2 Cables used to supply one load.

[TALK]

While inspecting a premises I have come across...a 3 phase cold room with 40amp mcb protection.The problem is...the supply cable is 2x2.5mm 4 core and a separate 4mm earth run along with the 2 cables.The 2.5mm cables are doubled at the mcb,(does this create a 5mm supply).
A 5mm supply cable will never carry 40amps if required,although circuit running ok.Whats everyones thoughts on this?especially 2 cables used for 1 supply.

 

[Des 56]

Whats everyones thoughts on this?especially 2 cables used for 1 supply.


523.8 may be worth reading 2 Swansea 4
sorry got sidetracked there

 

[TALK]

Whats everyones thoughts on this?especially 2 cables used for 1 supply.


523.8 may be worth reading 2 Swansea 4
sorry got sidetracked there

Cheers Des,just did that.I'll keep my bs closer from now on.

 

[Deleted member 26818]

This is quite common where very large supplies are required.
Often 2 x 240mm² cables are used.

 

[Adam W]

Des I know the reg you're on about and I've got no real problem with that, but:

A 5mm supply cable will never carry 40amps if required,although circuit running ok.

In which case either the cable size will need beefing up or the protective device... erm... 'beefing down'?
The cable needs to be able to take the fault current (without degradation).

 

[Deleted member 26818]

I'd ask how you have determined that 2 X 2.5mm² conductors will not carry 40A.
What cable is used and what is the reference method?

 

[Adam W]

I'd ask how you have determined that 2 X 2.5mm² conductors will not carry 40A.
What cable is used and what is the reference method?

Yeah based on table 4E2A (p286) it might be alright depending on the circumstances, but it's a close call.

 

[TALK]

Reference Method E. We call the cable hi/tuff,probably the same as thermoplastic,not swa.Also cable run is app 50mtrs.Table 4E2A states 2.5mm cable will carry 32amp alone .Doubled up to 5mm should be ok then.

 

[Engineer54]

Talk,

To be honest, i'm a little bewildered, ...have you never heard of paralleled supplies?? Paralleling such small sized cables in this case, is i suppose a little unusual, but quite acceptable. Paralleling supply cables, is normally reserved for larger cables/load carrying, and has many advantages.

These are some of them....

1/ The use of 2 or more smaller sized cables instead of one large sized cable.

2/ Cost saving on use of the smaller sized cables often quite significant.

3/ Generally a higher current carrying capacity is provided by multiples of the smaller sized cables.

4/ Lower values of volt drop, compared with one appropriate sized large cable.

5/ Ease of Installation compared with the size of cable needed if only one cable being used....

There are however certain rules that must be observed, two of them being that all paralleled cables must be of the same CSA, and be of the same length...

 

[Cyberprog]

There is one obvious disadvantage, which is that if one cable were to break, you could potentially overload the remaining cable, as well as both ends of the cable would remain live.

 

[phazed out]

? How can two 2.5 mm² csa cables equate to one 5 mm² csa cable.
6 1 mm² csa cable in parallel will each take load current/6 .
This is a greater current rating than 1 6 MM² CSA cable

 

[Des 56]

phazed out

Posted by engineer 54
3/ Generally a higher current carrying capacity is provided by multiples of the smaller sized cables.

 

[ian]

Could be wrong in this, but from memory I thought that torun cables in parallel they must be the same size, the same length and conductorsmade from the same material and have a minimum CSA of 50mm.

Cant give you the Reg No because I have left the BRB in thevan, and the vans being serviced.

Ian

 

[phazed out]

Yes I understand this but how can you say that 2 2.5mm² csa cable are the same as one 5 mm² csa cable

 

[Marvo]

There can be PSC implications if a supply is run as two parallelled cables instead of a single cable, especially on very short runs.