Cable Sizing

Umiq88

Can someone please go through whats required when installing a cable.

I know first you need to find the designed current i.e. the current of the load and then select your breaker and your cable is sized from the breaker with whatever correction factors (grouping, temp, etc.) is there any other considerations?

Also there's mention about loop impedance in other threads what is this about and should this be done on every cable? Are there any other checks required?

M cebee

the 'loop-impedance' needs to match the 'protective device' for the cable

to ensure 'automatic disconnection' within the required time

(basically a short- circuit)

'overcurrent protection' will still work even if the loop impedance is wrong

mazthespark

psittacosis wrote: »

Can someone please go through whats required when installing a cable.

I know first you need to find the designed current i.e. the current of the load and then select your breaker and your cable is sized from the breaker with whatever correction factors (grouping, temp, etc.) is there any other considerations?

Also there's mention about loop impedance in other threads what is this about and should this be done on every cable? Are there any other checks required?

ya you have mentioned most of the main ones there voltage drop is another one to consider.

etci regs are very handy for it and the sequence to do it which of course isnt in them! should have a sample question somewhere from my fas days if you need it

also if you are anywhere near the max values of a cable go up to the next size will allow for expansion etc.

M cebee

433.1 in the rules shows a simple formula

Ib<In<Iz

thats 'Design Current' less than/equal to rating of 'Protective Device '

less than/equal to 'CCC' of cable(including correction factors)

The 'Fault Loop Impedance' will have to be appropriate for protective device and type of circuit(distribution or final)

and 'VD' is 4%max within installation- at FLA or design current

you already have the most of it anyhow