volt drop rule of thumb

[johnster80]

Hi Guys,

could anyone explain to me the 1.2 volt drop rule of thumb? i understand the 3% for lighting and 5% for other circuits.
any help would be welcome studying for 2391.


Thanks in advance.

 

[Silly Sausage]

Hi Guys,

could anyone explain to me the 1.2 volt drop rule of thumb? i understand the 3% for lighting and 5% for other circuits.
any help would be welcome studying for 2391.


Thanks in advance.

I'd forget that rule of thumb job. Just do the calculation...length of cable run x design current x mV/m drp(from the appropriate big book table)

your %s are right.

Make up some imaginary circuits...100m run with 300Watt load, RFC in less than 100mm2(??)
then post your results.

 

[tony mc]

You are right re the volt drop percentage.

The rule of thumb is for calculation ZS.

In the BGB it will give you ZS values for fuses mcb's ect you then multiply by 0.8 which will give you a lower figure.
This takes into account the the increase of resistance of the conductor with the increase of temperture due to load current and errs on the side of safety.

 

[brucelee]

Hi mate volt is 3% lights and 5% power ie 6.9v lights and 11.5v power


asfor 1.2 do you not mean MF multiplying factor for increase in resistance due to operating temp for 70C cables when bunched or a core in a multicore Ie T/E
and 1.28 for xlpe 90C swa

 

[Silly Sausage]

the old 1.2 factor, I think he's getting confused with working out Zs sss, sod all to with voltage drop calcs

 

[JUD]

the old 1.2 factor, I think he's getting confused with working out Zs sss, sod all to with voltage drop calcs

Depends how you're working it out.

If using volt drop tables you don't need it.

If you're working out by resistance then you do i.e ((mΩ/m x 2 x 1.2) x L x Ib) ÷ 1000

 

[Sparky Ninja]

I think he's getting his mΩ/m and mV/m formulas mixed up.

 

[brucelee]

yes as i found out in 2391 june 2010 exam lol the 1.2 factor

 

[La Poste]

The 1.2 is the multiplying factor used in volt drop.
If you measure resistance R1 + R2 at 20C or use tables measured at 20C you have to multiply the figure by 1.2 to give you the resistance at operating temperature 70C.
This R1+R2 *1.2 is then used in the equation V=IR.
.....I..... being the design current for the circuit or preferably the measured load current.

V is the volt drop along the cable.

 

[JUD]

The 1.2 is the multiplying factor used in volt drop.
If you measure resistance R1 + R2 at 20C or use tables measured at 20C you have to multiply the figure by 1.2 to give you the resistance at operating temperature 70C.
This R1+R2 *1.2 is then used in the equation V=IR.
.....I..... being the design current for the circuit or preferably the measured load current.

V is the volt drop along the cable.

Isn't that what I just said in post #6 :smiley2:

Depends how you're working it out.

If using volt drop tables you don't need it.

If you're working out by resistance then you do i.e ((mΩ/m x 2 x 1.2) x L x Ib) ÷ 1000

 

[La Poste]

It certainly looks like it Aye.

 

[i=p/u]

V=ixr ....

 

[La Poste]

V=ixr ....

What's the X for?

 

[johnboy6083]

its a multiplcation sign? ie, 1x2=2

 

[johnster80]

Guys thanks for all the input a big help.. bit clearer, im i right in saying the multiplying by 1.2 converts from ambient temp 20c to operating temperture of 70c?