Voltage Drop Equation

[Noob2013]

Hi,

I've been doing some revison and came across 2 different equations for voltage drop.

The first one is;

Vd = mV/A x Ib x L divided by 1000

The second one is;

Vd = (R1 + R2) x Ib x Multiplier

The multiplier is to compensate the difference in conductor temperature at the time of the test and the normal operating temperature of conductors when maximum resistance. 1.2 multiplier for 20 Degrees Celsius.

Are these both correct?

Thanks

 

[telectrix]

mV/A/m in the first example. this is taken from bgb or osg tables.

second example is basic ohms law V=IR. ( with the multiplying factor for temp. taken into consideration.

 

[Noob2013]

mV/A/m in the first example. this is taken from bgb or osg tables.

second example is basic ohms law V=IR. ( with the multiplying factor for temp. taken into consideration.

Ideal thanks

 

[Geoffsd]

The equations are the same because the

mV/A/m value IS one metre of (R1+R2) x 1.2.

 

[SirKit Breaker]

Hi,

I've been doing some revison and came across 2 different equations for voltage drop.

The first one is;

Vd = mV/A x Ib x L divided by 1000

The second one is;

Vd = (R1 + R2) x Ib x Multiplier

The multiplier is to compensate the difference in conductor temperature at the time of the test and the normal operating temperature of conductors when maximum resistance. 1.2 multiplier for 20 Degrees Celsius.

Are these both correct?

Thanks

Tel has explained it well, so i wont expand. But i will give you a couple of whys and what fors.

The (mV/A/M) x Ib x L /1000 would be the calculation used by the designer at the design stage of a new installation.

Method 2, (R1+R2) x Ib X 1.2, this would be used on a periodic inspection and test, when the tester would need to do the calculation and the circuit length is not known.

The 1.2 is to account for the temp difference between the live conductors.

Cheers............Howard

 

[Silly Sausage]

Tel has explained it well, so i wont expand. But i will give you a couple of whys and what fors.

The (mV/A/M) x Ib x L /1000 would be the calculation used by the designer at the design stage of a new installation.

Method 2, (R1+R2) x Ib X 1.2, this would be used on a periodic inspection and test, when the tester would need to do the calculation and the circuit length is not known.

The 1.2 is to account for the temp difference between the live conductors.

Cheers............Howard

That needs expansion!

 

[telectrix]

they will expand as they warm up.i'll get my coat. :waving:

 

[jc_150]

Hi all in the forum
I was browsing his thread, andas the OP appears to be a student I thought I would correct the errors beingmade in the answers regarding the second formula. The first is perfectlyok for calculating volts drop but the second is not. R1 is the resistance ofthe line cable ad R2 is the resistance o the cpc., therefore R2 is not part ofthe volts drop calculation. It should be replaced with Rn (the resistance ofthe neutral cable). The 1.2 multiplier is used in a 70 degree thermoplasticcable to take into account that the cable could be loaded to itsmaximum current carrying capacity. I believe that R1+R2 equations canbe used when evaluating Zs

 

[telectrix]

Hi all in the forum
I was browsing his thread, andas the OP appears to be a student I thought I would correct the errors beingmade in the answers regarding the second formula. The first is perfectlyok for calculating volts drop but the second is not. R1 is the resistance ofthe line cable ad R2 is the resistance o the cpc., therefore R2 is not part ofthe volts drop calculation. It should be replaced with Rn (the resistance ofthe neutral cable). The 1.2 multiplier is used in a 70 degree thermoplasticcable to take into account that the cable could be loaded to itsmaximum current carrying capacity. I believe that R1+R2 equations canbe used when evaluating Zs

well spotted there. should be Rn. however the tabulated value of R1+R2 can be used where the cpc is the same size as the line conductor, e.g. with SWA or singles.