SWA cable rating U/G

[driverman]

Not got regs with me. Installing sub-main to detached garage using SWA method "D". Propose using 6mm 3 core SWA cable. From memory takes about 46/48amps. Don"t know about 17th regs..but in the 16th regs were you allowed to add 10% to the tabulated figure given if u/g. Basically I'm trying to find what is the max current that could be carried. The SWA cable will also installed within the house behind the ceiling/insulation. Thanks

 

[davesparks]

Eh? Cable selection starts with working out how much current it needs to carry, you don't start with a cable size then work backwards to the current.

You can't apply any factors for part of the cable being underground if the rest of it is run through the ceiling void
The whole length of the cable should be rated according to the most onerous of the conditions it is to be installed in.

 

[Richard Burns]

what is the max current that could be carried. The SWA cable will also installed within the house behind the ceiling/insulation. Thanks

This installation would bring your max current down to about 38A.

 

[old_sparky_RE]

Hi Richard,
I'm just asking and not poking!

I take it you used .88 as the Ci and the cable is a 90degree? This being the best case, .51 being the worse case and would thus give a reading of only 21A !
Cg = 1, Ca = 1, Ci = .88, Cs = .9, Cc = .9 & Cd = 1 It =53A.

 

[Richard Burns]

Hi Richard,
I'm just asking and not poking!

I take it you used .88 as the Ci and the cable is a 90degree? This being the best case, .51 being the worse case and would thus give a reading of only 21A !
Cg = 1, Ca = 1, Ci = .88, Cs = .9, Cc = .9 & Cd = 1 It =53A.

I was not doing a calculation just giving an idea of approximate value.

Cable run in insulation touching a wall surface is reference method A and 6mm² unarmoured multicore cable has a CCC of 32A, 90°C SWA has CCC of 42A.
70°C SWA generally has a slightly higher CCC than unarmoured multicore and slightly lower than 90° SWA so I went for about 38A.

Sorry not to be more technical:shame:

 

[Besoeker]

Eh? Cable selection starts with working out how much current it needs to carry, you don't start with a cable size then work backwards to the current.

You can't apply any factors for part of the cable being underground if the rest of it is run through the ceiling void
The whole length of the cable should be rated according to the most onerous of the conditions it is to be installed in.

Totally agree.

 

[davesparks]

Hi Richard,
I'm just asking and not poking!

I take it you used .88 as the Ci and the cable is a 90degree? This being the best case, .51 being the worse case and would thus give a reading of only 21A !
Cg = 1, Ca = 1, Ci = .88, Cs = .9, Cc = .9 & Cd = 1 It =53A.

You can't use the 90degree ratings unless you have equipment designed for 90degree operation connected to both ends of the cable

 

[driverman]

Totally agree.

Yes, badly worded question, sorry about that. customer has vague idea of his requirements but not too sure.

 

[driverman]

You can't use the 90degree ratings unless you have equipment designed for 90degree operation connected to both ends of the cable

Table 4D4A @ 70 degrees..states 46 amps Method D then it will be d/rating to suit environment. ie behind plaster board / insulation.

 

[Richard Burns]

Table 4D4A @ 70 degrees..states 46 amps Method D then it will be d/rating to suit environment. ie behind plaster board / insulation.

Consider each section of cable that has its own installation method independently, then calculate the current carrying capacity using the worst case scenario.

If it is installed to Method D then it is unlikley that there will be plasterboard and insulation underground, so consider the section underground and calculate the CCC for that.
Then consider the section in plasterboard and insulation (possibly method A) and calculate the CCC.

The lowest of the two values you get is the one that is limiting your installation design current.