Rating factor within an enclosure?

[lurch]

I seem to recall there is a rating factor that can be used within an enclosure effectively uprating the current capacity of the cable. Does any one know what this is and can it be found in BS7671 or is this a rule of thumb (?) for switchboard manufacturers?

 

[buzzlightyear]

is this a rule of thumb

which hand!

 

[lurch]

Hilar

 

[westward10]

I don't really understand what you are asking.

 

[Spoon]

Is this an existing enclosure mate or are you designing it?
A bit more info please.

 

[lurch]

Is this an existing enclosure mate or are you designing it?
A bit more info please.

Existing or new. Either or .....

 

[7029 dave]

I recall if you are running cables lets say in trunking, (metal containment) there is a factor to apply , that will uprate the ccc. But I cannot give you figures it's a long time ago.

 

[buzzlightyear]

rating factors pages 326 onwards .BYB.

 

[Lagentium]

Do you mean where a smaller cable can be fed from a higher rated supply, as long as it is mechanically protected?

 

[lurch]

Do you mean where a smaller cable can be fed from a higher rated supply, as long as it is mechanically protected?

Yes . . . Within an enclosure such as a bus bar chamber, switchboard of metal construction

 

[Bobster]

It doesn't up rate the CCC of the cable at all.

I think you're referring to a fault free zone, which is limited to 3meters.

If I understand what your asking, an example. 25mm cable connected to a bus bar system backed with 800A fuses. So long as that 25mm is terminated into a suitable ocpd within 3 meters and is mechanically protected along it's route. It's all good.

 

[westward10]

No conductor has a determined ccc it is all dependent on installation methods.

 

[lurch]

It doesn't up rate the CCC of the cable at all.

I think you're referring to a fault free zone, which is limited to 3meters.

If I understand what your asking, an example. 25mm cable connected to a bus bar system backed with 800A fuses. So long as that 25mm is terminated into a suitable ocpd within 3 meters and is mechanically protected along it's route. It's all good.

yep I'm fine with that one, but thanks for feed back. (BTW ...ocpd = obsessive compulsive personality disorder? ;-) )

For instance when I look at a domestic cu with a 100a mainswitch fed from a DNO cut out with BS88 100a fuse we would put 25mm tails in. I look at the neutral lead and sometimes the live lead within the board supplied by manufacturers these are no where near 25mm and I doubt if there rated at 100a but clearly somewhere there is some rating factor that switchboard manufacturers use that allows this. . . .

Also may moons ago I had a electrical clerk of works condemn a switchboard in a commercial scenario that and been supplied with same. . . we engaged the manufacturers and met with said CoW and they pointed him in the direction of this "uprating" factor, for want of a better word, and it was accepted.

hope this helps/clarifies where I'm coming from

 

[Bobster]

Yes, I see where you're coming from.

As far as I am aware there is no rules for "up rating"

However, BS7671 CCC ratings have a huge safety margin written into them. Ever seen how much current is needed to raise 2.5mm to above 70deg in open air.

If a manufacturer can prove through type testing that a smaller CCC is safe for use, that will overrule 7671

 

[lurch]

Rob. Agreed with regards to the safety margins, and in a lot of cases well over the top.

Off at a tangent a little. . . but still on current carrying capacity. If one doubles up on two cables would this simply double the current carrying capacity ?

 

[westward10]

Generally, yes if they are the same construction and csa.

 

[Bobster]

Rob. Agreed with regards to the safety margins, and in a lot of cases well over the top.

Off at a tangent a little. . . but still on current carrying capacity. If one doubles up on two cables would this simply double the current carrying capacity ?

If you're using normal cable calculations then yes effectively.

Delve a little deeper into it and then no, there will be a difference. Nothing to worry about at the level we work at.

However 2 x 70mm wires will take more than 1 x 140mm wire.

 

[lurch]

If you're using normal cable calculations then yes effectively.

Delve a little deeper into it and then no, there will be a difference. Nothing to worry about at the level we work at.

However 2 x 70mm wires will take more than 1 x 140mm wire.

that was my understanding also. But lets keepit simple !!!!!!!