Non Domestic 20A Radial

[Nelder]

Hi,

I'm curious to get peoples opinions on this non domestic situation please:

A 20A SP&N radial​

Fed from a 20A MCB​

70 degree singles, grouped in metal trunking fails with 2.5mm singles and even 4mm singles.​

I vaguely remember reading something that, if a cable is loaded to less than 30% its CCC, grouping can be ignored. In this instance the circuit is feeding a door access control panel with a negligible load.

Thanks

 

[littlespark]

Fails, as in trips the mcb?
Have you checked IR? Singles in metal trunking may have stripped against a sharp edge, and causing l-e fault.

how packed is the trunking, if grouping may be the issue?

 

[Ian1981]

Hi,

I'm curious to get peoples opinions on this non domestic situation please:

A 20A SP&N radial​

Fed from a 20A MCB​

70 degree singles, grouped in metal trunking fails with 2.5mm singles and even 4mm singles.​

I vaguely remember reading something that, if a cable is loaded to less than 30% its CCC, grouping can be ignored. In this instance the circuit is feeding a door access control panel with a negligible load.

Thanks

Have a look at this example regarding grouping involving lightly loaded circuits.

 

[Nelder]

Fails, as in trips the mcb?
Have you checked IR? Singles in metal trunking may have stripped against a sharp edge, and causing l-e fault.

how packed is the trunking, if grouping may be the issue?

The 2.5mm or 4mm cables fail on CCC on a 20A radial.
[automerge]1587592848[/automerge]

Have a look at this example regarding grouping involving lightly loaded circuits.
View attachment 57653

Thanks Ian1982

 

[davesparks]

What is the background to this, are you designing a new circuit?

If the load is negligible why is it being put on a 20A MCB, wouldn't a smaller one be more appropriate.

I can't find it at the moment, but there used to be a line in the regulation stating that conductors operating at less than 30% of their FLC can be excluded from grouping calculations. This is the reason a lot of lighting and small power circuits in commercial installations have 2.5mm lighting conductors and 4mm power conductors, this automatically excludes them from grouping calculations.

 

[Ian1981]

What is the background to this, are you designing a new circuit?

If the load is negligible why is it being put on a 20A MCB, wouldn't a smaller one be more appropriate.

I can't find it at the moment, but there used to be a line in the regulation stating that conductors operating at less than 30% of their FLC can be excluded from grouping calculations. This is the reason a lot of lighting and small power circuits in commercial installations have 2.5mm lighting conductors and 4mm power conductors, this automatically excludes them from grouping calculations.

If due to known operational loading of Less than 30% of it’s grouped rating then it can be ignored.
I posted an attachment on post no 3

 

[Nelder]

What is the background to this, are you designing a new circuit?

If the load is negligible why is it being put on a 20A MCB, wouldn't a smaller one be more appropriate.

I can't find it at the moment, but there used to be a line in the regulation stating that conductors operating at less than 30% of their FLC can be excluded from grouping calculations. This is the reason a lot of lighting and small power circuits in commercial installations have 2.5mm lighting conductors and 4mm power conductors, this automatically excludes them from grouping calculations.

Yeah I'm an M&E building services designer.

I thought about lower the breaker size, but its not something i have seen done before, and it will probably be picked up by the checking consultant.

Thanks
[automerge]1587636519[/automerge]
As Ian mention. I'm just going to omit the group factor.

 

[davesparks]

Yeah I'm an M&E building services designer.

I thought about lower the breaker size, but its not something i have seen done before, and it will probably be picked up by the checking consultant.

Thanks
[automerge]1587636519[/automerge]
As Ian mention. I'm just going to omit the group factor.

But surely 20A is not a suitable In if the Ib is as low as you suggest?
What is the Ib for the circuit?

 

[Ian1981]

If it’s door access, then it will be milli amps at the most unless it’s powering more than one unit , you’ll probably have a 3 amp fused connection unit before the PSU anyway but 20 amps for the ocpd is unnecessary. The psu will have its own ocpd internally as well.