Cable Selection

Question 1

What standard do we calculate cable to?

Answer 1

BS7671

Question 2

What are the design stages?

Answer 2

Determine design current (Ib)
Selecting your protective device (In)
Select correction factors
Divide (In) by the the corrective factors
Choose suitable cable (Iz)
Calculate voltage drop
Check shock risk constraints
Check thermal constraints

Question 3

How do you determine design current?

Answer 3

P/V
or
(P / V x eff% x pf)
or
(√3 x Vl x eff x pf)

Question 4

How to select a protective device?

Answer 4

Go to the fuse table quoted in the question

Question 5

How do you calculate cable rating?

Answer 5

Design current / correction factors

I.e
Ca - ambient temp
Ci - insulation
Cg - grouping factor
Cf - rewireable fuse

Question 6

Where to select cable size?

Answer 6

Look at the cable type table and select cable higher than design current

Question 7

How to calculate volt drop?

Answer 7

(mV/A/m) x Ib x Length / 1000

Question 8

How to check for shock constraints?

Answer 8

R1 + R2 = (r1 + r2) x L x I3 factor / 1000

Question 9

How to check for thermal constraints?

Answer 9

S = √I² x t / k