?

 

[Sintra]

Because voltage drop should be calculated prior to installation of the cable.

 

[confused-fuse]

its like using a paper bag to carry water it will only carry so much before it breaks down .. so you do the VD calcs so the circuit in question will not fail or overload ...

 

[acat]

Why all the complicated formulea for working out voltage drop?
Why not just connect the load and measure the voltage with a meter?

So what happens when you have installed the new circuit etc etc infront of the customer and then when you connect the load find the voltage drop is to much so infornt of the customer you have to remove the incorrect cable and put in the next size up just to find out thats wrong as well. Need I say more.


Chris

 

[Golden_Boy]

So what happens when you have installed the new circuit etc etc infront of the customer and then when you connect the load find the voltage drop is to much so infornt of the customer you have to remove the incorrect cable and put in the next size up just to find out thats wrong as well. Need I say more.


Chris

oops......

I Blame the beer..............

 

[acat]

OK you got that one covered must remember the beer answer myself


Chris

 

[SirKit Breaker]

Why not just stick to the maximum circuit length as per the BRB. Problem solved!

 

[scotsparky]

because the maximum circuit lenght in BRB does not account for ambient temps,insulation derating,circuit protective devices, gouping,installation nmethods etc

 

[Rob Smith 643]

Sorry for raking it up again but it's interesting. Table 7.1(1) latest OSG gives a maximum length of cable as 59m for 1.0mm with ref method 101 on a type B 6a mcb. Table F6 OSG gives ccc of 1.0mm with ref meth 101 of 10.5a and vd of 44mV/A/m. But the cable length permitted doesn't work out if you calculate the vd with an Ib of 6a and cable length of 59m. 44 x 6 x 59 / 1000 = 15.58v which is way in excess of 6.9v (3%). Or have I picked this up all wrong?

 

[Knobhead]

Your figures are correct. Hope you've not pulled the cable in, you need 2.5

 

[Rob Smith 643]

Cheers Tony, it's a hypothetical question thankfully. What I was trying to fathom out was why OSG reckons you can have a 59m cable run on a 6 amp breaker installed with 1.0mm with ref meth 101. How do you interpret the tables?

Rob

 

[Knobhead]

The others will tell you of my views on the reg's and recomendations. I love to prove them wrong. I'll just give the theory, facts and figures.

 

[Rob Smith 643]

Mind you I've done an install/alteration recently whereby an R1+R2 value on a landing light measured 1.59 which indicates a cable run in the region of 43m on a circuit with a design current of 4.4a (diversity taken into account). There was no way it was 43m long. 30m max. The value didn't drop significantly after a circuit 'tightening' up exercise either and the tester is fine...

 

[Jonny66]

Hi Rob, I dont think the values in the table have been calculated using 6amp as the design current or lighting load. It also will take into account that the lighting load will be spread along the full length of the circuit evenly and not all at the far end.

 

[Rob Smith 643]

Forgive me Jonny, could you clarify further for me? Am I confusing the purpose of the two tables as they appear to conflict. If I calc correctly you could only have a load of around 2.5a on a 59 m cable run on 1.0mm on ref meth 101? Sure I'm calculating incorrectly somewhere. Cold you review the string from where I came in if you have the time. Cheeky I know!

 

[Jonny66]

Sorry for raking it up again but it's interesting. Table 7.1(1) latest OSG gives a maximum length of cable as 59m for 1.0mm with ref method 101 on a type B 6a mcb. Table F6 OSG gives ccc of 1.0mm with ref meth 101 of 10.5a and vd of 44mV/A/m. But the cable length permitted doesn't work out if you calculate the vd with an Ib of 6a and cable length of 59m. 44 x 6 x 59 / 1000 = 15.58v which is way in excess of 6.9v (3%). Or have I picked this up all wrong?

What I'm trying to say is that the values in Table 7.1(1) for maximum length have probably..... taken into account that the lighting load will most likely be a lot less than 6amps and that this load will be spread out along the length of the circuit and not all lumped at the end. Hence using the standard volt drop calculation with full load and maximum distance will not equate to this standard circuit value. At the end of the day it is only a guide and doing your own calculations will give you a much more accurate value.

 

[Rob Smith 643]

There's nowt straightforward in this field...

 

[Knobhead]

This may explain it.
The voltages and % are cumulative
View attachment 14749

 

[Rob Smith 643]

The drop increases along the cable run dependent upon the load applied to it i.e., lighting points and longer cable unlike a shower circuit where all the load is applied at the far end? And the tables for vd in lighting circuits account for this probability?

 

[Knobhead]

You've got it. As you go along the cable the load drops in stages, each stage gives a lower voltage drop and %