Maximum Cable Length!

[videz6]

can someone please help me with this question

There is a 40A BS88 fuse, protecting a 10mm cable with a 4mm Earth, it is wired to Reference method C, supply is TNS and it is protected by a 30mA RCD. What is the maximum length the cable can be?

(thats all the info i have! I haven't been to collage for couple of years now and cant remember how to do it )

 

[ezzzekiel]

find max zs then deduct ze wich will leave r1+r2 rearange formula to make length the subject:

r=mva x length/1000

so

l=r x 1000/ mva

that should work

 

[JUD]

Ring circuit

4 x (Max Zs - Ze) ÷ ((R"1 + R"2) x Cr)


Radial

(Max Zs - Ze) ÷ ((R"1 + R"2) x Cr)


R"1 + R"2 = Resistance per meter of Line & Cpc conductors
Cr = Correction factor for conductor operating temp of 70°C

So lets assume your Ze is 0.8 (max for TN-S). The max Zs for a 40A BS88 fuse is 1.35Ω and the R"1 + R"2 of 10/4.0mm² cable is 6.44mΩ per meter.

The calculation would be:

(1.35 - 0.8) ÷ ((6.44/1000) * 1.2) = 71m

However most maximum circuit lengths are limited by Voltage Drop and the voltage drop of 71m of 10mm² cable carrying 40A is 12.5V which exceeds the 11.5V (5%) maximum VD for a power circuit.

VD = mV/A/m x Length x Current
VD = (4.4/1000) x 71 x 40 = 12.5V


So you would need to work out the max length for Voltage Drop

Ring

4 x (Max VD x 1000) ÷ (mV/A/m x Current)


Radial

(Max VD x 1000) ÷ (mV/A/m x Current)


So for your circuit the calculation for VD would be

(11.5 x 1000) ÷ (4.4 x 40) = 65m