What cable size and breaker would I need for two extractor fans in a public toilet? 1.5mm 3 core and a 20amp rcbo?

[timhoward]

Strictly speaking method C is 19.5 amps. But we won't haggle over 0.5 of an amp. In fact running a 1.5mm directly to the 2 fans is also perfectly safe on a 20amp rcbo. But is it conventional, good practice or advisable? No

Is it not 17.5 amps for a 3 core or am I losing plot again?!

Table 4D2A:

 

[Soi disant]

Strictly speaking method C is 19.5 amps.

Not according to 4D5

 

[LastManOnline]

I was wondering if there were FCU's by the fans and that overload protection was provided that way.

If the rcbo feeds the fans directly than they can be treated like any "End of line" appliance and overload protection omitted. Short circuit protection only required and the 20 amp RCBO covers that

It's likely the RCBO is oversized but can't be sure without knowing more about the fans who knows.

It would be oversized if the 1.5mm was connected to a circuit where the loading can't be controlled directly, like a socket circuit socket

If the OP has a clamp meter it would be interesting to know the current draw with both fans on.

In principle that's not an issue. Both fans won't take more than an amp or 2.Well under the capacity of the cable and the rcbo. The issue is, is the cable going to be protected properly under overload and short circuit conditions. It is Interesting how unconventional situations (like the OP, s) force us to look at a situation that we all know is instinctively wrong but can't immediately explain why (talking about myself here). Then we end up "going back to the drawing board" and having to figure out again.. "Why is this wrong?". Good exercise though!!

 

[Soi disant]

Is it not 17.5 amps for a 3 core or am I losing plot again?!

That's 3-core as in 3 phase conductors, not as in T&E.

 

[LastManOnline]

Is it not 17.5 amps for a 3 core or am I losing plot again?!

Table 4D2A:

View attachment 84376

You are most definately not losing the plot. It's there in black and white (or purple and white). But I think the key is that that 3 core is referring to 3 "loaded" conductors, as in 3 phase, whereas the 3 core referred to by the OP is containing just 2 loaded conductors

 

[LastManOnline]

Not according to 4D5

Does it not say 19.5 amps in the table "Tim Howard" posted?

 

[Soi disant]

Does it not say 19.5 amps in the table "Tim Howard" posted?

It does, but that's Table 4D2A, not 4D5

 

[timhoward]

The issue is, is the cable going to be protected properly under overload and short circuit conditions. It is Interesting how unconventional situations (like the OP, s) force us to look at a situation that we all know is instinctively wrong but can't immediately explain why (talking about myself here). Then we end up "going back to the drawing board" and having to figure out again.. "Why is this wrong?". Good exercise though!!

Thank you for talking that through. I was certainly thinking in terms of overload and fault protection. I get that overload protection could technically be omitted. If (as in the case of many fans) the manufacturer advises 3 amp protection it still doesn't sit right with me that that 60 amps could theoretically flow (B curve) before it shuts off.
Is there a remote possibility of more current being drawn if the if the fan jammed or the windings were damaged and it failed to properly start and just hummed I wonder. Probably unlikely and academic.

 

[timhoward]

It does, but that's Table 4D2A, not 4D5

My bad - trouble with using app, too easy to choose wrong table.

 

[LastManOnline]

It does, but that's Table 4D2A, not 4D5

Then I stand corrected. I was, nt aware you had different tables for calculating current capacity.

 

[Soi disant]

Then I stand corrected. I was, nt aware you had different tables for calculating current capacity.

There are different cable types, so there have to be different tables.

 

[LastManOnline]

Is there a remote possibility of more current being drawn if the if the fan jammed or the windings were damaged and it failed to properly start and just hummed I wonder. Probably unlikely and academic.

That's a valid point actually as technically all motors should have overload protection. But the wattage of a standard 4" fan is very low.Should it jam its internal impedence will prevent large currents from flowing and if they did the fan windings would likely burn out long before the supply cable would be troubled. In fact you will sometimes come across fans that have "stuck" and on being released they carry on working. High internal impedence prevents the from burning out

 

[Soi disant]

Do they make special fans for the UK, different from the millions they make for countries where you can't get any overload protection beyond the circuit OPD?

 

[LastManOnline]

There are different cable types, so there have to be different tables.

Interestingly 4D2A conforms exactly with the table we use here (ROI) for pvc insulated cables

 

[Soi disant]

Ah - here they reverse-engineered 4D5 to keep 2.5mm² 32A ring finals workable. ?