Calculating max loads for radial and looped circuits

[4xpaws]

Say for example, you've got T&E 4mm cable

Volt drop is 11.0 mV/A/m

wiring regs say that your voltage drop must not be more than 4%.

4% of 230V is 9.2V.

Current rating is 36A clipped direct.

For a 32A breaker, 32 x 11mV = 352mV drop per metre of cable.

9.2 ÷ 0.352 = 26m.

How do you then calculate the max kW loads for:
-26m radial circuit
-& 26m looped circuit in this case?

 

[Adam_151]

You've got it for the radial circuit... 26m

For the ring final multiply it by 4... 26mx4 = 104m

(the logic behing it is, the worst case is the midpoint, the cabling length to the midpoint is half of the total cable length, so we halve the resisance of the full length of cable, and we have two such bits in parrallel, so we halve that resistance again, so the resistance from orgin to mid-point on a ring final circuit is one quater of a radial circuit using the same length and CSA of cabling)

Remember though, that the load on a circuit serving socket outlets is not going to be concentrated all on the mid point or the the far point, and the standard circuits in the OSG do make an allowance for this

Thats just for the volt drop side of things, sometimes EFLI bites before VD!

 

[4xpaws]

Thanks Adam,
How would you then calculate the max kW loads for my example above?

 

[ricicle]

As you have based the VD on 32A then

32A @ 240V = 7.68 kW

32A @ 230V = 7.36 kW

(Sorry Adam, didn't want to steal your thunder)

 

[4xpaws]

Of course, P=IV !!
many thanks again!