10mm swa

[sparkattack]

hi, ive got a length of 10mm swa 3core to install partly underfloor, clipped along outside wall and underground. my big red book states it can carry 67a clipped direct so i was going to use a metal clad fused switch with a 63a fuse so it is protected but ive seen on some websites it is rated at 56a so obviously wouldnt be protected properly, do you know were this 56a comes from? The length of run is 35m.

Also im not sure weather it would be better to use a metal clad consumer unit and isolate swa into that or use an adaptable box, isolate swa into that with gland and bajo and then into dual rcd cu, what do you think? no real mechanical protection is required.

also i will be exporting earth, no debate on this please:run

thanks

 

[Andy™]

dont have BRB to hand, but the lower ratings are usually after de-rating. also depends if its XPLE or not

 

[spinlondon]

You sure it's not 53.6A, 67 X 0.8?

 

[sparkattack]

think il use a 50a fused switch to be safe. hows the proper way to isolate 3core swa into a plastic consumer unit with the earth being exported into an outbuilding? should a box be use beforehand or can the swa be glanded into the plastic with a banjo?

 

[Andy™]

depends on the plastic... IP rated enclosures can usually have SWA terminated into them. other plastic (most CU's) cant

 

[spinlondon]

Why would you want to isolate the armour, if you are exporting the earth?

 

[sparkattack]

sorry i mean terminate

 

[ianmacd]

Look for pirhanna nuts. They are much better then banjo's.

 

[sparkattack]

is it ok to terminate swa into a plastic c/u with a metal gland and either a banjo or pirhanna nut then

 

[steptoe]

yes,

as long as the plastic is physically up to the job,

 

[spinlondon]

If the plastic of the CU is too flimsy and you do not intend isolating the armour, you can use 2" metal trunking placed adjacent to the CU above, below or even perhaps along side.

 

[sparkattack]

ive been told that a 60a fuse is too high to protect 10mm swa, but my regs book on page 280 says its good for 67a clipped direct and 60a underground. Im not making a major mistake here an I?

 

[spinlondon]

My Regs. book says the same as yours.

However when designing a circuit, it is usuall to apply correction for the increased resistance when the conductor is at operating temperature.

This is achieved by either multiplying the tabulated current carrying capacity by 0.8.

As such 67A would become 53.6A.

Or dividing the value by 1.2, which would provide the result 55.83A.

This would correspond with the value 56A given in your first post.

Myself, I prefer to use the lower figure, to allow some leeway.

What is your design load?

 

[sparkattack]

about 8.5kw, but obviously i want to allow for the electric heater which could be plugged in etc and get the most of the cable as safely possible

 

[spinlondon]

So with a 3kW fire, you'd be looking at about 50A, you have a cable that when at operating temperature can carry also about 50A.

What rating of protective device do you think would be appropriate?

 

[sparkattack]

obviously 50-55a, think im going to play it safe, spend at extra 50quid and go 16mm on a 65a switch fuse. can safely have three 20a radials and a 6a lighting circuit then with no issues

 

[spinlondon]

Applying diversity to your radials, you are looking at 36A. That's based on the CPD rating, not the actual current demand, which is likely to be less.

With your lighting again just on the CPD rating you have a total of 42A.

 

[Chr!s]

My Regs. book says the same as yours.However when designing a circuit, it is usuall to apply correction for the increased resistance when the conductor is at operating temperature.

This is achieved by either multiplying the tabulated current carrying capacity by 0.8.

As such 67A would become 53.6A.

Or dividing the value by 1.2, which would provide the result 55.83A.

This would correspond with the value 56A given in your first post.

Myself, I prefer to use the lower figure, to allow some leeway.

What is your design load?

Spin the ratings in appendix 4 are taken at 70 c maximum operating, so why are you applying 0.8?

 

[spinlondon]

I use 0.8 because it gives slightly more leeway than 1.2.

 

[Chr!s]

I use 0.8 because it gives slightly more leeway than 1.2

.I maybe missing something here, but why are you applying a factor of 0.8 to the ccc of the cable?